SolarVeri Data Team
Irradiance data and off-grid sizing
Solar panels work in winter, but they make much less energy. In December, a panel in London produces about 27% of what it makes in June, one in Chicago about 31% and one in Phoenix about 78%. For a grid-tied home that is a seasonal dip; for an off-grid system it is the number that decides everything.
Below is month-by-month PVGIS data for 14 US and 16 European cities, what tilt and snow do, and what the winter drop means for off-grid systems. For the full design process see the off-grid solar sizing guide; for storage, how many batteries you need; and for vans and motorhomes, how much solar an RV needs.
wb_twilightWhy solar output drops in winter
- check_circleShorter days: fewer daylight hours means fewer hours of production.
- check_circleLower sun angle: sunlight passes through more atmosphere and hits flat or shallow panels at a glancing angle.
- check_circleMore clouds: in many climates winter is also the cloudiest season, which matters more than temperature.
- check_circleSnow cover: a covered panel produces almost nothing until the snow slides or melts off.
- check_circleCold is not the problem: panels are actually more efficient when cold. Losses come from less light, not low temperature.
flagWinter vs summer output in the US
| City | December (kWh per kW) | June (kWh per kW) | December as % of June | December per day |
|---|---|---|---|---|
| Miami, FL | 119 | 121 | 98% | 3.84 kWh |
| Phoenix, AZ | 124 | 160 | 78% | 4.00 kWh |
| Austin, TX | 103 | 138 | 75% | 3.32 kWh |
| Albuquerque, NM | 124 | 165 | 75% | 4.00 kWh |
| Atlanta, GA | 92 | 140 | 66% | 2.97 kWh |
| Denver, CO | 103 | 156 | 66% | 3.32 kWh |
| Nashville, TN | 74 | 134 | 55% | 2.39 kWh |
| New York, NY | 69 | 126 | 55% | 2.23 kWh |
| Boston, MA | 68 | 128 | 53% | 2.19 kWh |
| Chicago, IL | 44 | 140 | 31% | 1.42 kWh |
| Minneapolis, MN | 59 | 136 | 43% | 1.90 kWh |
| Bozeman, MT | 43 | 151 | 28% | 1.39 kWh |
| Portland, OR | 40 | 127 | 31% | 1.29 kWh |
| Seattle, WA | 34 | 135 | 25% | 1.10 kWh |
Latitude explains most of the pattern, but clouds explain the outliers. Seattle and Portland drop to about 25% and 31% of their June output because Pacific Northwest winters are both dark and overcast, while Denver, at a similar latitude to New York, holds about 66% thanks to clear, dry winter skies. Miami barely has a winter at all from a solar point of view.
publicWinter vs summer output in the UK and Europe
| City | December (kWh per kW) | June (kWh per kW) | December as % of June | December per day |
|---|---|---|---|---|
| Seville, Spain | 108 | 160 | 68% | 3.48 kWh |
| Athens, Greece | 85 | 158 | 54% | 2.74 kWh |
| Lisbon, Portugal | 87 | 158 | 55% | 2.81 kWh |
| Madrid, Spain | 96 | 159 | 60% | 3.10 kWh |
| Rome, Italy | 79 | 155 | 51% | 2.55 kWh |
| Lyon, France | 49 | 144 | 34% | 1.58 kWh |
| Vienna, Austria | 40 | 138 | 29% | 1.29 kWh |
| Munich, Germany | 45 | 129 | 35% | 1.45 kWh |
| Paris, France | 41 | 133 | 31% | 1.32 kWh |
| Berlin, Germany | 26 | 135 | 19% | 0.84 kWh |
| Amsterdam, Netherlands | 26 | 131 | 20% | 0.84 kWh |
| London, UK | 34 | 125 | 27% | 1.10 kWh |
| Dublin, Ireland | 32 | 119 | 27% | 1.03 kWh |
| Manchester, UK | 26 | 113 | 23% | 0.84 kWh |
| Edinburgh, UK | 21 | 113 | 19% | 0.68 kWh |
| Stockholm, Sweden | 11 | 145 | 8% | 0.35 kWh |
Northern Europe sees the steepest drop. In the UK, December output is roughly a quarter or less of June: about 27% in London, 23% in Manchester and 19% in Edinburgh. Stockholm falls to about 8%. Southern Spain, Portugal and Greece keep roughly half of their summer output, which is why the Mediterranean is the easiest part of Europe to live off-grid year-round.
straightenTilt matters most in winter
The low winter sun favors steeper panels. Flat panels, common on RVs and some flat roofs, lose the most. The comparison below uses the same 1 kW of panels laid flat and tilted to 30 degrees.
| City | December, flat (kWh per kW) | December, 30° tilt | Gain from tilting | June, flat | June, 30° tilt |
|---|---|---|---|---|---|
| Phoenix, AZ | 73 | 124 | +70% | 181 | 160 |
| Denver, CO | 53 | 103 | +94% | 168 | 156 |
| New York, NY | 38 | 69 | +82% | 133 | 126 |
| Seattle, WA | 17 | 34 | +100% | 137 | 135 |
| Madrid, Spain | 50 | 96 | +92% | 169 | 159 |
| Munich, Germany | 22 | 45 | +105% | 132 | 129 |
| London, UK | 14 | 34 | +143% | 126 | 125 |
Tilting barely changes June output but lifts December output substantially, often by half or more. Off-grid arrays built for winter are commonly set even steeper, around latitude plus 10 to 15 degrees. Steeper panels also shed snow faster.
weather_snowySnow on solar panels
A light dusting often melts off within hours because dark panels absorb sunlight and warm up. Heavy snow on a shallow or flat panel can stay for days. Steep tilt, a clear area below the panels for snow to slide into, and ground mounts you can reach with a soft roof rake all help. Avoid metal tools and hot water, which can scratch or crack the glass. The PVGIS figures on this page do not model snow cover, so in snowy regions real December output can be lower than the tables show.
cabinWhat the winter drop means for off-grid systems
For a grid-tied home, a weak December is balanced by a strong June over the year. Off-grid there is no balancing. We simulated the same cabin system everywhere (1,600 W at 45°, a 10,240 Wh battery, 3,000 Wh per day): the battery hit its cutoff on 0% of December days in Phoenix, 55% in Chicago, 86% in London and 100% in Stockholm, against at most 0% of July days at those same locations. The battery guide explains why adding storage does not fix this, and the sizing guide shows how to size the array for your worst month.