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Old 09-10-2019, 07:10 AM
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Optimum geometry of roof solar panels ?


So I am very happy to see some houses, where I live, get solar panels. The solar panels rarely cover all the roof since the roofs are typically trapezoidal or triangular in shape.

So assume the following :

1. Manufacturing costs of solar panels is not effected by the size/shape of panels
2. Transportation/Installation costs of solar panels is not effected by the size/shape of panels

Just like hexagonal leather pieces are the best geometry to make a spherical soccer ball, is there is a particular geometry that would give the best coverage of the median US house roofs ?

Please keep the analysis to a pure geometry problem.

Last edited by am77494; 09-10-2019 at 07:10 AM.
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Old 09-10-2019, 07:32 AM
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ISTM, that if the shape doesn't change the cost, then the best thing to do would be to have either the big squares like we already have for the field, and a variety of other shapes to fill in the boarders. Whether it's panels that are trapezoidal with one size being square and the other side at an angle that lets it follow the roof line, or even smaller squares to fill the gaps along the edges.

Come to think of it, even just having them all made of squares would get you a much better fill, like finding the area under a curve, the smaller the panels, the closer to the edge you can get.

As I'm writing this, I'm thinking that it wouldn't surprise me if over the next few decades, the panel sizes became standardized (if they're not already) and people kept that in mind while building houses. Even if you don't fit put them on while building the house, it would be easier to retrofit if the roof was designed in such a way that you could get edge to edge coverage.
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Old 09-10-2019, 09:14 AM
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You could get better coverage, regardless of shape, by making the panels smaller. But that would presumably carry its own decreases in practicality, or they'd be doing it already.

And while some roofs are trapezoidal (with various angles), some are still rectangular, and some are other, more complicated shapes. And not all roofs would be equally well-suited for solar panels, due to their orientation and environment. A proper answer to this question would require some sort of statistical analysis of typical roof shapes and sizes, probably cross-tabbed by latitude and climate.
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Old 09-10-2019, 09:56 AM
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A proper answer to this question would require some sort of statistical analysis of typical roof shapes and sizes, probably cross-tabbed by latitude and climate.
Agree. I was hoping such an analysis would have already been done and was available.
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Old 09-10-2019, 10:20 AM
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Originally Posted by am77494 View Post

So assume the following :

1. Manufacturing costs of solar panels is not effected by the size/shape of panels
I take this to mean that it costs a fixed amount to manufacture X square feet of panel, no matter how many panels are manufactured to cover that area. Given this (extraordinarily unrealistic) assumption, the answer is very simple: make them as small as possible. The shape doesn't really matter as long as it tessellates the plane. Panels that are 1-inch squares will completely cover any roof, with only a tiny perimeter less than one inch wide uncovered.
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Old 09-10-2019, 11:09 AM
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Alternatively , if you follow the constraints of the OP, you make each facet of the roof a single panel since manufacturing costs won't matter. I think squares would be the best bet overall for reasons mentioned above. Keep in mind you will only be covering the areas of the pitch that face southerly as covering the entire thing wouldn't be worthwhile. This problem is the whole idea driving Solar Roof tiles, BTW. Tesla's version is still not here but supposedly other companies are trying to make it economically reasonable.
https://arstechnica.com/science/2018...nt-from-tesla/
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Old 09-10-2019, 11:50 AM
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I realize my mistake in the OP. I think it is a trivial solution to make the panels extremely small or just the shape of the roof.

If you may permit, my original intent was to ask to use the minimum number of panels to cover the most area of the median US houses.
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Old 09-10-2019, 11:56 AM
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Even with that clarification, I think you're still up against roof shape/pitch and southern exposure to come up with any meaningful answer. The pedantic answer is "1, custom fit to fill the available space that has the proper orientation," but that's not really going to get you anywhere. Knowing the square footage of the median US home doesn't help because you won't know how that relates even to a hypothetical median roof structure, because you won't know how many floors it's spread across. A ranch-style house of 2600 sq. ft. has a hell of a lot more roof than a four-story townhouse of 2600 sq. ft.

