# If all rooftops were used, how much solar power could be generated? **URL:** \ **Category:** Great Debates\ **Created:** [October 15, 2009, 8:08pm UTC](https://boards.straightdope.com/t/if-all-rooftops-were-used-how-much-solar-power-could-be-generated/513948 "2009-10-15T20:08:25Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![Magiver](https://avatars.discourse-cdn.com/v4/letter/m/4491bb/32.png) [@Magiver](https://boards.straightdope.com/u/Magiver)\ **Post date:** [October 15, 2009, 11:58pm UTC](https://boards.straightdope.com/t/if-all-rooftops-were-used-how-much-solar-power-could-be-generated/513948/21 "2009-10-15T23:58:33Z")
> [@Snowboarder\_Bo](#): > > Who says we can’t [do that](http://www.physorg.com/news171545860.html)? You can do anything with enough money. Compare an $80K Tesla against a $15K stock Honda Civic. You can buy a lot of gas with $65K. The trick is to build a $15K electric vehicle. Standard solar cells fall into this mindset of spending more and getting less. The [thin film cells](http://greeninc.blogs.nytimes.com/2009/10/07/dow-unveils-solar-shingles/) that replace shingles make more sense because they use less material and replace another commodity. Subtracting the cost of a new roof is a way of lowering the cost of solar cells without making any other technological improvements in cost efficiency. ---
**Author:** ![Snowboarder\_Bo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/snowboarder_bo/32/229_2.png) [@Snowboarder\_Bo](https://boards.straightdope.com/u/Snowboarder_Bo)\ **Post date:** [October 16, 2009, 12:18am UTC](https://boards.straightdope.com/t/if-all-rooftops-were-used-how-much-solar-power-could-be-generated/513948/22 "2009-10-16T00:18:07Z")
> [@Sam\_Stone](#): > > In 2005, the U.S. consumed 29000 TWh of power. So the number above equals about 18% of U.S. power consumption. Got a cite for that 29000 TWh figure? ---
**Author:** ![Sam\_Stone](https://avatars.discourse-cdn.com/v4/letter/s/ecccb3/32.png) [@Sam\_Stone](https://boards.straightdope.com/u/Sam_Stone)\ **Post date:** [October 16, 2009, 2:20am UTC](https://boards.straightdope.com/t/if-all-rooftops-were-used-how-much-solar-power-could-be-generated/513948/23 "2009-10-16T02:20:34Z")
> [@Snowboarder\_Bo](#): > > Got a cite for that 29000 TWh figure? That’s a number in my own files, but 2 seconds of googling turns up the number for Electricity generation in 2007. From the [EIA](http://www.eia.doe.gov/basics/energybasics101.html): 2007 electrical generation (net): 4,156,745 million KWh A number significantly higher than my 2005 number. It could be that my number is only the amount produced by electrical utilities - in 2007, it was 2,504,131 million KWh. I assumed we were talking about replacing current electrical generation. As I said, all of these numbers are ballpark anyway, because when you’re talking \*net/i] power production, differences like where the power is made matter. If you’d like, we can use the EIA’s number. That will make solar look worse. ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [October 16, 2009, 2:28am UTC](https://boards.straightdope.com/t/if-all-rooftops-were-used-how-much-solar-power-could-be-generated/513948/24 "2009-10-16T02:28:09Z")
> [@Bryan\_Ekers](#): > > I guess the first step would be to try to calculate all the rooftop space in, you didn’t specify what region, but I’ll assume the continental United States. A very high percentage right off the bat can be eliminated because the amount of sunlight they get isn’t enough to pay for the energy used to produce and install the solar panels in the first place. I heard a report – I think it may have been on KPLU (an NPR affiliate) in Seattle – about a German company using solar panels on houses in Germany. The gist of it was that Germany is not an obvious location for solar power, but that the benefits outweighed the costs; and if it works in Germany, then it will work in Seattle. ---
