Solar Panels in Winter: Real Numbers for Maryland and DC

An 8 kW system in Silver Spring produces about 510 kWh in December and 1,220 kWh in July. That gap drives most of the winter solar questions we hear from homeowners in Bethesda, Rockville, Takoma Park, and Northwest DC. Cold weather actually boosts panel output, but short days and a low sun angle overwhelm the gain. Net metering carries you through the rest.

What follows is what really happens to your panels between November and March, the production numbers a typical Maryland or DC home sees, and the few things actually worth doing during winter.

Does Cold Weather Help or Hurt Solar Panels?

Short answer: Cold helps. Solar panels are semiconductors, and like most electronics they perform better cool. A 35°F morning in Bethesda with bright sun can briefly push panels above 100% of their rated output.

The wattage on the back of every panel is measured at 25°C (77°F). For every degree above that, output drops by roughly 0.3 to 0.5 percent. Below it, output goes up.

The catch: panel temperature is not the limiting factor in DC and Maryland winters. The sun is.

Industry standard temperature coefficient guidance, NREL and panel manufacturer datasheets; range -0.3% to -0.5%/°C is typical for monocrystalline silicon modules.

Why Does Winter Production Drop So Much?

The DMV sits at roughly 39 degrees north latitude. On the December solstice, Washington gets 9 hours and 26 minutes of daylight; on the June solstice, 14 hours and 53 minutes. The sun also sits much lower in the sky in winter, so light hits your roof at a steeper angle and travels through more atmosphere before getting there.

Combine fewer hours with a flatter angle, and average peak sun hours (the industry shorthand for usable solar production) fall from about 5.5 per day in June to about 3.0 per day in December and January. A typical residential system in Maryland or DC produces roughly half as many kilowatt-hours in December as it does in June. Cloudy days still produce something. Diffuse light from a heavy overcast sky usually delivers 15 to 25 percent of clear-day output, which is why monthly totals never collapse to zero even in a bad weather stretch.

timeanddate.com Washington DC sunrise/sunset tables; University of Maryland Extension FS-2023-0655; NREL PVWatts v8 modeling.

What Does an 8 kW System Actually Produce by Month?

Modeled in NREL PVWatts for a typical 8 kW system in Montgomery County, south-facing panels at roughly a 25-degree pitch:

• January — 530 kWh. Year's low point: shortest days, possible snow cover
• February — 690 kWh. Days lengthen noticeably by late month
• March — 940 kWh. Big jump as sun climbs higher in the sky
• April — 1,090 kWh. Strong shoulder-season production
• May — 1,170 kWh. Often one of the year's best months
• June — 1,190 kWh. Long days, but hotter panels trim some efficiency
• July — 1,220 kWh. Peak production despite the heat
• August — 1,130 kWh. Still strong, gradual decline begins
• September — 970 kWh. Comfortable temps, shorter days
• October — 890 kWh. Fall drop becomes visible
• November — 630 kWh. Winter curve starts
• December — 510 kWh. Shortest days of the year
• Annual total — about 10,960 kWh

The summer-heavy curve is why many properly sized systems in Maryland and DC end the year with a net-metering credit balance. You bank kilowatt-hours from May through September, then draw them down through the winter.

NREL PVWatts v8, Silver Spring zip 20910, 8 kW DC, fixed roof mount, 180° azimuth, 25° tilt, 14% system losses. Actual production varies with shading, exact azimuth, and module efficiency.

What About Snow on the Panels?

Short answer: Most snow on rooftop solar in this region clears itself within a day or two of any sun, and even partial snow cover lets through enough light to keep production going. Climbing on a snowy roof is rarely worth the risk.

Washington averages about 14 inches of snow per year, Baltimore about 19, mostly in January and February. Panel glass is dark, absorbs sunlight, and warms above the surrounding roof, so snow tends to slide once the sun comes out, even when air temperature stays below freezing. A 25 to 35 degree roof pitch helps it along.

A thin layer of light snow still lets through enough light for panels to produce roughly 30 to 50 percent of normal output, which itself accelerates melting. If snow does stick for three or four days after a heavy storm, that is normal in our climate. It does not require a service call. Production catches up the next clear week.

