Clean Energy SF: San Francisco, fog then sun, est. 2026

Vol. 1 - No. 14 - Thursday, September 24, 2026 - Free to readers

Solar ยท Analysis

Fog Takes Some Solar Hours. Less Than the Lore Says.

San Francisco's microclimates are real, but the fog penalty for west-side rooftop solar is smaller than the sidewalk wisdom claims. Here is the neighborhood math, and what to ask your installer.

Filed October 8, 2026 By the Clean Energy SF editors Reading time: 6 minutes

I. How fog actually cuts production

Panels run on light, not heat, so they keep working under a bright gray sky, just at a reduced rate. On a thick marine layer morning, a west-side roof might produce somewhere in the range of a third to two-thirds of its full-sun output, depending on how dense the fog sits. The lost hours tend to cluster in the late morning, when the layer is still parked over the avenues before it burns off.

Timing is what saves the math. San Francisco's foggiest stretch runs roughly June through August, the same weeks the days are longest, so some of the lost light is offset by sheer hours of daylight. Winter, when short days would worry you most, is often the clearer season near the coast, and panels run a little more efficiently in the cool air the city offers nearly year round.

II. The neighborhood math, honestly

NREL's free PVWatts calculator is the honest starting point: enter your address, roof tilt, and direction, and it pulls a typical weather year for the area. Run a sunny east-side address against a foggy west-side one and the annual gap is usually modest, on the order of several percent up to maybe the mid teens at the extremes. The tool leans on a limited set of weather stations, so it smooths over block-by-block differences, and your actual roof could land above or below its estimate.

Bigger swings almost always come from the roof itself, not the fog: a taller building next door throwing afternoon shade, a north-facing slope, a chimney, or the broken rooflines common on Victorian and Edwardian flats. A clean south-facing roof in the Outer Sunset often beats a shaded south roof in a sunnier district. Get an address-specific estimate before you trust a neighborhood's reputation.

III. What west-side installers should tell you

A good installer models your actual address rather than a citywide average, and shows you the month-by-month production curve so the summer morning dip is visible instead of buried. Ask them to run PVWatts or their own tool for your roof, and to explain how they sized the system to your usage. CleanPowerSF and PG&E customers alike interconnect their panels through PG&E, so the hookup process is the same either way.

Current rules changed the calculus more than fog ever did. New systems fall under the state's net billing tariff (the current compensation rules, sometimes called NEM 3.0), which pays less for power you export to the grid than the older net metering did, so the economics now favor using your own power or adding a battery. SFPUC's CleanPowerSF pages are worth a look for any local solar or storage incentives before you sign.

IV. Worth watching this month

1. The CPUC's ongoing proceedings on the net billing tariff, where any change to export credit rates would move payback math for new west-side systems.

2. SFPUC and CleanPowerSF program pages for updates to local solar or battery incentives, which can open and close with little notice.

3. PG&E's interconnection timelines, which can stretch past the routine few weeks when application volume runs high.

4. The fog season easing through October, a good window to compare your real fall production against the PVWatts estimate for your roof.

5. CAISO grid conditions and any flex alerts, routine during heat events but a reminder that a home battery earns its keep when the grid is tight.

Frequently asked questions

Does fog cut my solar production in half?

No. A bright overcast sky still makes power, often roughly a third to two-thirds of full sun. Annual losses on a foggy west-side roof are usually single digits to low teens of percent against a sunny roof, not half.

Which neighborhoods are foggiest?

Generally the Outer Sunset, Outer Richmond, and the western edge near Ocean Beach see the most summer marine layer. The Mission, Bernal Heights, and Potrero Hill tend to be sunnier. Your own block can still differ from its neighbors.

Is solar still worth it under the new rules?

It depends on your usage and whether you add a battery. The net billing tariff pays less for exported power, so systems sized to your own use, or paired with storage, tend to pencil out better than ones built to sell a lot back to the grid.

How do I get a number for my exact roof?

Use NREL's PVWatts calculator with your address, roof tilt, and direction, then ask any installer to model the same roof. An address-specific estimate beats a neighborhood's reputation every time.

References