**Author:** ![Sam\_Stone](https://avatars.discourse-cdn.com/v4/letter/s/ecccb3/32.png) [@Sam\_Stone](https://boards.straightdope.com/u/Sam_Stone)\ **Post date:** [October 16, 2009, 2:30am UTC](https://boards.straightdope.com/t/if-all-rooftops-were-used-how-much-solar-power-could-be-generated/513948/25 "2009-10-16T02:30:30Z")
> [@Snowboarder\_Bo](#): > > Wrong. Laughably wrong. > > It may be that in your country, the only buildings with a roof are single family homes, but in the US, we have roofs on businesses, housing duplexes, townhouses, storage facilities… in fact, damn near every building we make down here has a roof. Did you miss the part where I said this is all a WAG? Did you note that I was rounding all my numbers up or down? Do you understand how to do quick estimates of this sort? The best you can hope for is some sort of ‘reasonableness’ number, to see if you’re even in the ballpark. No, I didn’t include industrial buildings, malls, etc. But I also gave every home credit for 1,000 square feet of usable rooftop - a number that’s certainly going to be way too high. It also doesn’t include rooftops that are in shade because of trees or taller homes around them. It doesn’t consider the fact that most roofs are peaked, which means good chunks of them are at bad angles and/or in shade half the time. It doesn’t consider how many of them are oriented in the wrong direction. It doesn’t include the space on the roof taken up by chimneys, ventilation, antennas, and by the need to be able to move around the solar panels for maintenance and cleaning. If you wanted to do a REAL analysis, you’ve got a hell of a lot of work ahead of you. You could probably start with Google Maps and some clever software to calculate roof sizes. By looking at maps from two different times of day, you might be able to get a sense of how much shading is going on. Have fun with that. All I was shooting for was an answer along the lines of, “We could _easily_ replace all our power needs” vs “we’re orders of magnitude away from being able to meet our power needs this way.” Sorry if that doesn’t meet your stringent needs. I look forward to your mathematical analysis. ---
**Author:** ![bri1600bv](https://avatars.discourse-cdn.com/v4/letter/b/9fc29f/32.png) [@bri1600bv](https://boards.straightdope.com/u/bri1600bv)\ **Post date:** [October 16, 2009, 2:30am UTC](https://boards.straightdope.com/t/if-all-rooftops-were-used-how-much-solar-power-could-be-generated/513948/26 "2009-10-16T02:30:39Z")
> [@Johnny\_L.A](#): > > I heard a report – I think it may have been on KPLU (an NPR affiliate) in Seattle – about a German company using solar panels on houses in Germany. The gist of it was that Germany is not an obvious location for solar power, but that the benefits outweighed the costs; and if it works in Germany, then it will work in Seattle. Not at all. The issue is subsidies and the inherent intermittency of solar power. It’s probably a tiny percentage of power in Germany, for one thing. What do you do when there’s no sun in Seattle? Run back up power. Well that alone makes solar power much more expensive. If it’s such a no brainer, then why aren’t people putting them on their roofs without subsidies? Is it a conspiracy? Are they being bribed by “big coal” or “big oil”? ---
**Author:** ![Sam\_Stone](https://avatars.discourse-cdn.com/v4/letter/s/ecccb3/32.png) [@Sam\_Stone](https://boards.straightdope.com/u/Sam_Stone)\ **Post date:** [October 16, 2009, 2:37am UTC](https://boards.straightdope.com/t/if-all-rooftops-were-used-how-much-solar-power-could-be-generated/513948/27 "2009-10-16T02:37:39Z")
According to this [Wikipedia](http://en.wikipedia.org/wiki/Solar_power_in_Germany), Germany gets about 1% of its electricity from solar. According to [this related page](http://en.wikipedia.org/wiki/Feed-in_tariffs_in_Germany), the feed-in tariff that stimulates this costs all German consumers about 3% of their household electrical bill. Paying 3% of your bill for 1% of solar power doesn’t seem like a rousing success. ---
**Author:** ![The\_Tao\_s\_Revenge](https://avatars.discourse-cdn.com/v4/letter/t/73ab20/32.png) [@The\_Tao\_s\_Revenge](https://boards.straightdope.com/u/The_Tao_s_Revenge)\ **Post date:** [October 16, 2009, 2:46am UTC](https://boards.straightdope.com/t/if-all-rooftops-were-used-how-much-solar-power-could-be-generated/513948/28 "2009-10-16T02:46:34Z")
> [@Bryan\_Ekers](#): > > Well, la-dee-daaaaa… Hey man, don’t hate on our pointed roof pride. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [October 16, 2009, 2:47am UTC](https://boards.straightdope.com/t/if-all-rooftops-were-used-how-much-solar-power-could-be-generated/513948/29 "2009-10-16T02:47:26Z")
