Every November, the same message turns up in inboxes along the Front Range: “My solar production has dropped by half. Is something broken?”
Usually, nothing is broken. Winter output falls for reasons that have nothing to do with your panels, and it comes back on its own in spring. But sometimes a real problem hides inside the seasonal dip, and it is worth knowing how to tell the difference.
This article explains why winter output drops, what normal looks like, and what to check before you call anyone.
Why does output drop in winter?
Four things stack up, and all of them are about the sun, not your system.
Shorter days. Around the winter solstice, Denver gets far fewer hours of daylight than in late June. Fewer hours means fewer kilowatt-hours, even on a perfect day.
A lower sun. In December, the sun crosses the sky low in the south. Its light reaches your panels at a steeper angle, so more of it reflects off the glass and less lands on the cells.
More atmosphere. Low-angle light travels through more air to reach you, which takes the edge off its strength, especially early and late in the day.
Snow. A covered panel makes almost nothing until the snow slides or melts off.
Put those together and a winter month can easily produce a fraction of a summer month. That is what the system was designed for, and your installer’s original production estimate almost certainly showed the same curve.
Doesn’t cold weather help panels?
It does, a little. Solar cells work more efficiently when they are cool, which is one reason a crisp, clear January morning can produce surprisingly well for a few hours.
But cool cells cannot make up for half the daylight and a low sun. The cold helps each hour of sunlight. Winter simply has fewer of those hours.
What about shade?
Shade changes with the season, and it catches people out. A neighbor’s house, a chimney or a tall evergreen that never touches your array in June can throw a long shadow across it in December, when the sun is low.
Deciduous trees work the other way. Once the leaves drop, a big maple or cottonwood lets more light through than it did all summer.
If your winter numbers look oddly low for only part of the day, walk outside at that time and look at where the shadows fall. It may be a building, not a fault.
What is normal for a winter month?
Normal is whatever your own system did last winter, in the same month, adjusted for the weather.
That is the comparison that matters. December against June will always look alarming. December against last December tells you something.
Here is a simple way to check:
- Open your monitoring app’s monthly view. Most apps keep at least a year or two of history.
- Compare this month with the same month last year. Look at the total and at a few clear days.
- Allow for weather. A snowy, cloudy month will come in lower than a dry, sunny one.
- Look at the shape of a clear day. A smooth arch peaking near midday is healthy.
Our guide to reading your solar app shows where to find these views in most apps.
What does a healthy winter day look like?
On a clear December day, a healthy array draws a short, smooth arch in your app. It starts later in the morning than in summer, peaks a little after midday, and tails off well before dinner.
The peak is lower and the arch is narrower, but the shape is the same. That shape is the thing to look for.
A few shapes that are not a worry:
- A late, slow start. Frost or a thin layer of snow melting off the glass.
- A dent in the afternoon. A shadow from a chimney, a neighbor’s roof or an evergreen.
- A ragged line. Passing clouds, which are common on winter afternoons along the foothills.
A clear day with a flat line near zero, or an arch that stops abruptly at the same time every day, is worth a second look.
When is a winter drop not normal?
A few patterns point to something worth a closer look:
- Zero on a clear day. If the sun is out and the app shows nothing, check whether the inverter is on and showing an error. That is a call for your installer.
- One panel far below its neighbors. If you have panel-level data, one module that lags on every clear day while the rest track together suggests shade, snow, dirt or a fault.
- A flat line that does not recover after snow. When the rest of the array is clear and producing, one section stuck near zero deserves a look.
- A big drop compared with last winter. Same month, similar weather, much less output.
A published method gives a useful yardstick. Researchers at UTS, UNSW and Diagno Energy, writing in Solar Energy, flag major underperformance when output stays below 60% of expected for three non-cloudy days, and minor underperformance below 80% for seven days. The key word is expected. In winter, expected means winter, not July.
What should you do about snow?
Mostly, nothing. Panels are tilted and dark, and once the sun hits them, snow usually slides off within a day or two of a storm. Our page on snow and ice on solar panels explains what to expect.
Please do not climb onto the roof to clear them, and do not use a roof rake on the glass. Rakes scratch modules and loosen hardware, and roofs in winter are dangerous. We do not remove snow from rooftop arrays either.
We wrote a longer answer in should you clear snow off solar panels?, including what to do with a ground mount you can reach safely.
Does dirty glass matter more in winter?
It can. With the sun low and the light already weak, a film of dust or droppings takes a bigger bite out of what is left. A dropping on the glass blocks light the same way in December as in June, but December has less light to spare.
That does not mean a winter wash is always worth it. Production is low anyway, so the dollar value of a cleaner panel is small. If your glass is badly filmed from a dry autumn, though, a wash on a dry day above freezing can make sense. The output check helps you decide with your own numbers.
When does output come back?
Gradually, from late December onward, as the days lengthen and the sun climbs. Most Front Range systems climb steadily through February and March, with spring snowstorms causing short dips along the way.
By April, the curve is back near its spring shape. If it is not, that is the time to look harder.
Our Front Range solar year maps what each month tends to bring, from December snow to spring pollen.
Who watches this for you?
You can, with a few minutes in the app once a month. Or someone else can. Our output watch plan puts the monitoring desk on your data with read-only access. They compare each month with what your system should be making, allow for weather and season, and tell you plainly if something looks off.
If you would like more detail on what the monitoring desk does, our performance monitoring service page covers it.
The short version
- Winter output drops because of shorter days, a low sun, more atmosphere and snow.
- Cold helps a little but cannot make up for lost daylight.
- Compare this winter with last winter, not with summer.
- Zero on a clear day or one panel lagging its neighbors are worth a look.
- Leave snow on rooftop panels to the sun.
If your numbers look strange this winter and you are not sure why, the output check is a good first step. Or send us a screenshot of your app, and the monitoring desk will tell you whether it looks like winter or something else.