N.b.: the linked stat, I realize in retrospect, is for new homes not all homes. Alas, I have to go offline now.

Last edited by KneadToKnow; 09-10-2019 at 11:59 AM.
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Old 09-10-2019, 12:08 PM
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I think that, even if we're assuming that all shapes are equally easy to make for any given area, we still want to assume a minimal number of different shapes, because the panel factory will have to re-tool in some way for each shape, which is usually expensive, so you want each of your models to be produced in as high a quantity as possible.
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Old 09-10-2019, 12:18 PM
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The people who are proposing tiny panels are forgetting one thing: we live in the “real world.”
That means, each panel must have connections to all the other panels. The more panels, the more connections, which means more points of failure, and more holes in the roof to leak. And, more expense to install.
That’s one of the reasons solar shingles have never caught on.
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Old 09-10-2019, 01:40 PM
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Originally Posted by beowulff View Post
The people who are proposing tiny panels are forgetting one thing: we live in the “real world.”
That means, each panel must have connections to all the other panels.
Yes we do, but the OP's question is not about the real world. They specifically said:

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Originally Posted by am77494 View Post
Please keep the analysis to a pure geometry problem.
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Old 09-10-2019, 01:56 PM
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Yes we do, but the OP's question is not about the real world. They specifically said:
Then the obious answer is: dust.
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Old 09-10-2019, 02:23 PM
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I agree the OP's conditions are crazily unrealistic -- both of them are clearly not true. And the OP is neglecting one of the more important components: solar panels produce DC current, and so each panel needs a DC-to-AC converter built in. It's far more expensive than the glass & silicone that makes up most of the panel, so bigger panels are cheaper.

But I would say that you could adequately cover most roofs with about 4 sizes:
- 4' x 8' rectangular
- 2' x 6' rectangular
- 4' x 4' right triangle (4' x 4' square divided in half diagonally)
- 2' x 2' right triangle (2' x 2' square divided in half diagonally)

These shapes could be patched together to cover most roof geometries. You would start with the biggest sizes, and then place smaller ones around them to cover the rest of the roof. (Assuming US measurements here; the rest of the world would use metric equivalents.)
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Old 09-10-2019, 03:02 PM
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Originally Posted by Tim@T-Bonham.net View Post

But I would say that you could adequately cover most roofs with about 4 sizes:
- 4' x 8' rectangular
- 2' x 6' rectangular
- 4' x 4' right triangle (4' x 4' square divided in half diagonally)
- 2' x 2' right triangle (2' x 2' square divided in half diagonally)
There's a slippery slope here. Yes, that set of 4 panels would do a good job. But I could do an even better job with a set of 6 smaller panels. Somewhere (in the real world) there's a point of diminishing returns where you have to stop making them smaller.

The OP's attempted clarification makes the classic mistake of asking to optimize several things at once:
Quote:
Originally Posted by am77494
If you may permit, my original intent was to ask to use the minimum number of panels to cover the most area of the median US houses.
It's a well-defined question to ask for the minimum number of panels that exactly cover a given set of roof shapes. It's also a well-defined question to ask for the maximum area of roof that can be covered by a given set of panels. It's not well-defined to ask for a minimum set of panels that will cover the maximum area of roof. For example you may find that you can cover 99% of roof area with 15 panels, but require 47 panels to cover 100%. In that case most people would say the 15 panel set is the best answer. But then what if reducing it to 14 panels drops the coverage to 85%? Is that set of 14 panels that covers 85% "better" than the set of 15 panels that covers 99%? You have to weigh a trade-off between number of panels and coverage and the OP hasn't given us any guidance for how to do that.
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Old 09-10-2019, 03:51 PM
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Yes, you need a DC to AC converter, but I don't think there's any reason why that must be built into the thing that goes onto the roof. More likely, you'd have the things on the roof producing DC, all of that DC going into the house, and then being converted to AC in a single box in a closet somewhere.
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Old 09-10-2019, 04:20 PM
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Yes, you need a DC to AC converter, but I don't think there's any reason why that must be built into the thing that goes onto the roof. More likely, you'd have the things on the roof producing DC, all of that DC going into the house, and then being converted to AC in a single box in a closet somewhere.
In some places where the roof may get un-uniform shade or some panels may not get light while others do, they do put in individual inverters to improve net power production.