Quoth **Sam Stone** : > [@](#): > > In 2005, the U.S. consumed 29000 TWh of power. So the number above equals about 18% of U.S. power consumption. > [@](#): > > 2007 electrical generation (net): 4,156,745 million KWh > > A number significantly higher than my 2005 number. It could be that my number is only the amount produced by electrical utilities - in 2007, it was 2,504,131 million KWh. The second figure is 4147 TWh, significantly smaller than your first figure of 29000 TWh. ---
**Author:** ![Delayed\_Reflex](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/delayed_reflex/32/3172_2.png) [@Delayed\_Reflex](https://boards.straightdope.com/u/Delayed_Reflex)\ **Post date:** [October 16, 2009, 3:05am UTC](https://boards.straightdope.com/t/if-all-rooftops-were-used-how-much-solar-power-could-be-generated/513948/30 "2009-10-16T03:05:47Z")
> [@Sam\_Stone](#): > > That’s a number in my own files, but 2 seconds of googling turns up the number for Electricity generation in 2007. From the [EIA](http://www.eia.doe.gov/basics/energybasics101.html): > > 2007 electrical generation (net): 4,156,745 million KWh > > A number significantly higher than my 2005 number. It could be that my number is only the amount produced by electrical utilities - in 2007, it was 2,504,131 million KWh. > > I assumed we were talking about replacing current electrical generation. As I said, all of these numbers are ballpark anyway, because when you’re talking \*net/i] power production, differences like where the power is made matter. > > If you’d like, we can use the EIA’s number. That will make solar look worse. 4,156,745 million kWh is 4,156 TWh, so it is in fact only a portion of the ~29,000 TWh average annual energy usage in the United States (the figure in the top right from 2008 lists the total energy usage in the US to be 99.3 quadrillion Btu, or 29,102 TWh - not far from the 2005 figure). Thus, if we are looking at just electrical generation, your 5,475 TWh is greater than the total electricity generation in the States - if you can only get 20% of “optimal” recovery, we are still talking over 25% of the country’s current electrical generation. on preview: What Chronos said. ---
**Author:** ![Snowboarder\_Bo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/snowboarder_bo/32/229_2.png) [@Snowboarder\_Bo](https://boards.straightdope.com/u/Snowboarder_Bo)\ **Post date:** [October 16, 2009, 3:24am UTC](https://boards.straightdope.com/t/if-all-rooftops-were-used-how-much-solar-power-could-be-generated/513948/31 "2009-10-16T03:24:45Z")
> [@Sam\_Stone](#): > > Did you miss the part where I said this is all a WAG? Did you note that I was rounding all my numbers up or down? Well, ISTM it’s not really a WAG. It seems to me that it’s more your attempt to make up shit to back up your attitude that solar isn’t worth the effort. > [@](#): > > Do you understand how to do quick estimates of this sort? The best you can hope for is some sort of ‘reasonableness’ number, to see if you’re even in the ballpark. No, I didn’t include industrial buildings, malls, etc. But I also gave every home credit for 1,000 square feet of usable rooftop - a number that’s certainly going to be way too high. It also doesn’t include rooftops that are in shade because of trees or taller homes around them. It doesn’t consider the fact that most roofs are peaked, which means good chunks of them are at bad angles and/or in shade half the time. It doesn’t consider how many of them are oriented in the wrong direction. It doesn’t include the space on the roof taken up by chimneys, ventilation, antennas, and by the need to be able to move around the solar panels for maintenance and cleaning. Do you understand that you’re just making shit up? > [@](#): > > If you wanted to do a REAL analysis, you’ve got a hell of a lot of work ahead of you. You could probably start with Google Maps and some clever software to calculate roof sizes. By looking at maps from two different times of day, you might be able to get a sense of how much shading is going on. Have fun with that. > > All I was shooting for was an answer along the lines of, “We could _easily_ replace all our power needs” vs “we’re orders of magnitude away from being able to meet our power needs this way.” > > Sorry if that doesn’t meet your stringent needs. I look forward to your mathematical