Photons still deliver power—even when the sky is gray.
On overcast days, it’s easy to assume solar slows to a crawl. We hear from Localyst subscribers who aren’t against solar but are skeptical that it produces enough energy to be worth it where they live. One subscriber from Michigan recently said, “Inefficient. Last December there was not a single sunny day.” So we looked into it.
“Last December, there wasn’t a single sunny day.” — Localyst subscriber, Michigan
Clouds don’t shut solar off. The simple physics: panels eat photons, not “sunbeams.” Clouds scatter light, but that scattered light still hits your panels and turns into electricity. Even grid operators and engineers plan for this; standard tools account for cloudy-day light and temperature when estimating output. pvwatts.nrel.gov+1
Why overcast ≠ “off switch”
- Daylight works. The UK’s National Grid puts it plainly: panels don’t need direct sunlight—any level of daylight can generate power (output just varies with brightness). The Energy Saving Trust echoes the same point. National Grid+1
- Cooler can help. Solar electronics prefer cooler temperatures. As panels heat up, power typically falls by about 0.2% to 0.5% per °C; cooler, cloudy weather can claw back a few percent compared with hot, clear days. (This temperature effect is baked into mainstream models.) pv-tutorials.github.io+1
Proof from cloudy places
If solar needed endless blue skies, gray-climate countries would lag. Instead, Germany—not famous for year-round sunshine—set records in 2024: 72.2 terawatt-hours of solar, supplying about 14% of its electricity. That’s real, annual grid production across every season and sky condition. Fraunhofer ISE
“How much will panels make when it’s cloudy?”
It depends on cloud thickness, time of day, and your system. In practice, many systems still produce a meaningful share of their clear-sky output on partly cloudy days, and steady, usable power on overcast days. That’s exactly why utilities and planners use tools like NREL’s PVWatts—they roll cloudy-day light and temperature into the forecast for your location. pvwatts.nrel.gov
Kitchen-table takeaway: Solar doesn’t “turn off” when it’s cloudy—it just makes less than on a bright, clear day, and sometimes works a bit more efficiently per photon because it’s cooler. pv-tutorials.github.io
Sources
National Grid (UK), Solar power: your questions answered (panels don’t need direct sunlight). National Grid
Energy Saving Trust, Debunking solar myths (works on cloudy days). Energy Saving Trust
NREL, PVWatts / US DOE, Understanding Solar PV System Performance (models include temperature and cloudy-day light). pvwatts.nrel.gov+1
Fraunhofer ISE (Germany), Public Electricity Generation 2024 (72.2 TWh solar; ~14% of electricity). Fraunhofer ISE
Sandia PV Performance Modeling Collaborative (PVPMC), Sky-diffuse irradiance models (how engineers model cloudy-day light). PVPMC