An analogy is like the lowest strength (highest internal resistance) battery in a series circuit decides the maximum current. So a string of solar panels gets limited by the production of the “shadiest” panel.

It’s not common though and very expensive and has shorter life.

Last edited by am77494; 09-10-2019 at 04:20 PM.
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Old 09-10-2019, 04:38 PM
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Yes, you need a DC to AC converter, but I don't think there's any reason why that must be built into the thing that goes onto the roof. More likely, you'd have the things on the roof producing DC, all of that DC going into the house, and then being converted to AC in a single box in a closet somewhere.
Because to be efficient, the inverter has to be closely matched to the output current from the solar panels. So you'd need lots of different versions of this closet inverter to match various numbers of panels on each roof (or an adjustable one (more expensive, less efficient)). Plus during the day, as the sun moves and some of the panels fall into shadow, an efficient inverter would have to adjust for the reduced production -- even more complexity. Also, as inverters get bigger to handle more current, they get more expensive. It's much cheaper & easier to have a finely-tuned inverter built into each panel. That also makes inter-connecting them simpler, so less chance that your average construction worker could mess it up.

Also, modern inverters also pump the voltage up to standard household level. That reduces the size of the wires needed to carry it. And copper is expensive.
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Old 09-10-2019, 04:48 PM
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There are now people who, in addition to running their house on solar, also charge a large capacity battery for use when the sun goes down. The easiest way to do this is to have all power from the panels go to the battery and then run the house off that. In which case, you want DC going straight to the battery and then convert the power as it's used. Whether they do it this way, I don't know.

ETA: After reading T-Bonham's post, it looks like they don't.

Last edited by dtilque; 09-10-2019 at 04:52 PM.
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Old 09-10-2019, 04:59 PM
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In some places where the roof may get un-uniform shade or some panels may not get light while others do, they do put in individual inverters to improve net power production.

An analogy is like the lowest strength (highest internal resistance) battery in a series circuit decides the maximum current. So a string of solar panels gets limited by the production of the “shadiest” panel.

It’s not common though and very expensive and has shorter life.
When I had my system installed, all of the companies (plus the general consensus of the rabble on internet forums) I met with recommended going with micro inverters (the one inverter per panel solution), being that that solution was the wave of the future. I understand that there have been some innovations with string inverters that have allowed them to stay relevant, but I'm surprised to hear that micro inverters are not common. The three installs on my street (from three different companies) are all so configured.
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Old 09-10-2019, 05:05 PM
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Because to be efficient, the inverter has to be closely matched to the output current from the solar panels. So you'd need lots of different versions of this closet inverter to match various numbers of panels on each roof (or an adjustable one (more expensive, less efficient)). Plus during the day, as the sun moves and some of the panels fall into shadow, an efficient inverter would have to adjust for the reduced production -- even more complexity. Also, as inverters get bigger to handle more current, they get more expensive. It's much cheaper & easier to have a finely-tuned inverter built into each panel. That also makes inter-connecting them simpler, so less chance that your average construction worker could mess it up.

Also, modern inverters also pump the voltage up to standard household level. That reduces the size of the wires needed to carry it. And copper is expensive.
This is much more in alignment with my actual experience having solar installed and the input I got from all directions.

Aside: Does it affect the solution to the proplem posed if we have to consider local fire and building codes? For example, I could not install solar panels to fill the space the way I had hoped for due to the requirements for clear space between the panels and my ridge line, and the panels to any other roof edge. If I understand correctly, clear walkable access in case of fire or whatever is required almost anywhere.
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Old 09-11-2019, 05:57 AM
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Just like hexagonal leather pieces are the best geometry to make a spherical soccer ball, is there is a particular geometry that would give the best coverage of the median US house roofs ?
It is not true that hexagonal leather pieces are the best geometry to make a spherical soccer ball. Most modern footballs consist of twelve regular pentagonal and twenty regular hexagonal panels positioned in a truncated icosahedron spherical geometry.