analysis. Glad to help out with some real numbers. Try 710,000 MW. [cite](http://www.ef.org/documents/PV_pressrelease.pdf) (pdf! but a small one) > [@Energy Foundation](#): > > Rooftop space is not a constraining factor for solar development. **Residential and commercial rooftop space in the U.S. could accommodate up to 710,000 MW of solar electric power (if all rooftops were fully utilized, taking into account proper orientation of buildings, shading from trees, HVAC equipment, and other solar access factors)**. For comparison, total electricity-generating capacity in the U.S. today is about 950,000 MW. > [@](#): > > The state-by-state analysis, the first of its kind, concludes that **the potential U.S. market for grid-connected solar rooftop PV could reach 2,900 MW per year by 2010** , assuming that the solar industry can achieve a “breakthrough” price of $2.00-$2.50 per installed watt. This would be enough new electricity, brought online in just one year, to power more than 500,000 average U.S. homes. Moreover, the study found there is enough suitable rooftop space on residential and commercial buildings to sustain this annual level of growth. We can already produce solar power for less than $1/watt. [cite](http://www.popularmechanics.com/science/research/4306443.html) The folks at Solar Roadways have also given us some [real numbers](http://www.solarroadways.com/The%20Numbers.htm). > [@](#): > > In the contiguous 48 states, there are over 25,000 square miles of impervious surfaces (roads, parking lots, driveways, sidewalks, etc.), not including actual buildings and structures. Continuing development adds another quarter of a million acres each year. If these impervious surfaces were replaced with Solar Road Panels™, how much electricity could we produce? > > Let us make these very conservative assumptions: > > · We use solar cells that have a mere 15% efficiency (there is technology available that actually doubles this number) > > · We average only 4 hours of peak daylight hours per day (4 x 365 = 1460 hours per year) > > A popular manufacturer of solar panels offers a 200 Watt model rated at 15% efficiency. Its surface area is 15.16 square feet. If we covered the entire 25,000 square miles of impervious surfaces with solar collection panels, we’d get: > > ((25,000 mi²) x (5280 ft / mi)²) / (200W/15.16 ft²) = > > ((25,000 mi²) x (27,878,400 ft² / mi²)) / (200W/15.16 ft²) = > > (696960000000 ft²) / (200W/15.16 ft²) = 9194722955145.118733509234828496 Watts ≈ 9.19 Billion Kilowatts > > If we average only 4 hours of peak daylight hours (1460 hours per year), this gives us: 9.19 Billion Kilowatts x 1460 hours = 13424295514511873.350923482849604 Kilowatt-hours (or) ≈ 13,424 Billion Kilowatt-hours of electricity. > > Now, keep in mind that this is a conservative estimate. > > According to the Energy Information Administration, the United States (all 50) used just over 4,372 Billion Kilowatt-hours of electricity in 2003, while the entire world (including the U.S.) used approximately 14,768 Billion Kilowatt-hours of electricity total. It is easy to see that the Solar Roadways™ could produce over three times the electricity that we currently use in the United States. Slightly increasing the conservatively low 15% efficiency would allow the U.S. (the “lower 48”) to produce the entire world’s electricity needs. There you go. Real numbers. Not made up fantasy bullshit. Solar is worth the time, the money, and the effort. ---
**Author:** ![Sam\_Stone](https://avatars.discourse-cdn.com/v4/letter/s/ecccb3/32.png) [@Sam\_Stone](https://boards.straightdope.com/u/Sam_Stone)\ **Post date:** [October 16, 2009, 4:59am UTC](https://boards.straightdope.com/t/if-all-rooftops-were-used-how-much-solar-power-could-be-generated/513948/32 "2009-10-16T04:59:33Z")
> [@Chronos](#): > > Quoth **Sam Stone** : > > The second figure is 4147 TWh, significantly smaller than your first figure of 29000 TWh. Actually, the number I had was for energy consumption, which is what we’re talking about. My mistake for saying production. Of course, the U.S. imports a lot of energy as well, so the domestic production value isn’t really meaningful. According to the [EIA](http://www.eia.doe.gov/emeu/states/sep_sum/html/pdf/rank_use.pdf), the U.S. consumed 101,468 Trillion BTU of energy in 2007, which is 