Why your solar system produces less in winter

Solar production in DC, MD, and VA swings significantly across the year. A system that puts out 50 kWh on a sunny June day might produce 15 kWh on a cloudy December day. That is not a sign of damage. That is the system working exactly as designed.

Here is what causes the swing.

Day length

DC gets about 14 hours, 50 minutes of daylight at the summer solstice and about 9 hours, 25 minutes at the winter solstice. That is a five-hour gap. Even with no clouds, no snow, and a perfectly clean array, you simply have less time to generate.

Sun angle

In summer, the sun sits high in the sky, close to directly overhead at solar noon. Light hits your panels close to perpendicular, which is the most efficient angle. In winter, the sun arcs low across the southern sky. Light hits your panels at a steeper angle, and a portion of the energy is reflected away rather than absorbed.

Cloud cover and weather

DC's average annual cloud cover is around 53%. November through February typically run cloudier than May through September. A high-quality monocrystalline panel still produces in cloudy conditions, usually 10% to 25% of its rated output, but that is a steep drop from a clear summer day.

Snow

Light snow usually clears off panels within a day or two. Heavy snow that sits on the array stops production until it melts or slides off. Most DC installs are pitched at 20 to 30 degrees or sit on flat roofs at a low tilt, so snow rarely sticks for long. A multi-day blizzard can show as a flat line on your monitoring app.

What "normal" looks like across the year

For an 8 kW system in DC:

• June average: about 1,100 kWh per month
• December average: about 500 kWh per month
• Annual total: about 10,500 kWh

The monthly numbers can swing 20% to 30% in either direction based on weather. What matters is that the year-over-year totals stay close.

When to flag something

The pattern is wrong, not the absolute number. Watch for:

• Production drops to near-zero on sunny days
• One panel or string consistently underproducing in your monitoring app
• A sudden drop with no weather to explain it

If any of those show up, contact us. Otherwise, a low December reading is just December.