Of course, most roofs are not spherical so hexagons would work. In fact the solution to your problem might be a combination of hexagons and half-hexagons or triangles.

Last edited by bob++; 09-11-2019 at 05:57 AM.
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Old 09-11-2019, 06:16 AM
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The optimum fill shape for a solar panel will be the size of a single cell, created from a mono-crystalline silicon ingot. I think right now they are about ?? 10" across?
They need to be connected in series to get an efficient voltage for either your bulk inverter or your roof-top micro inverters. The cost and difficulty of mounting individual cells is worse than the problem of interconnecting them, and the low cell voltage makes per-cell inverters inefficient even if they were economic.

People who actually want to fill roofs use poly-crystalline cells, which can be cut to match the desired shingle-size. I don't know how they are interconnected, but they are mounted like roof tiles. I don't think they are using micro-inverters at all, because the cell performance is not comparable to the performance of a large mono-crystalline panel.

Traditional panels are defined by three things: the matching characteristic of a lead-acid battery, the size of the silicon ingot the cell were cut from, and the size and shape of the kind of panel people wanted to manufacture and handle. 32 cells gave about the right voltage, could be mounted in a 4x8 pattern, in a panel small enough to handle but big enough for economy.
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Old 09-11-2019, 07:25 AM
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Perhaps the better alternative is to start building homes with solar as part of a total equation of eco design. Don't put roofs with odd shapes and try to have a good southernly aspect with the right pitch for that area. Yes it's a longer term solution, but it seems like a sounder option then what you suggest.

But back to your inquiry, they already have solar roof tiles but from what I heard their lifespan is not been the greatest. Perhaps we could make the panels very long and skinny of varying lengths, or maybe can be cut on site to fit, perhaps with certain indicator lines indicating where cuts can be made.
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Old 09-11-2019, 08:58 AM
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The people who are proposing tiny panels are forgetting one thing: we live in the “real world.”
That means, each panel must have connections to all the other panels. The more panels, the more connections, which means more points of failure, and more holes in the roof to leak. And, more expense to install.
Each panel also has an inert perimeter. A smaller panel will have a higher perimeter-to-area ratio; make your panel small enough, and the actual area of photovoltaic power output would fall to zero. So bigger panels should be more efficient in terms of power per installed area. But if your panels are too big, they're a pain to transport and install, and you do in fact end up with a bunch of wasted perimeter on typical household roofs.

What are the odds that current panel manufacturers are way off of the mark as far as optimal panel size and geometry?

Quote:
Originally Posted by am77494
I was hoping such an analysis would have already been done and was available.
It wouldn't surprise me if solar panel mfrs have done exactly this sort of analysis. It also wouldn't surprise me to learn that such analyses were proprietary and confidential.
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Old 09-11-2019, 02:28 PM
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And I'd be surprised if anyone but the panel manufacturers had ever done such a study. It'd be a hard study to do, and hence expensive, and nobody else would have the financial incentive to fund it.
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Old 09-11-2019, 04:10 PM
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Aside: Does it affect the solution to the proplem posed if we have to consider local fire and building codes? For example, I could not install solar panels to fill the space the way I had hoped for due to the requirements for clear space between the panels and my ridge line, and the panels to any other roof edge. If I understand correctly, clear walkable access in case of fire or whatever is required almost anywhere.
Yes, this has to be considered in the real world. Local building inspectors have been dealing with the local electric company for years, and are real friendly. They tend to put whatever barriers they can against solar panels.