29,730 TWh. Notice that my last sentence says 5% of energy consumption. Not production. > [@Snowboarder Bo](#): > > Glad to help out with some real numbers. Try 710,000 MW. cite (pdf! but a small one) 710,000 MW? And I said 1.5 TW, and then said you’d have to reduce that number by at least half or a quarter. Clearly I managed to get into the ballpark, which is all I was shooting for. Your cite, however, sucks. It’s the equivalent of citing an NRA study on handguns or a tobacco industry study on the safety of tobacco. BTW, I tried to follow their links back to the full report, and couldn’t find it on either web site. > [@](#): > > Do you understand you’re just making shit up? No, I don’t. I was doing a back-of-the-envelope reasonableness estimate, which I suitably disclaimered. > [@Snowboarder Bo](#): > > The folks at Solar Roadways have also given us some real numbers. Which has nothing to do with rooftop solar, the subject of discussion. You seem to be mistaking me for someone who’s opposed to solar power. I’m not. I was just honestly trying to figure out if putting solar on rooftops could provide enough energy for the U.S. It can’t. But there are much better candidates for solar. Solar thermal using molten salt looks very promising. I think there’s a potential for some large solar plants to become cost-effective if you factor in carbon taxes of some sort for its competition. Rooftop might help. If thin-film cells can be manufactured into shingles, and a shingle-solar system can be sold at a competitive price, you could see that becoming a major product, gaining all kinds of market share in the shingle industry and eventually and slowly replacing some reasonable percentage of roofs within 30-40 years. Conventional solar cells are a non-starter. They’re an eyesore, they require additional maintenance, they add weight to the roof, and they’re bloody dangerous. Falls are one of the leading causes of accidental death in the United States. How many people do you think will die clearing leaves off their solar roof or brushing snow off it? This is a dangerous application of this technology. They’ll remain a niche product purchased by technologically skilled homeowners and other early adopters, and they’ll have a solid market in remote power, RVs, traffic lights, all kinds of special applications. Solar energy will part of our future energy mix. It just won’t be the largest part. ---
**Author:** ![Delayed\_Reflex](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/delayed_reflex/32/3172_2.png) [@Delayed\_Reflex](https://boards.straightdope.com/u/Delayed_Reflex)\ **Post date:** [October 16, 2009, 6:12am UTC](https://boards.straightdope.com/t/if-all-rooftops-were-used-how-much-solar-power-could-be-generated/513948/33 "2009-10-16T06:12:20Z")
> [@Snowboarder\_Bo](#): > > Well, ISTM it’s not really a WAG. It seems to me that it’s more your attempt to make up shit to back up your attitude that solar isn’t worth the effort. > > Do you understand that you’re just making shit up? > > Glad to help out with some real numbers. Try 710,000 MW. [cite](http://www.ef.org/documents/PV_pressrelease.pdf) (pdf! but a small one) > > We can already produce solar power for less than $1/watt. [cite](http://www.popularmechanics.com/science/research/4306443.html) > > The folks at Solar Roadways have also given us some [real numbers](http://www.solarroadways.com/The%20Numbers.htm). > > There you go. Real numbers. Not made up fantasy bullshit. Solar is worth the time, the money, and the effort. Uh, **Sam’s** super-rough estimates are in fact MORE aggressive than your first cite (he estimated 1.5TW of generation for rooftops, while the cite only estimates 710,000MW - less than half!). The solar panel capacity factor seems to be [around](http://solarpanelrebate.com.au/blog/?p=251) [25%](http://www.dailyreckoning.com.au/supercapacitors/2008/02/28/), so the average daily energy generation of the panels would actually be ~6Wh/W installed capacity (Sam used 10Wh/W, again more generous than typical). Your “Solar Roadways” cite may show how much _potential_ there is for solar power in the country, but is extremely sketchy about the feasibility of such a scheme. The ocean has [4.5 billion tons](http://peakoildebunked.blogspot.com/2006/01/207-uranium-from-seawater-part-1.html) of uranium - enough to