For an ordinary residential house with “average” efficiency, is it possible to generate enough electric power annually from solar panels on the roof of that house that at the end of a year, 0 net electricity was consumed? (that is, the electric grid received as many KWh back from the house as was provided to the house)
Assume this is a retrofit, and only half the roof faces west.
This assumes no energy conservation measures, as well. A/C, heating, etc. Heating would be done via electric heat pump.
I’m aware of how you’d solve this problem rigorously, it’s a straightforward math problem, and I’m wondering if some of the SDers have already done the exercise and might know if this would work or not.
Where does the other half face? East? Neither is a good choice for a Northern Hemisphere house, at least for a passive setup; South is the preferred orientation.
You didn’t say your location and your latitude. Both are important for calculation.
If all you are looking at is kWh, then yes it is easily possible to produce more than you use in total. The OP assumes selling it back to the grid during sunlight hours and buying it back again from the grid on dark hours. Storage batteries can also be used. Some of this does depend on where you are at, though. If you live someplace bright and sunny with a temperate climate it’s no problem. If you live someplace where there isn’t so much sun and you need a lot of heating in the winter, then you will likely not be able to produce enough energy.
If you are talking about economics instead of kWh though, then it gets a bit murky. Even in the best of places (bright sun, pleasant climate), it is difficult to beat the local utility company in cost. Other variables include how much the utility will pay you for electricity produced. In many areas they will be glad to buy your electricity (assuming you have all of the proper equipment) but they won’t pay you the same rate that they charge when you buy it from them. So you lose money using the grid as your temporary storage. But then, that may or may not cost you more than using your own batteries in the long run. To make it even more confusing, a lot of places are offering incentives for you to go solar. With the incentives, if you are careful, it is possible to beat the utility company’s price per kWh for electricity in the long run. Sometimes that requires a more efficient house than your “average” home though.
A lot of folks are able to do it though. If you are interested in that sort of thing, find out what local, state, and federal incentives there are to help ease the cost. Those are generally what makes the difference between you making or losing money in the long term by going solar.
There is no easy answer as every situation is different. I would advise that you have an energy audit done of your house and get familiar with any tax credits or deductions.
Things to consider:
I wouldn’t borrow money to do it. However, if you can pay in cash you might get a greater return than the lousy interest the banks are paying.
If you are environmentally conscious you are offsetting oil consumption and CO2 emissions.
Long term, the cost of energy for heating and cooling is bound to increase. If and when that happens the math works more and more in you favor.
Don’t think about getting off the grid. If you do then you can’t sell excess when the system is maxing out. Also, you have lost a backup.
A propane backup system will let you sleep better at night and is probably more cost effective than a battery backup system.
At least consider aesthetics if you are so inclined. If, when you see your house, you think it is ugly it might weigh into the equation.
I’ve done it. I like it. I call it my electric company. I love the fact that I don’t burn any oil which is about the only other way I could heat the house since I can’t get natural gas.
In the process of adding solar to my home. Early process still the planning phase.
I do not think it is pratical to end up with a net zero system. My average power consumption is in the area of 1150 KWH per month. AC system and Spa. My southern roof is estimated to produce around 8700 KWH per year or around 725 KWH per month. To increase the number of panels would require making stands to face the panels south.
Earlier in the year I had a solar company give me a bid where I purchased the system. The buy back was in the range of 13 years with tax credits. And I would own the system so any problems were mine. And the life of the panels was rated at 15 years.
The system that will be put onto my house will be owned by the solar company, I will purchase power from them at $0.195 per KWH. In the top end of my power bill tier 4 and 5 I am purchasing at $0.50per KWH range. And my tier 1 rate from noon to 6 PM is $0.31 per KWH. The off peak tier 1 rate is $0.08 per KWH. To have a zero KWH system would cost me more than I would save. But I also have the advantage in being able to sell power back to PG&E at tier 1 on peak rate of $0.31 per KWH during on peak time.
The advantage of a solar system is eliminating the tier 3, 4, &5 levels or any on peak rates.
By “practical” I meant “with off the shelf solar panels that are no more than midrange in cost for panels”. Obviously, the exotic multi-junction cells that they use in satellites produce significantly more power/surface area, but the costs are prohibitive.
A friend of mine has a solar/wind business, and every year or so, I call him up and ask if the costs have come down enough to make a residential solar and/or wind installation practical economically. So far, his answer has been “Not yet, but just wait.”
This is for 45 N lat, a house that has ample southern exposure for solar and a respectable wind location if the tower was high enough.
Mother Earth News is a good source for cost-benefit analysis of solar installations. Their website includes this article from 2012 about declining costs of solar energy systems. They also have information on the viability of solar power by state, which article includes a link to this interesting map, as well as links to other resources for calculating whether solar power is right for your area.
We just received their current dead-tree issue, which is not reflected on the website. It contains an article updating solar panel pricing to reflect current levels.
I don’t think either an east- or west-facing roof surface is feasible unless you live on or very near the equator. Our house has roof surface facing in all directions, but if we installed solar panels we’d be forced to place them at some distance from the house due to tree cover, and they’d have to be adjustable to allow for seasonal changes in sun position. All in all, we’re better off just taking more basic energy-saving precautions, as beowulff mentioned above.
