Do Solar Panels Work in Winter? What 100W Really Gives You in December
Solar panels do work in winter in the UK, but not in the way the summer figures suggest. A 100W panel that comfortably tops up a leisure battery in June will struggle to keep pace with even modest daily use in December. The panel is not faulty. It is being asked to work with roughly half the daylight, a much lower sun angle and considerably more cloud.
Understanding the scale of the drop is what stops you either overestimating solar and flattening a battery, or dismissing it and missing the one job it is excellent at.
Why winter output falls so far
Three separate effects stack up, and it is the stacking that produces the difference.
- Day length. Southern England gets roughly sixteen and a half hours between sunrise and sunset in late June and around eight in late December. Scotland is more extreme still. That alone roughly halves the available window.
- Sun angle. In midwinter the sun stays low. A flat roof-mounted panel presents a poor angle to a low sun, so even in clear conditions the light striking each square metre is far weaker than in summer. This is usually the largest single factor.
- Cloud and shorter useful hours. UK winters are cloudy, and the hours near sunrise and sunset contribute very little even on a clear day.
Panels are rated under standard test conditions, at a defined light intensity and cell temperature. Those conditions do not occur on a caravan roof in Yorkshire in January. The rating is a comparison figure between panels, not a promise of output.
One factor works the other way: panels are more efficient when cold. A cold, bright, still winter day can give a very useful figure. It just does not happen often enough to plan around.
What to actually expect
As a planning rule of thumb rather than a guarantee, a fixed roof panel in the UK might return something in the region of three to four hours of equivalent full output per day across the summer, and closer to half an hour to one hour in December. Site conditions, shading, panel angle and weather will move that considerably in either direction.
| Panel | Rough summer day | Rough December day |
|---|---|---|
| 100W | around 300 to 400 Wh | around 50 to 100 Wh |
| 150W | around 450 to 600 Wh | around 75 to 150 Wh |
| 300W | around 900 to 1,200 Wh | around 150 to 300 Wh |
Treat those as order-of-magnitude estimates for planning, not as specifications. The point is the ratio: expect something in the region of a fifth of the summer figure, and be pleasantly surprised rather than disappointed.
What that buys you
A 12V system's daily draw adds up faster than people expect. Lighting, the water pump, a control panel, a fridge on 12V and phone charging can easily come to several hundred watt-hours a day. In summer a 150W panel covers that. In December it does not come close.
So in winter, solar becomes a supplement rather than a supply. It extends how long you can stay off-grid, it does not make you independent.
The job winter solar is really excellent at
Maintaining a battery in storage. This is where a modest panel earns its money.
A leisure battery left over winter self-discharges, and anything left connected such as an alarm or a tracker pulls a small continuous current. A flat lead-acid battery sulphates and may never recover its capacity. Replacing it costs far more than a panel and controller.
The maintenance load is small, on the order of tens of watt-hours a day, and that is well within reach even in December. A Foldable Solar Panel MSFO-100 at 100W, paired with a controller, will comfortably hold a battery topped up through the winter. A fixed roof panel does the same job without you having to set anything up.
You still need the right controller
A panel cannot be connected straight to a battery. The controller regulates the charge, and in low light an MPPT controller extracts noticeably more from the same panel than a simpler PWM one, because it converts the panel's higher voltage down rather than dragging it to battery voltage. That advantage is largest exactly when light is weak, which is the winter case.
Match the controller to the panel rather than buying the biggest one:
| Controller | Max charge current | Max solar input |
|---|---|---|
| MSC-4010 | 10A | up to 130W |
| MSC-4020 | 20A | up to 260W |
| MSC-4030 | 30A | up to 400W |
All three work on 12V or 24V systems, carry an LCD display, and include protection against overload, excessive discharge, short circuit, high voltage and reverse polarity. Set the controller's charge profile to match your battery chemistry, because lithium and lead-acid want different voltages.
Getting more out of a winter panel
- Keep it clean. Winter road film, salt and leaf debris cut output, and there is far less output to lose.
- Clear snow immediately. A covered panel produces essentially nothing.
- Avoid shade completely. Partial shading on one part of a panel disproportionately reduces the whole panel's output.
- Tilt if you can. A portable or foldable panel angled towards a low winter sun will out-produce a flat roof panel of the same rating by a wide margin. This is the biggest single gain available in winter.
- Park facing south where the pitch allows, and away from buildings and trees that cast long winter shadows.
That tilting point is why a foldable panel is worth considering alongside a fixed one for winter use. A fixed panel cannot be aimed, and in December aim matters more than area.
The honest summary
Do not plan a winter off-grid trip around solar. Do use solar to keep a stored battery alive, to extend a weekend, and to reduce how often you need to run an engine or find a hook-up. Size the controller to the panel, keep the glass clean, and tilt it if you have the option.