power the earth for [5 billion years](http://www-formal.stanford.edu/jmc/progress/cohen.html), so why don’t we just pan the ocean for uranium and use fast breeder reactors? Let’s look at the scale of replacing all the asphalt in the US with solar road - **global** PV manufacturing capacity was only [15,200MW](http://en.wikipedia.org/wiki/Solar_PV). At [12 watts/sq ft](http://www.solarpanelstore.com/) (4304MW/sq mile) - that would, if ALL that production went to making solar roads, pave the equivalent of ~3.5 square miles. Even if solar road manufacturing capacity was **10x the current global capacity** , you could pave 350 square miles per year - it would take over 700 years to replace all the asphalt in the US! “Not made up fantasy bullshit” indeed. > [@Sam Stone](#): > > Actually, the number I had was for energy consumption, which is what we’re talking about. My mistake for saying production. Of course, the U.S. imports a lot of energy as well, so the domestic production value isn’t really meaningful. > > According to the EIA, the U.S. consumed 101,468 Trillion BTU of energy in 2007, which is 29,730 TWh. > > Notice that my last sentence says 5% of energy consumption. Not production. As noted, at 6Wh/W installed and 25% capacity factor, 710,000MW of installed panels would generate 1.06 TWh of energy, which is 3.6% of the country’s _total energy needs_ - that includes transportation (ie. gasoline), industrial and commercial (non-electric primary) consumption. It would make up ~9% of the country’s _power_ supply - not massive, but not insignificant. Your mistake was not relating to production - in all likelihood, much energy is currently wasted due to power plant inefficiencies. You will note that 11,750 TWh (40 quadrillion BTUs) were _consumed_ for electricity generation but only 4,147 TWh was produced - this is not due to massive electricity import (where would it come from?), but from the inefficiencies of thermal power plants - the rest of the energy is wasted as heat (I assume heat harnessed from cogen plants is accounted for in the commercial/industrial blocks). The 1.06 TWh of energy from solar panels covering all the roofs in the nation already takes into account all inefficiencies. That being said, we are still a far cry from being capable of putting a solar panel on every roof. Global manufacturing capacity of solar panels would have to increase sixfold in a year to cover all the roofs in the US within a decade. [The US](http://www.greentechmedia.com/articles/read/boom-time-ahead-for-u.s.-solar-manufacturing-says-analyst/) manufactured 499 MW of solar panels in 2008, and the article suggests that it could grow to 2.13 GW by 2012, and growth of manufacturing capacity cannot remain exponential for long - I would be impressed if there was a solar panel on every roof by the end of the century. ---
**Author:** ![Snowboarder\_Bo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/snowboarder_bo/32/229_2.png) [@Snowboarder\_Bo](https://boards.straightdope.com/u/Snowboarder_Bo)\ **Post date:** [October 16, 2009, 7:10am UTC](https://boards.straightdope.com/t/if-all-rooftops-were-used-how-much-solar-power-could-be-generated/513948/34 "2009-10-16T07:10:06Z")
> [@Sam\_Stone](#): > > Your cite, however, sucks. It’s the equivalent of citing an NRA study on handguns or a tobacco industry study on the safety of tobacco. BTW, I tried to follow their links back to the full report, and couldn’t find it on either web site. Yeah. Sure. The Pew Charitable Trusts, the Rockefeller Foundation, the McKnight Foundation, the Orlando Utilities Commision, PSE&G, ConEd NY, Dow Chemical, Duke Energy, San Diego Gas & Electric… yeah, all highly partisan organizations with only their own self-interest in mind. None of them serve the public interest at all. :rolleyes: The [original study](http://www.ef.org/documents/EF-Final-Final2.pdf), from 2004, which I found at [www.ef.org](http://www.ef.org) without much effort. I think it was 2 mouse clicks. [Updated study](http://www.navigantconsulting.com/downloads/knowledge_center/SmartGridPVMulti-Client_US_EG.pdf) found at [www.navigantconsulting.com](http://www.navigantconsulting.com) with maybe 3 mouse clicks. [Article about FedEx installation](http://www.environmentalleader.com/2009/07/31/fedex-installs-24mw-rooftop-solar-array-largest-in-us/). > [@](#): > > The