Why Is My Solar Production Low in Winter?
Last updated: 2026-04-06 · Solar Benchmark
Why Is My Solar Production Low in Winter?
Winter solar production drops 50–75% compared to summer in most US states, and the physics behind it is straightforward. December daylight in Boston runs about 9 hours; June runs 15. The sun sits 26 degrees lower in the sky at noon on the December solstice than the June solstice. Both effects reduce the irradiance hitting your panels, and there's nothing wrong with your system when this happens.
Expected Winter vs. Summer Production by Region
How much your system should produce in December relative to June depends entirely on latitude. Southern states have less seasonal variation; northern states have more.
| Region | Example States | June 6kW Output (kWh) | December 6kW Output (kWh) | Ratio |
|---|---|---|---|---|
| Southwest Desert | AZ, NV | 1,050–1,200 | 500–560 | ~2.1:1 |
| Southeast | FL, TX, GA | 950–1,080 | 390–500 | ~2.2:1 |
| California Coast | coastal CA | 980–1,075 | 375–450 | ~2.5:1 |
| Mid-Atlantic | NJ, MD, VA, NC | 960–1,010 | 270–340 | ~3.1:1 |
| Midwest | OH, IL, MO | 940–980 | 240–300 | ~3.5:1 |
| New England | MA, NY, CT | 950–972 | 230–260 | ~3.8:1 |
| Pacific Northwest | WA, OR | 750–840 | 120–180 | ~5.5:1 |
(Source: pvlib physics modeling, Open-Meteo ERA5 weather data)
Why Winter Production Is Low: The Physics
Two factors cut winter production: fewer daylight hours and a lower sun angle.
Daylight hours: From June to December, the day shortens by 4–6 hours depending on latitude. A 6kW system generating 1,000 kWh in a 15-hour June day has about 9 hours of daylight in December, reducing available collection time by 40% before accounting for anything else.
Sun angle (solar altitude): The sun's maximum elevation angle at noon in Boston is 72 degrees in June but only 26 degrees in December. At a 26-degree sun angle, sunlight passes through 3–4 times more atmosphere than at 72 degrees, reducing irradiance at ground level. Panels tilted toward the south at 30–40 degrees receive much less direct irradiance when the sun is that low.
Together, these effects reduce December irradiance to 25–35% of June irradiance in most of the Northeast and Midwest. A properly functioning system produces proportionally less.
When Low Winter Production Signals a Real Problem
Winter underperformance is worth investigating when:
- Your December production is more than 20% below the regional benchmark for December (not June)
- Production drops suddenly to near-zero during non-storm daylight hours
- You see fault codes in your monitoring app
- Your production is significantly lower this December than last December with no obvious reason (no new shading from trees, no system changes)
Snow accumulation can temporarily reduce production to zero while panels are covered. This is expected and typically resolves within 1–5 days as snow slides off or melts. The real concern is when December production is consistently low year over year, not when a snowstorm causes a brief spike to zero.
Checking Against the Real Benchmark
The mistake most homeowners make is comparing December production to September or October production and concluding something is wrong. The correct comparison is December actual vs. December expected given this December's actual weather at your address.
A physics model using ERA5 actual hourly data for your address calculates what your system should have produced in December based on the specific cloud cover, temperature, and irradiance that actually arrived. A December that was 30% cloudier than average will have a lower expected production; a healthy system will show production close to that lower expected figure, not close to a sunny-year benchmark. Learn more at /resources/methodology.
What Affects Winter Production Beyond the Calendar
- Snow on panels: Flat or low-tilt arrays retain snow longer. Systems at 30-degree+ tilt shed snow faster. Steep-roof installations in New England typically recover within 1–3 days of a snowfall.
- Inverter cold-start behavior: Some string inverters take longer to reach operating temperature on cold mornings, briefly delaying production ramp. This is typically 15–30 minutes of reduced output and has minimal annual impact.
- Shading at low sun angles: Winter's low sun angle creates new shading patterns that don't exist in summer. A chimney or rooftop HVAC unit that never casts a shadow in July may shadow a significant portion of the array at December's 26-degree sun angle. If your system underperforms in winter but not summer, check for winter-specific shading.
- Soiling: In regions without winter precipitation, dust and debris accumulate on panels through winter months. Dry western climates (California, Nevada, Arizona) can see 3–5% soiling-related losses in winter.
Frequently Asked Questions
Q: My solar system produced 250 kWh last month but 900 kWh in summer. Is it broken?
A: Not necessarily. For a 6kW system in the Mid-Atlantic or New England, producing 250 kWh in December and 950+ kWh in June is expected. The 3.8:1 ratio is built into the physics of latitude, not a system defect. Compare your December output to the December benchmark for your region, not to your summer output.
Q: Should I clean my panels before winter to improve winter production?
A: Cleaning helps if soiling is present, but its impact on winter production is limited by reduced irradiance regardless. The bigger benefit of fall cleaning is starting winter without accumulated summer soiling, which would otherwise compound through the low-production months. In dry climates, a fall cleaning makes sense; in rainy climates, rainfall typically handles panel cleaning naturally.
Q: My neighbor's system seems to produce more than mine in winter. Why?
A: Small differences in roof orientation and tilt become more significant in winter. A 5-degree difference in tilt angle has a larger impact in December than in June because the sun is already at such a low angle. A south-facing 35-degree tilt outperforms a south-facing 15-degree tilt by about 12% in winter, but only 3% in summer.
Q: When should I worry about low winter production vs. accept it as seasonal?
A: Accept it as seasonal if December production is within 15–20% of the regional December benchmark. Investigate if it's more than 20% below benchmark, if it dropped sharply compared to last December, or if your monitoring shows fault conditions. A system producing 180 kWh in December when the regional benchmark is 250 kWh is telling you something different than normal seasonality.
Data: pvlib physics modeling + Open-Meteo ERA5 weather data | Last updated: 2026-04-06 | Solar Benchmark