We do not recommend climbing on a snowy roof to clear panels manually. The fall risk and the risk of scratching the glass with metal tools both far outweigh the production you would gain over the day or two it takes to clear naturally.

NOAA NCEI Climate Normals 1991–2020, Reagan National and Baltimore-Washington International stations.

What Should You Watch in Your Monitoring App?

Watch the monitoring app, not the bill. A January electric bill can rise just because you used more electricity for heating, lighting, and longer evenings indoors. That's a usage story, not a solar story. The app tells you what your system actually produced.

If your system has Enphase microinverters, the Enphase app shows per-panel production. Winter shading patterns are different from summer ones: leafless tree branches, chimneys, plumbing vents, and neighboring rooflines all cast longer shadows when the sun is low.

A useful habit is to open the app once in mid-December and again in mid-January. If you see one panel consistently producing 60 percent or less of its peers, there is probably a shading issue. Usually nothing alarming, often a vent stack throwing a long winter shadow on one module, but worth flagging at the next service window.

Year-over-year comparison is more useful than absolute numbers. Last December's production at the same system tells you what to expect this December. Year-over-year drops of 5 to 10 percent are normal weather variance, but a 25 percent gap during clear-sky weeks is worth a closer look.

Enphase Enlighten monitoring documentation; Aduu Solar field practice on service calls.

How Does Net Metering Carry You Through?

In both Maryland and DC, your utility credits you at the full retail rate for every kilowatt-hour you push back to the grid during a billing month. Pepco serves DC and Montgomery and Prince George's counties; BGE serves Baltimore and most of central Maryland. The retail-rate monthly credit applies to both.

The annual reconciliation rules differ:

In Maryland, the default is a 12-month cycle ending in April, with any leftover excess paid out at the lower commodity rate. Maryland's Net Metering Flexibility Act, passed in 2023 and codified at Md. Public Utilities Code § 7-306, gives eligible customers an opt-in alternative: indefinite rollover of credits at full retail value. The opt-in is the financially better choice for most homeowners, but it has to be elected. Your utility will not switch you over automatically. Confirm enrollment with Pepco or BGE after your system is operational.

In DC, monthly excess is credited at the full retail distribution rate and rolls forward month to month. At the end of the calendar year, only excess generation beyond 100% of annual consumption is paid out, at the lower generation rate. For systems sized to actual annual usage, which is most residential installs, the credits earned over the summer ride into winter intact and offset the higher winter draw.

A correctly sized system covers close to 100 percent of your annual electricity use even though it covers only 30 to 50 percent of December's day-by-day demand. The kilowatt-hours you over-produced in July are sitting on your account in January.

Md. Public Utilities Code § 7-306; Maryland PSC implementation of the Net Metering Flexibility Act, 2023; D.C. Code § 34-1518; 15 DCMR Chapter 9, § 900 et seq.

What Should You Actually Do in Winter?

Almost nothing. The best thing for your system between November and March is to leave it alone.

• Do not climb on the roof to clear snow.
• Do not clean the panels. Winter rain handles that in our climate.
• Do open the monitoring app once a month. Look for any panel offline or significantly under-producing relative to its peers.
• Do compare this winter's production to last winter's same months. Small year-over-year differences are normal; a 25 percent gap warrants a service call.

If your November or January production is more than 25 percent below the same month last year and the rest of the array looks fine, send us a screenshot from your monitoring app and we will take a look. Most of the time, in January as in July, the system is doing exactly what it should and you should not be thinking about it.

Frequently Asked Questions

Why is my January electric bill higher even though my solar system is working?

Two separate things are happening. Your panels really are producing less in January, about half of what they make in July. At the same time, your home is using more electricity for heating, lighting, and longer evenings indoors. The bill reflects both. The monitoring app is the right place to check whether the system itself is healthy.

Does cloudy weather kill production completely?

No. Diffuse light from a heavy overcast sky usually delivers 15 to 25 percent of clear-day output, which is why monthly totals never drop to zero. A run of grey days will reduce production but not stop it.

Should I add a battery to handle winter?

For most Maryland and DC homeowners on standard net metering, no. The summer credits already cover most of the annual usage gap, and a battery costs $10,000 or more without changing your annual production. Batteries make sense mainly for backup power during outages or for homeowners on time-of-use rates with large peak/off-peak spreads, not as a winter production aid.