When my solar panels were installed, I expected resistance & delay & silly fees from the electric company (I'd been warned of that), but I was surprised by the passive opposition & added 'requirements' by the City building inspector office. Maybe it's wishful thinking, but you always seem to feel that corrupt government officials are somewhere else, not in our town!
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Old 09-11-2019, 06:50 PM
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What are the odds that current panel manufacturers are way off of the mark as far as optimal panel size and geometry?
Traditional panel sizes are traditional. They are based on a time when solar panels were far more expensive than they are now.

Part of the reason why solar panels are so cheap now is that solar panels are produced by hundreds of Chinese companies that got into the business in a typical Chinese-economy boom-bust bubble. Those companies weren't doing research on the optimum panel size -- they were just trying to get into a business where it was apparent that demand was growing and that a good margin was available.

But panel size and geometry isn't set by manufacturer research anyway. It's set by installers. Right now installers are using bigger panels, because they're easier to install and work well with bulk inverters.

If the world actually moves to micro-inverters (and the world hasn't done that yet), we may see another change. Or we could move to roof-replacement, instead of on-roof panels.

Last edited by Melbourne; 09-11-2019 at 06:52 PM.
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Old 09-12-2019, 08:06 AM
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There are a lot of other constraints involved in designing a useful solar array. Covering the roof seems to assume the panels lay flat on the roof, no matter what the pitch of the roof. This leads to all manner of inefficiencies. Putting any panels on the shaded side of a pitched roof is simply wasting money. But other matters come into play.

The average Joe has a day job. So power generated during weekdays isn't going to be used in the household. You either sell it at whatever feed-in rate you can get - which can vary wildly depending on government policy/subsidy and your supplier, or you buy a battery, with the significant costs that comes with. So you may want to optimise panel layout and angles so that they optimise efficiency when the power is most useful to you. This starts to drive the layout towards both optimal elevation angles, and and a mix of azimuths. If you have a flat roof the one thing you will never do is lay the panels flat. If you have a pitched roof your ability to optimise the angles is more limited, but optimise you will. Simply coating the roof in panels flat to the roof line will not provide an optimal solution, even if you can cover the entire roof.

Inverters are going to get more interesting. I'm pretty sure we will see a major swing to inverters built to provide "inertia" to the grid, rather than slaving themselves to the grid. The technology already exists and there have been real world trials. It makes for more complex inverter technology, but solves some significant problems when the grid starts to move to dominant renewables.
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Old 09-12-2019, 06:53 PM
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When my solar panels were installed, I expected resistance & delay & silly fees from the electric company (I'd been warned of that), but I was surprised by the passive opposition & added 'requirements' by the City building inspector office. Maybe it's wishful thinking, but you always seem to feel that corrupt government officials are somewhere else, not in our town!
These extra fees, and the labor and liability with roof installation hugely increase the cost of solar panels.

Huge cost increases. Per the source, solar done at an individual rooftop level is $81-$170 per megawatt-hour but only $36-$44 done at utility scale.

This is because the utility company can get the necessary permits/do the engineering work for a huge, square kilometer or larger plot of land. Where the plot is somewhere the land is cheap and flat. So about the same amount of work in terms of planning and permits as it takes for one house, and then they cover a square kilometer with panels instead. Also they get bulk discounts on the hardware, and it's on flat land, so the labor and installation is far quicker and cheaper. (an electrician, rather than coming to one house at a time, gets paid to walk down entire rows of the array doing the connections and inspections)

Basically, rooftop solar doesn't make much sense. Perhaps if it was a plug and play product*, where Federal law forces the locals to accept products made to a certain standard, and it was only on new houses/during new roof installs it would make sense.

(*this is a thing. Consider this - you buy a microwave oven from costco. Feds and UL approve it. Your local jurisdiction doesn't get a say whether you can plug that oven in)
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Old 09-12-2019, 07:13 PM
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As for panel geometry: I went here and choose an area in Clear Lake City Texas, where I used to live.

I picked a house similar to the one I used to live in. Using the calculator, the South facing part of the roof can support 100 square meters of panels, easily, with some room leftover. This is 15 kW DC of panels with the cheapest kind available, at 15% efficient.

