How Much Does Solar Production Drop in Winter? Measured in Southern California
On the same rooftop system, December produces a median 45% of June in our Los Angeles-area fleet; the middle half of system-years ran from 37% to 54%. November through February together make about 23% of a year’s solar. The winter drop on real roofs is deeper than models predict: in December our systems reached 68% of the PVWatts estimate for an ideal roof, against 96% in early summer.
How deep the winter drop is on each system
December as a percent of the same system’s June.
| Measure | p10 | p25 | Median | p75 | p90 |
|---|---|---|---|---|---|
| December as % of June | 30% | 37% | 45% | 54% | 61% |
| December as % of July | 30% | 35% | 42% | 49% | 54% |
| Nov–Feb share of the year | 19% | 21% | 23% | 25% | 26% |
| Best month ÷ worst month | 1.9× | 2.1× | 2.5× | 3.1× | 4.0× |
https://cali-energy.com/research/winter-solar-production-southern-california. Aggregated data are free to use under CC BY 4.0 with a link to this page.The spread is the finding. On a typical system December makes a bit under half of June. On one in ten it makes less than 30%, and on the best tenth more than 61%. The difference between those systems is not the weather — they share it — but the roof: which way it faces, how steep it is, and what stands between the panels and a low winter sun.
Real roofs lose more in winter than the models say
Measured median as a percent of the PVWatts ideal-roof estimate.
| Month or period | Measured median, kWh/kW | PVWatts ideal roof | Measured as % of ideal |
|---|---|---|---|
| June | 153 | 164 | 93% |
| July | 163 | 172 | 95% |
| December | 68 | 100 | 68% |
| January | 81 | 110 | 74% |
| Best-to-worst month | 2.4× | 1.7× | deeper swing on real roofs |
| All twelve months: see the monthly benchmark. | |||
Three things make winter harder on real roofs than in the model. Shade: in December the sun stays low in the southern sky, so trees, a neighbor’s second story or a chimney can shade panels that are clear all summer; PVWatts assumes no shade. Direction: east- and west-facing arrays lose a larger part of the short winter day than south-facing ones. Weather: winter storms and cloudy weeks come in some years and not others, while the model uses a typical year. Our data cannot split the gap among these causes system by system, but the pattern — close to the model in summer, well below it in winter — is consistent across the fleet.
Where does your winter drop sit?
Place your December-to-June ratio among the measured systems.
| December as % of June | Where it sits among measured systems | Common reasons on real roofs |
|---|---|---|
| Above 54% | Above the middle half of observed seasonal ratios | South-facing panels with little winter shade |
| 37–54% | Within the middle half of observed seasonal ratios | Typical mix of roof direction and seasonal shade |
| 30–37% | In the lowest quarter | East- or west-facing panels, winter shade from trees or buildings |
| Below 30% | In the lowest tenth | Heavy winter shade, or something that changed after June |
| A seasonal ratio is a screening signal, not a diagnosis of the equipment. | ||
Suggested next step. Use the same system’s June and December of the same year. In the lowest quarter, compare with last winter and look at what may have grown or been built nearby. In the lowest tenth, also check the daily graph in your app for flat or zero days and any inverter alerts, and consider a check. A ratio that falls from one winter to the next while the weather was similar points to something new: a tree, a dirty or damaged panel, or equipment. For a single month against all LA systems, use the monthly check.
What the winter dip means for your bill
Winter bills come back
Lower solar output plus heating and holiday use means more grid power in winter. See solar panels in winter.
Credits are banked in summer
Under net metering, summer credits offset winter use at true-up. See the NEM true-up bill.
Size batteries for winter too
A battery that fills every summer day may not fill in December; see the battery sizing calculator.
Shade is fixable
Trimming a tree can restore winter output; see solar, shade and trees.
Methodology
- Data source
- Monthly AC energy recorded by SolarEdge inverters in Cali Energy’s monitoring fleet. These are readings from the monitoring system, not from utility revenue meters.
- Systems
- 96 monitored systems; 4 excluded (1 customer system we service but did not install, 1 expanded mid-period, 2 whose output does not match the capacity recorded in the monitoring account). 89 systems have at least one complete system-year.
- Complete system-year
- One system over one calendar year, 2019–2025, with production recorded in all 12 months. Years below 75% of the same system’s own median were treated as downtime and kept out of the benchmark (3 of 388). Result: 385 complete system-years. 2026 is incomplete and not used.
- Unit
- kWh per kW: yearly or monthly energy divided by the system’s DC nameplate rating (kW) as recorded in the monitoring account. It removes system size, so a 4 kW and a 12 kW system can be compared.
- Statistics
- Medians and percentiles (p10, p25, p75, p90), not averages, so a few unusual systems do not move the result. “Middle half” means p25–p75.
- Geography
- Assigned by ZIP code: 49 systems in the San Fernando Valley and Burbank, 21 elsewhere in Los Angeles County, 4 in the Santa Clarita Valley, 4 in Ventura County and 11 elsewhere in Southern California.
- Limits
- Not a random sample of Los Angeles roofs: these are systems we monitor, weighted toward the San Fernando Valley. Roof direction, tilt and shade vary from system to system and are not adjusted for. Customer names and addresses are never published; only aggregates are.
- Seasonal ratios
- Computed within each system-year (December ÷ June of the same system and year). 4 system-years where December exceeded June or July — a sign of a summer outage — were excluded.
Frequently asked questions
Do solar panels work in winter in Southern California?
Yes, at reduced output. In our measured fleet December produced a median 68 kWh per kW, about 45% of the same system’s June.
How much less do solar panels produce in winter?
On the same system, December makes a median 45% of June (middle half 37–54%). November through February together make about 23% of the year’s output.
Why is my solar so low in December?
Short days and a low sun. Shade from trees and nearby buildings reaches much farther in December, and east- or west-facing panels lose more of the day.
What is a normal winter drop for solar in LA?
Among the systems we measured, the middle half made 37–54% of their June output in December. Below 30% puts a system in the lowest tenth — a screening signal worth a closer look, not a diagnosis.
Is solar production lower than PVWatts estimates in winter?
In our data, yes: about 68% of the ideal-roof PVWatts figure in December versus 96% in early summer, because the model assumes no shade.
More research
Next steps
Planning for winter, or your December looks too low?
Prepared by Cali Energy, October 7, 2026. Figures describe systems in our monitoring fleet and are not a guarantee of what any specific roof will produce; orientation, tilt, shade, equipment and weather change results. Not engineering, financial or utility advice. See our Content Disclaimer.