Do Solar Panels Work in Fog?

By Matthew Wheeland - July 28, 2010

Solar panels and fog in San Francisco

Can solar panels work in the fog?

The short answer: YES, solar panels do work when it’s foggy. Fog and clouds, unlike shade, allow some sunlight to reach your roof. The amount of power you’ll generate depends on the thickness of the fog; if the cloud cover is at its worst, your panels might produce only 5-10% of the energy they normally do, but a thin layer of fog has much less impact on solar energy output. Even if you live in an area with lots of foggy days, what really matters is how much sunshine your roof receives over the course of a whole year, not on a given day. Your utility company will look at the yearly data in order to calculate how much power you’ve generated overall.

To see how this works, take a look at the famously foggy city of San Francisco.

Solar power in San Francisco

Because it’s surrounded by water on three sides, inland heat tends to draw cool ocean air across San Francisco, often enveloping it in fog. There’s actually an advantage to the relatively cool weather in SF: solar panels work their best when they don’t get too hot. In cities that constantly bake in the sun, solar panels operate slightly less efficiently. And SF isn’t always foggy. Summertime fog often burns off by mid-day– conveniently timed with the sunniest part of the day, when solar panels can work their hardest– and not return until nighttime. In other seasons, like spring and fall, bright blue skies are common. Overall, SF compares well with other cities in the sunny state of California, and a solar panel system there can actually produce more power over a year than the hot inland city of Sacramento (see the San Francisco Solar Task Force presentation, PDF). We checked in with a PURE member in the foggiest part of San Francisco, near the Pacific Ocean, to see how his solar panels were performing.

Producing extra energy, even in the fog

PURE member Thomas Beutel lives in the Outer Sunset District and keeps careful tabs on his solar panel output,

 Member Thomas Beutel's solar installation. Note the Golden Gate Bridge in the background!

Member Thomas Beutel's solar installation. Note the Golden Gate Bridge in the background!

even posting daily stats on Twitter. His 3.85 kW system can produce a maximum of 28kWh of electricity each day, and on foggy days it typically produces 10-15kWh. The sunny months of March, April, and May easily balance out the foggier summer. Last year, Thomas receieved a production credit of $83 from his utility, PG&E. Proof that even in the foggiest part of one of the foggiest cities in the country, solar panels work well enough to reduce your utility bill to zero!

One advantage of fog

Depending on the type of fog you experience, it may even give your solar panels an advantage. If you live near the coast and marine fog rolls out around 10AM and back in around 4pm, that bright layer of fog in the early morning can actually help boost production. That’s because the early morning sun hits your roof at an obtuse angle, and the fog helps more light reach the panels than would if the sky was clear, according to solar expert Doug Livingston.

Do I need to calculate the fogginess of my neighborhood when choosing the size of my solar system?

Your solar installer will know how to correctly size your system based on your location, so it’s not something that you’ll have to worry about. The “insolation tables” that organizations like the National Renewable Energy Laboratory produce show figures for particular areas that include factors like fog (if you happen to live in San Francisco, you can also just type your address into this handy map to find out details about how much power you can produce). If you live in an exceptionally foggy location, you may have a slightly bigger system than someone who lives in a sunnier city.

Don’t forget to sign up for PURE’s home solar group discounts: we have discounts running in San Francisco, the rest of the Bay Area, and across the country.

Image credit

Leave a comment Comment Arrow