These houses (2600 square feet, hot climate) consume about 1000-1500 kWh per month. (more if occupied by a full family and using electric hot water heating)

This means that every month, the panels would be producing 1821 kWh but the home only consumes 1000-1500.

So there is not a need for some fancy technology to squeeze more performance out of the roof area.

But if you use better panels - "premium" on PVwatts is only 19% efficient but you can routinely buy 22% efficient panels for a modest cost increase - you can produce 2783 kWh per month. If both adults work, and have 60 mile commutes, and both drive Tesla model 3s that get 240 watt-hours per mile, and are charging at 93.3% efficiency.

Then the household would be using 616 kWh to "fuel" their vehicles and thus need to produce ~1500-2100 kWh per month to be "net zero".

Optimal sizing then depends on local electric company policies, the cost of the solar installation, and so on.

In reality while it's cool to do these numbers, the most economically efficient thing to do, in many career fields, is to be flexible and move often for better compensation. So solar panels and other fancy upgrades don't make any sense, you won't live in a place long enough to benefit.

Instead what makes sense is to rent a place to live, and just deal with the extra cost of the utilities, they end up being small potatoes. (compared to how much buying/selling a house or apartment rent is)
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Old 09-12-2019, 07:29 PM
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For the above: I used PVwatts, entered the address, and drew on the satellite picture of the roof. https://pvwatts.nrel.gov/pvwatts.php

Try it for your area, then find out what the annual solar production for your area actually is. I bet you'll be surprised: if you live in a single family home, and are in the continental USA, no matter where, you can probably generate all of the electricity the house needs with just the solar on the South facing side.

Some folks might need to upgrade their home to reduce consumption: upgrades include:

a. Go from 12-15 SEER (central) to 39 SEER (so 1/3 the power used) mini-split air conditioners. (Gree or other brands)
b. Upgrade to on-demand electric hot water. (or on-demand gas if you live in the North and have natural gas available)
c. All LED lighting
d. Energy star appliances
e. More recent TVs and computers (huge decreases in power consumption in recent years)
f. Cellulose spray the attic for a thicker insulation layer
g. Seal air leaks to the outside

Note that for me, just doing (a) made an enormous difference. I was using 1500 kWh in the summer and this dropped it down to 600 or less. There was no need to bother with sealing air leaks or better insulation because I had simply made cold/hot air so cheap it didn't matter.

Last edited by SamuelA; 09-12-2019 at 07:30 PM.
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Old 09-12-2019, 07:44 PM
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These extra fees, and the labor and liability with roof installation hugely increase the cost of solar panels.

Huge cost increases. Per the source, solar done at an individual rooftop level is $81-$170 per megawatt-hour but only $36-$44 done at utility scale.

This is because the utility company can get the necessary permits/do the engineering work for a huge, square kilometer or larger plot of land. Where the plot is somewhere the land is cheap and flat. So about the same amount of work in terms of planning and permits as it takes for one house, and then they cover a square kilometer with panels instead. Also they get bulk discounts on the hardware, and it's on flat land, so the labor and installation is far quicker and cheaper. (an electrician, rather than coming to one house at a time, gets paid to walk down entire rows of the array doing the connections and inspections)

Basically, rooftop solar doesn't make much sense. Perhaps if it was a plug and play product*, where Federal law forces the locals to accept products made to a certain standard, and it was only on new houses/during new roof installs it would make sense.

(*this is a thing. Consider this - you buy a microwave oven from costco. Feds and UL approve it. Your local jurisdiction doesn't get a say whether you can plug that oven in)
The problem here is that a square kilometre of solar farm can’t be used for anything else, the land value disappears. With solar on a roof top, I can have a perfectly useful house holding it off the ground. There is no other value to sloped roof area.
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Old 09-12-2019, 09:27 PM
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The problem here is that a square kilometre of solar farm can’t be used for anything else, the land value disappears. With solar on a roof top, I can have a perfectly useful house holding it off the ground. There is no other value to sloped roof area.
This would be a valid comment except there's large tracts of land that aren't doing much right now and the land is nearly worthless.

https://www.bloomberg.com/graphics/2018-us-land-use/

Basically just grazing land, and I think it's possible to keep using land with solar panels as grazing land (since water is the rate limiting factor, not sunlight) with the panels on poles.
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Old 09-12-2019, 09:35 PM
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Basically, rooftop solar doesn't make much sense.
Then invest in a solar fence. Yep, a fence made of solar panels. Lots easier to install, I expect.