I bloody hope it is, or somebody in the British government has made a massive miscalculation, because every other roof here is covered in solar panels, under some subsidy scheme that is apparently designed to take advantage of Britain’s famously sunny climate.
A friend near San Francisco signed on with an outfit who leases the solar equipment. After one year, his “true up” billing was a net profit to him of about sixty bucks for the year. It’s an almost ideal location for residential solar - like many homes in SF, I don’t think his has air conditioning, despite all of the computers and servers he has. On the flip side, all those computers reduce the amount of heating needed in what passes for winter in SF.
The one thing I don’t know is if the cost of installation is rolled into the leasing, or if he paid for that separately.
Here’s the thing- it doesn’t have to be zeroed out to be worth it. At least around here, you pay cheaply for so many KWH, more for the next bunch and *thru the ass *from then on.
On a hot day, it’s really easy to get into "thru the ass’ level. So, install some panels which then on a hot day synergistically keep your KWH down to base line.
I’m wondering if a rethink on the whole process is in order. What if solar power was a combination solar electrical panels and solar thermal. And instead of electrical batteries the energy was stored mechanically which would allow the homeowner to exercise their option to add to the battery.
More expensive to have either solar thermal storage or storage batteries. If there are utility lines already connected to the home why add the additional expense. My brother has looked into the expense of adding storage batteries it is cheaper to just pay the fuel bill for his generators. Using thermal storage vs batteries really becomes expensive.
Say 1100kwh/month consumption.
Let’s say a 2000sf house, so about 900sf of panels facing the sun, or 100sq yards. Sun produces about .7kw/sq yard, so panels under decent sunlight, 6.3kw per hour.
The sun shines, let’s say 30x10 hours a day (welcome to the bright sunny desert) or 300 hours.
300x6.3=1900kw/hr produced/
So you are close to producing more than you need, assuming sunny conditions. Too far north, or too British a climate, forget it.
Then there’s maintenance, etc.
Another point - minimize your consumption. A smart thermostat (like the one Apple sells) claims to learn your temperature requirements - why heat or cool as much when nobody’s home, let the heat go down when you are sleeping under thick blankets, etc. Insulate, get triple-pan argon-filler windows, stop direct sunlight from coming in the windows in summer, etc.
Put your equipment on timers, get a LED TV, energy efficient appliances, LED or CFL lights, etc. Set your computer to go to sleep aggressively. Use the microwave instead of heating elements.
If you goal is absolute minimal energy consumption, conservation has better payback I’m sure.
First thing is will the electric company credit you for energy you put back into the system? A friend mine put up 12 panels on a gently pitched roof on an outbuilding at his house. Here in Virginia he calculated an 8-year payback for the cost of the panels vs. electric bill savings. His electric company will not give him credit for power put back onto the grid, so the best he could do is take his usage to zero, but even then his electric bill would still be ~$50 for “distribution charge” and fees. And of course zero usage is not possible because of overnight electric usage where the panels won’t help. He could power things overnight with a bunch of batteries to store excess power generated during the day, but that gets a lot bigger and expensive, and batteries only last so long.
On a sunny day recently he showed me his electric meter running backwards, when clouds moved in front of the sun the meter actually stopped, the house usage equaled the panel output. He then turned on the electric oven and the meter ran pretty quickly forwards.
In about 27 months his panels have produced 7.39MWh of power.
Doh!
I forgot to calculate in the efficiency of conversion. You may get close to a Kw of sunlight per sq meter, but the best you can hope for with commercial panels is about 170w/m^2, maybe 20% conversion at very best.
So instead of say, 1900kWhr you might get at best, 380kw-hr. And you house would use 1100kWhr. You can see where aggressive energy-saving is your friend.
People don’t really grasp the extent of energy consumption by the average first-world person. There’s a reason why the oil business seems to be a major money tree and a geopolitical obsession. If you had to chop enough trees or pick-axe the necessary amount of coal by hand, you would have time for nothing else.
We’re talking 400kW for a 1200-sf area; a lower-powered 30hp car would need 22kW, in bright sunlight would haul a trailer of solar cells 12 feet by several blocks… Or spend a LOT of time parked and charging batteries.
Annual electric savings: 1920
SREC income/year 2778 (for first 10 years)
Break even after 5 years, given SREC income and fact that I get full credit toward electric bill on every Kw I generate. Surplus from over production months goes toward deficit in late Fall, winter.
Panels and Inverters guaranteed for 25 years
Installation /Labor guarantee 10 years.
My roof faces south, is about 22% elevation, which is less than ideal of 37%.
I am in MA. So far, since we turned on system in June , I’ve generated more electricity than I have used.
If not for the Federal tax subsidy and the State SREC credits, the payback would be much longer. Our electric use about 850 - 950 KwH /month, no central air, no electric stove or dryer. We don’t scrimp on power use, but are going to easily cover our usage with PV Array.
65x39 inches for a 265-watt solar panel. (so roughly 5.5x3.25 feet, or 17.6sf).
32 of these is 563sf. 32x265= 8480 watts
8.5kwx31days x 10 hrs= 2635kwhr/month; of course, MA not AZ means you can’t expect this especially in winter, but you probably save a lot in summer air condition requirements versus the deep south.
Nice. One spot I found sold these for $340, so I assume installation, setup, electrical work, etc. is what really cost?
