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Does Fog in Bay Area Interfere with Solar Power?

By Scott Sklar
September 19, 2006 | 12 Comments

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I live in the San Francisco area, does the fog interfere with using solar? -- several readers

The National Renewable Energy Laboratory experts say, "The rule of thumb is that the solar resource during foggy or low-cloud conditions is approximately 10% of the value under clear sky conditions." And that fit's my rule too. I size the systems with 10 percent more photovoltaics or solar thermal panel area, and the same for storage (battery bank or water tank). And where applicable, I add small wind or some other clean distributed energy to potentially compensate for less sunlight. Now solar has to work in this City; in 2004 city officials announced their use of the City's "Community Choice" law in conjunction with the City's 2001 voter-approved solar bond authority to support more solar and wind installations, and a great database exists so you can review the type of solar installations in the SF area (see link below). -- Scott Sklar Scott Sklar is President of The Stella Group in Washington, DC, a distributed energy marketing and policy firm. Scott, co-author of "A Consumer Guide to Solar Energy," uses solar technologies for heating and power at his home in Virginia.

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12 Reader Comments

Comment
1 of 12

evan-haskins-53111

September 20, 2006

I represent Bay Solar Power Design; we could send a representative out in both a foggy day and on a clear day to show you the difference. To give you a number, like 10%, would be a disservice. It depends on how much light comes threw. Sometimes you can only see 10 feet and other times a 100 yards, it will make a difference... If you would like service and a consultation please contact Evan@baysolarpower.com

Comment
2 of 12

lee-salberg-54822

September 20, 2006

September 20, 2006

Can you tell me please
What would be the five U.S. solar energy companies that can build a large COMPLETEinstallation in an international market project.
Thank you for your interest.

Kindest Regards,

Lee

Lee Salberg
President
Lee SalbergInternational Group

(847) 965-0409

leesalberg@cyberlink.com

Comment
3 of 12

doug-sherman-46666

September 20, 2006

In checking the handout materials from Doug Livingston's Intro to Grid-tied PV course, he suggests that fog de-rates the system by 15% (30% in the most extreme cases)

Quoting without permission - 'The reason fog does not derate your production as much as you might think is a matter of geometry of the sun in a clear sky over the course of the day. At noon under marine fog you might only be producing (from the whole diffuse sky's light) 50% of what the clear valley not far inland might be producing, BUT, at 9 in the morning, the sun is so obtuse to the array that it might be only doing 10 or 20% of its noon sun output AND the fog bound system is still doing close to it's 50%. The net effect over the course of the whole day for typical California foggy location is only a 15% derating of production. A location where the fog rolls back at 10am and rolls in at 4pm will actually produce more than a nearby site with no fog!'

Comment
4 of 12

rob-wilder-56178

September 20, 2006

I'm pretty passionate about maximizing efficiency and so i monitor live our workplace solar irradiance and solar PV output (we're solar powered here). Anyway, this morning was very foggy and i saw that the irradiance (sunshine) was down by a very sizable amount as was our PV output. If interested, this is at http://wildershares.com/solar.php

We were generating less than one kW this morning, but now under just partial fog (483 M/W2) we are making far better PV power at 2,621 kW at this moment (9:18 AM). So my anecdotal suggestion is that full fog really can kill PV output, while partial fog allows for substantially better PV output. Hence your results in the Bay Area may depend on your own micro-climate and whether you receive full fog, vs. partial fog for big portions of the day.

Thanks,
Rob W.

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5 of 12

james-pace-56066

September 20, 2006

I belive a check of weather records is in order. My memory from living in the bay area is; there are usually sunny mornings and not much fog untill 4:00 PM or so. except for the infrequent winter "tule" fogs.

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david-cutter-43483

September 20, 2006

Maximizing Energy Efficiency and Renewable Energy is the domestic epicenter in the War on Terror and it is imperative that we maximize the partnerships between the public and private sectors in new and creative ways with a sense of seriousness, national purpose and the urgency the situation merits.

A commitment to clean energy would reduce pollution, create millions of high tech jobs,
diversify our energy sources, add to global energy security and save billions of dollars. A much-needed transition to solar, wind power and other <a href="http://www.pleiades-enterprises.bigstep.com/generic24.html" target=_blank>renewable energy</a> in every region of the world holds the
promise of a better future for us, our children and future generations.

Comment
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thomas-detreville-55218

September 20, 2006

Ian, he meant that you LOSE 90% of PV collection in fog or low cloud cover- yes, you would need to pony up 900% more PV for that period or go to complimentary indirect solar such as wind. Solar thermal evacuated tube collectors, on the other hand, only lose about half power in similar weather. And, there is no annual severe degradation either (for solar thermal- also, you dynamically suspend waste heat from nuclear fission power plants and others from using solar thermal unlike doofus PV...

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ian-page-36034

September 20, 2006

Scott- didnt understand the percentages. Do you mean that with fog 90% of the solar is available, so you oversize the system by about 10% , or do you really mean that 10% of the solar is available in which case you would have to oversize by ten fold or 1000%. Despite San Francisco being one of my favorite cites , I have no intuition as to which it is , as its gets pretty chill under the fog!

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9 of 12

thomas-detreville-55218

September 27, 2006

Lee, Instead of PV you should be investing in low-temp heat conversion from evacuated-tube solar technology. Of course I do not know of any other 150-250C process other than our ECO-Cycle-powered rotary Expander-COndenser ECO-Engine...

Tom deTreville
President
ECO-Energy Group
www.eco-energygroup.com

Comment
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thomas-detreville-55218

September 27, 2006

Comment
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douglas-livingston-58237

November 22, 2006

In general, these "fog" de-rations are already built into the NREL (or whatever) insolation tables. You usually don't need to de-rate (or rather up your PV size) further for fog or overcast. If you receive substantially more fog than the NREL data site location you are using, you might consider an additional deration. When I was quoted as suggesting 15-30% deration, it was for locations which experience regular CA marine fog, very different from valley fog or overcast and where their NREL data location does not. Marine fog is usually very bright, and as I was quoted, can actually improve the system's output when the sun is obtuse. If the fog rolls out at 10am and rolls back in at 4, you would actually produce more than someone nearby who doesn't see any fog!

Douglas Livingston
LivingstonConsulting@hughes.net

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douglas-livingston-58237

November 22, 2006

Since I was quoted (fairly accurately), I've got to respond. Scott did say that fog can derate your output to 10% of its rated output. In the next sentence he says he would increase his array size by 10% (suggesting only 10% loss due to fog overall). Both can be right. Heavy fog or overcast can derate your output by more than 90%. I've seen gloomy rainy days where output was 5% of typical noon sun conditions. Most overcast and fog derates output far less. But you don't base your PV array size on some instantaneous output. In most cases, your PV array size is based on you local insolation values (cumulative daily light energy input).

Continued next comment (too long for one reply)...

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Scott Sklar

About: Scott, founder and president of The Stella Group, Ltd., in Washington, DC, is the Chair of the Steering Committee of the Sustainable Energy Coalition and serves... more »