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The problem here is that a square kilometre of solar farm can’t be used for anything else, the land value disappears. With solar on a roof top, I can have a perfectly useful house holding it off the ground. There is no other value to sloped roof area.
There is a company (German, I think) that will put solar panels above farmland, so you can still grow crops under them. The panels are on a framework that's like 3 or 4 meters above ground, high enough so a tractor can be driven underneath. You might wonder about how the plants get enough light. Well, the solar panels have enough gaps between them to let some light down. Plants don't use anywhere near 100% of sunlight, more like about 2%. And some crops will actually grow better in those conditions. Also, having plants underneath keeps the panels cooler, so they don't lose efficiency when it's gets hot. I'd give a link, but I don't remember the name of the company.
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Old 09-13-2019, 05:31 AM
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This would be a valid comment except there's large tracts of land that aren't doing much right now and the land is nearly worthless.

https://www.bloomberg.com/graphics/2018-us-land-use/

Basically just grazing land, and I think it's possible to keep using land with solar panels as grazing land (since water is the rate limiting factor, not sunlight) with the panels on poles.
Fair enough. Thanks for that Bloomberg link, very interesting info graphic!
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Old 09-13-2019, 09:13 AM
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I've driven past houses with solar panels on their roof. I don't know the size but they're large panels - 4'x6' or 4'x8' probably. Then there's one huge gap because there's something on the roof, typically the bathroom exhaust stack where they don't put any panel in that area. It seems to me that having a half-sized panel over whatever the standard size is could be used to fill in that gap as, percentage-wise, missing one large panel on a roof is a big hit. Why don't I see that? Is having two standard sizes so difficult to make/keep in inventory?
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Old 09-13-2019, 09:22 AM
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I've driven past houses with solar panels on their roof. I don't know the size but they're large panels - 4'x6' or 4'x8' probably. Then there's one huge gap because there's something on the roof, typically the bathroom exhaust stack where they don't put any panel in that area. It seems to me that having a half-sized panel over whatever the standard size is could be used to fill in that gap as, percentage-wise, missing one large panel on a roof is a big hit. Why don't I see that? Is having two standard sizes so difficult to make/keep in inventory?
It’s most likely because panels need to be matched pretty closely in terms of voltage and current output.
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Old 09-13-2019, 10:03 AM
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In reality while it's cool to do these numbers, the most economically efficient thing to do, in many career fields, is to be flexible and move often for better compensation. So solar panels and other fancy upgrades don't make any sense, you won't live in a place long enough to benefit.
According to this Zillow study, Homes With Solar Panels Sell for 4.1% More.

So with a little luck the next guy will pay some or all of the solar panel investment.
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Old 09-13-2019, 12:12 PM
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According to this Zillow study, Homes With Solar Panels Sell for 4.1% More.

So with a little luck the next guy will pay some or all of the solar panel investment.
Fair enough. It depends on how long you intend to live in one place. (And, really, your expected value of residency). We live in a world where your best guarantee of career stability is to have in demand portable skills such that you can get another position elsewhere easily. Loyalty to a single company is just being a victim.
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Old 09-13-2019, 12:48 PM
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I've driven past houses with solar panels on their roof. I don't know the size but they're large panels - 4'x6' or 4'x8' probably. Then there's one huge gap because there's something on the roof, typically the bathroom exhaust stack where they don't put any panel in that area. It seems to me that having a half-sized panel over whatever the standard size is could be used to fill in that gap as, percentage-wise, missing one large panel on a roof is a big hit. Why don't I see that? Is having two standard sizes so difficult to make/keep in inventory?
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It’s most likely because panels need to be matched pretty closely in terms of voltage and current output.
That's correct, if you do not have microinverters (one inverter dedicated to each panel; described above) all the panels have to be identical. The output on a traditional system will only be as good as the worst panel.