solar array will cover 3.3 acres of rooftop space, featuring approximately 12,400 solar panels. When completed, it should generate 2.6 million kilowatt hours of electricity annually. [It can be done in Canada, too](http://www.scientificamerican.com/article.cfm?id=neighborhood-rooftop-solar). Although this is about heating, not electricity. > [@](#): > > One of the first neighborhood-wide solar installations in the world was at the master-planned community of Drake Landing in the town of Okotoks in Alberta, Canada. The entire community, now with more than 50 homes built and occupied, is heated by a neighborhood-wide “borehole thermal energy” system designed to store abundant solar energy underground during the summer and distribute it to each home as needed for space heating throughout the winter. The system, which launched in June 2007, now fulfills some 90 percent of each home’s space heating needs, with any slack taken up by fossil fuels. [Duke Energy has several projects already underway](http://charlotte.bizjournals.com/charlotte/stories/2009/10/12/daily12.html). > [@](#): > > Duke will install 5,096 rooftop panels, with a capacity of 1.2 megawatts, at National Gypsum’s manufacturing plant in Mount Holly. And 5,616 panels, with a capacity of 1.3 megawatts, are planned for the roof of Food Lion’s distribution center in Salisbury. > > The only installation outside the Charlotte region will be in Greensboro. Duke will install 7,020 panels, with a capacity of 1.6 megawatts, on Highwoods Properties Inc.’s building in Enterprise Park. We can generate _a lot_ of power just from solar panels on rooftops. Enough power to make the investment worthwhile. With less pollution. And it’s cost effective to do so. At least, the California Energy Commission thinks so. Here’s a snippet from why they denied the permit for a new gas fired power plant at Chula Vista this year. [cite](http://www.energy.ca.gov/2009publications/CEC-800-2009-001/CEC-800-2009-001-CMF.PDF) (page 29-30) > [@](#): > > Photovoltaic arrays mounted on existing flat warehouse roofs or on top of vehicle shelters in parking lots do not consume any acreage. The warehouses and parking lots continue to perform those functions with the PV in place. (Ex. 616, p. 11.). Mr. Powers (expert for intervenor) provided detailed analysis of the costs of such PV, concluding that there was little or no difference between the cost of energy provided by a project such as the CVEUP (gas turbine peaking plant) compared with the cost of energy provided by PV. (Ex. 616, pp. 13–14.). PV does provide power at a time when demand is likely to be high—on hot, sunny days. Mr. Powers acknowledged on cross-examination that the solar peak does not match the demand peak, but testified that storage technologies exist which could be used to manage this. The essential points in Mr. Powers’ testimony about the costs and practicality of PV were uncontroverted. ---
**Author:** ![RTFirefly](https://avatars.discourse-cdn.com/v4/letter/r/c77e96/32.png) [@RTFirefly](https://boards.straightdope.com/u/RTFirefly)\ **Post date:** [October 16, 2009, 1:00pm UTC](https://boards.straightdope.com/t/if-all-rooftops-were-used-how-much-solar-power-could-be-generated/513948/35 "2009-10-16T13:00:11Z")
> [@Sam\_Stone](#): > > Of course, the U.S. imports a lot of energy as well, so the domestic production value isn’t really meaningful. I think you mean to say we import _petroleum and natural gas_ which we _turn into_ energy in vehicles, power plants, etc. So unless you Canadians are producing a huge amount of electricity which winds up getting transmitted into the U.S. and used up here, I’d expect the domestic production value to be quite meaningful. ---