Additionally, there are clearance rules in the code about how close a panel can some to specific roof features. I can't say specifically how those apply to penetrations like exhaust stacks, but I know the layout on my roof was constrained by several things, including distance to the ridgeline, distance to a chimney, and other things like that. So it could be even if the install was microinverter based, a part-sized panel (assuming they exist) couldn't even be deployed in that spot per regulatory stuff.
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Old 09-13-2019, 04:08 PM
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The panels we use at work are roughly 250W each and measure 2.5 x 4 ft on a single inverter. Since this is used to charge a battery bank as a back up when the genset isn't running using micro inverters would be a waste. They are either vertically mounted or on an angle roughly 15 degrees plus the latitude to maximize solar gain when it's winter.
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Old 09-13-2019, 05:31 PM
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The panels we use at work are roughly 250W each and measure 2.5 x 4 ft on a single inverter. Since this is used to charge a battery bank as a back up when the genset isn't running using micro inverters would be a waste. They are either vertically mounted or on an angle roughly 15 degrees plus the latitude to maximize solar gain when it's winter.
Why would the microinverters be a waste? In principle money per watt is money per watt, you need a certain amount of solar capacity to charge the battery backups at an acceptable rate.

If microinverters give you more watts generated/$, it is more optimal to use them, regardless of where the power is going.

Now, there is an option on Sunny Boy inverters called "Secure Power Supply". It allows for using the solar panels to drive a load without a battery bank. This is useful, it gives an emergency capability to grid-tied solar systems.

But, only the Sunny Boy Brand string inverters offer this feature. This, to me, is a solid reason to use them rather than microinverters. For every other purpose, I assume microinverters are better. (with oversized panels. Use 280 watt microinverters with a 300 watt panel so that the inverter gets more utilization)

Here's a page explaining the reason to oversize the panels:

https://www.yourenergysolutions.com/...rters/enphase/

Last edited by SamuelA; 09-13-2019 at 05:33 PM.
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Old 09-16-2019, 01:03 PM
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For several reasons:
1) We have the panels arranged in a single array.
2) The current set-up is almost ten years old now and as it is supplementary power, utmost efficiency isn't required. Don't let perfect be the enemy of good enough.
3) Simplicity of set-up.
4) The Outback Flexmax 80 controllers have been super reliable and are easy to datalog and set-up.
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Old 09-16-2019, 02:15 PM
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This would be a valid comment except there's large tracts of land that aren't doing much right now and the land is nearly worthless.

https://www.bloomberg.com/graphics/2018-us-land-use/

Basically just grazing land, and I think it's possible to keep using land with solar panels as grazing land (since water is the rate limiting factor, not sunlight) with the panels on poles.
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Originally Posted by dtilque View Post
There is a company (German, I think) that will put solar panels above farmland, so you can still grow crops under them. The panels are on a framework that's like 3 or 4 meters above ground, high enough so a tractor can be driven underneath. You might wonder about how the plants get enough light. Well, the solar panels have enough gaps between them to let some light down. Plants don't use anywhere near 100% of sunlight, more like about 2%. And some crops will actually grow better in those conditions. Also, having plants underneath keeps the panels cooler, so they don't lose efficiency when it's gets hot. I'd give a link, but I don't remember the name of the company.
I dunno the German company, but here are a few links about agrivoltaics:

Nature

futurity.org

the Yale School of Forestry & Environmental Studies

And this story has a quote from a farmer claiming that solar panels yield equivalent $-per-acre, if not more, than the almonds and pistachios he used to grow there:

L.A. Times

Pretty interesting that growing crops underneath solar panels can actually improve crop yields and reduce irrigation needs. Sounds like a win-win-win, but I know nothing about the economics involved.
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Last edited by mjmlabs; 09-16-2019 at 02:17 PM. Reason: add'l link
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