**Author:** ![RTFirefly](https://avatars.discourse-cdn.com/v4/letter/r/c77e96/32.png) [@RTFirefly](https://boards.straightdope.com/u/RTFirefly)\ **Post date:** [October 16, 2009, 1:12pm UTC](https://boards.straightdope.com/t/if-all-rooftops-were-used-how-much-solar-power-could-be-generated/513948/36 "2009-10-16T13:12:56Z")
> [@bri1600bv](#): > > If it’s such a no brainer, then why aren’t people putting them on their roofs without subsidies? Is it a conspiracy? Are they being bribed by “big coal” or “big oil”? OK, here’s the problem: 1. While electricity demand is highest during the day, 2. and solar panels generate electricity during the day, 3. that’s when people aren’t at home to use the electricity that their solar panels would be generating, and 4. because electricity storage is expensive, 5. it makes little sense for homeowners to put solar panels on their homes, unless 6. they can sell their excess electricity back to their local power company, 7. which, in general, they aren’t permitted to do. If we passed a law requiring power companies to buy excess power generated by their customers, at market rates, and people _still_ didn’t put solar panels on their roofs in substantial numbers, _then_ your questions would be quite applicable. ---
**Author:** ![emacknight](https://avatars.discourse-cdn.com/v4/letter/e/3ec8ea/32.png) [@emacknight](https://boards.straightdope.com/u/emacknight)\ **Post date:** [October 16, 2009, 1:30pm UTC](https://boards.straightdope.com/t/if-all-rooftops-were-used-how-much-solar-power-could-be-generated/513948/37 "2009-10-16T13:30:49Z")
> [@RTFirefly](#): > > I think you mean to say we import _petroleum and natural gas_ which we _turn into_ energy in vehicles, power plants, etc. > > So unless you Canadians are producing a huge amount of electricity which winds up getting transmitted into the U.S. and used up here, I’d expect the domestic production value to be quite meaningful. Canada exports a huge amount of power to the US, I’m going to try and dig up a site. ---
**Author:** ![emacknight](https://avatars.discourse-cdn.com/v4/letter/e/3ec8ea/32.png) [@emacknight](https://boards.straightdope.com/u/emacknight)\ **Post date:** [October 16, 2009, 1:33pm UTC](https://boards.straightdope.com/t/if-all-rooftops-were-used-how-much-solar-power-could-be-generated/513948/38 "2009-10-16T13:33:03Z")
> [@RTFirefly](#): > > I think you mean to say we import _petroleum and natural gas_ which we _turn into_ energy in vehicles, power plants, etc. > > So unless you Canadians are producing a huge amount of electricity which winds up getting transmitted into the U.S. and used up here, I’d expect the domestic production value to be quite meaningful. [In 2008, Canada exported 49.8 billion kWh to the U.S. while importing only 17.1 billion kWh from the U.S., according to Canada’s National Energy Board (NEB).](http://www.powermag.com/gas/Canada-Moves-to-Rebalance-Its-Energy-Portfolio_1933.html) ---
**Author:** ![RTFirefly](https://avatars.discourse-cdn.com/v4/letter/r/c77e96/32.png) [@RTFirefly](https://boards.straightdope.com/u/RTFirefly)\ **Post date:** [October 16, 2009, 3:04pm UTC](https://boards.straightdope.com/t/if-all-rooftops-were-used-how-much-solar-power-could-be-generated/513948/39 "2009-10-16T15:04:55Z")
> [@emacknight](#): > > [In 2008, Canada exported 49.8 billion kWh to the U.S. while importing only 17.1 billion kWh from the U.S., according to Canada’s National Energy Board (NEB).](http://www.powermag.com/gas/Canada-Moves-to-Rebalance-Its-Energy-Portfolio_1933.html) Sounds like a lot - and I guess it is. But the difference is still less than 1% of the 4.1 trillion kWh that the EIA says was generated in the U.S. So it’s not really ‘big’ in the sense of throwing anyone’s estimates from this discussion off by much. But it _is_ interesting. Had no idea we imported that much electricity from Canada. ---
**Author:** ![Polycarp](https://avatars.discourse-cdn.com/v4/letter/p/82dd89/32.png) [@Polycarp](https://boards.straightdope.com/u/Polycarp)\ **Post date:** [October 16, 2009, 3:10pm UTC](https://boards.straightdope.com/t/if-all-rooftops-were-used-how-much-solar-power-could-be-generated/513948/40 "2009-10-16T15:10:45Z")
> [@sweeteviljesus](#): > > That doesn’t factor in non-residential roofs. What difference would this make? Do WalMart and Amalgamated Widget get an exemption from the mandate to install solar power on their roof that everyone else has to comply with? (Sorry if this comes across unnecessarily sarcastic, but I don’t see the point – large roof areas on factories, malls, and big box stores should be a major component of whatever the answer actually is. [Previous page](https://boards.straightdope.com/t/if-all-rooftops-were-used-how-much-solar-power-could-be-generated/513948.md?page=1) [Next page](https://boards.straightdope.com/t/if-all-rooftops-were-used-how-much-solar-power-could-be-generated/513948.md?page=3)