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Key takeaways
- Winter output depends on location, weather and shade. A mid-latitude site can receive nearly three times more solar energy in June than in December, according to the US Department of Energy.
- Use a tool such as NREL's PVWatts for a month-by-month picture, then trust your own logged data over any estimate.
- Plan in layers: solar, then load management, then a charging backup for poor-sun stretches.
- Decide in advance what you will do after a run of dark days: move, plug in, drive or run a generator according to its manual.
- Keep heating loads out of the battery budget unless you have measured them.
What actually changes in December
Three things shrink your winter harvest. Days are shorter, the sun sits lower in the sky so its rays are more slanted, and weather tends to be cloudier in many places. The US Department of Energy's explanation of solar radiation says mid-latitude places such as the United States get more solar energy in summer because days are longer and the sun is nearly overhead, while winter rays are more slanted. It uses Denver, near 40 degrees latitude, as an example: that city receives nearly three times more solar energy in June than in December.
The same page notes that weather can cut direct sunshine by up to 10 percent on clear, dry days and up to 100 percent on thick, cloudy days. You do not need to memorize those numbers. The takeaway is that seasonal swings are large and cloud cover can erase the rest.
This is also why the same array can feel different depending on where you park. Move south and you change the sun angle and, often, the weather. Our winter tilt angle guide and solar camping in winter cover the panel-side tactics. This guide is about the planning side: what to expect and what to do when it is not enough.
Build a winter estimate from tools and from your own logs
There are two ways to size up December, and you should use both.
A planning tool. NREL's PVWatts calculator estimates solar production using location-based weather data. It is built for grid-connected systems, so treat it as a way to compare months for a place, not a promise for an RV that moves, tilts or sits in shade. Version 8 also lets you enter monthly losses that can represent soiling or snow, which is a useful reminder that real conditions cost output. Compare its December result with its result for the month you know well, and you have a rough ratio for the season.
Your own data. If your charge controller or monitor logs daily harvest, note it for a few December days in different weather. A controller's history is the best record of what your panels, parking habits and shade actually yield. See our guide to solar monitoring displays if you do not have one.
Battery monitor with history
A monitor that records amp-hours in and out and state of charge turns December from a guess into data. Check the maker's page for compatibility with your battery voltage and for what the display records.
This guide gives no sizing formula, because the right one depends on your system. Use the sizing guide and your equipment manuals, and ask a qualified RV technician or electrician to check any wiring change.
Set a seasonal routine instead of a daily number
Short days push you toward a routine. Think in states of charge rather than abstract totals:
- Know your lowest comfortable battery level. Use the battery maker's guidance, not the point where the inverter alarms.
- Find your typical winter morning and evening readings. Where is the battery when the sun comes up, and where does it get to by late afternoon?
- Sort your loads. Lights and the fridge run every day, and heating, cooking appliances and entertainment can be flexible. Our guide to 12V appliances covers low-draw choices.
- Shift flexible loads to the sunny hours. Run what you can in the middle of the day when panels are working, and cut back in the evening.
- Write down the point at which you switch to a backup. A rule like this removes the guessing at 9 p.m. in the dark.
On heating, a furnace and a space heater both change the picture. Measure the furnace with our furnace draw guide before you assume your solar covers it. If you are wondering about a space heater, read why a space heater is hard on batteries and inverters, and look at 12V blankets and heated pads for a lower-draw option.
Line up your backup charging before you need it
Every winter plan needs a layer for stretches when the sun does not deliver. Choose the options that fit how you travel, and set them up in advance.
| Backup | Fits best when | Set it up by |
|---|---|---|
| Shore power | You can stay at a hookup site when the weather turns | Checking your charger's settings in its manual, and carrying the right cord and a protector |
| Driving and DC-DC charging | You move the rig regularly | Following the DC-DC charger and battery makers' instructions for your equipment |
| A generator run per its manual | You boondock for long stretches | Following the generator manual on placement, fuel and carbon monoxide, and never running it in an enclosed space |
| Moving to a sunnier site | You have flexibility | Checking forecasts and having a route in mind |
| A portable panel | Your roof is shaded or tilted wrong | Matching connectors to your controller and following the panel maker's instructions |
For driving, see our DC-DC charger guide. For more on a portable panel that you can aim and move, see the suitcase panel guide. Whatever you choose, installation and wiring belong with your manuals and a qualified RV technician or electrician.
Portable or suitcase solar panel
A portable panel can be set in the sun and pointed toward it, which a roof panel cannot. Match the connector and specs to your controller's manual before buying.
A December checklist
- Clean panels of dirt and clear snow gently, with soft tools only, as your panel manual allows.
- Check your controller and monitor for logged data and note a few winter days.
- Review your lithium battery maker's cold-weather charging guidance; no figures are given here because they differ by battery.
- Decide your backup trigger and confirm that the backup equipment works before you rely on it.
- Ask a qualified RV technician or electrician about any wiring, fusing or charging change.
None of this is glamorous, but planning around December pays off when the weather turns. The panel that earns its keep in July is still the panel you own in December, so the aim is not to force it to do more. It is to arrange the rest of the system so a poor week is an inconvenience, not a crisis.
Frequently asked questions
How much less solar do I get in December?
It depends on where you are. The US Department of Energy says Denver, near 40 degrees latitude, receives nearly three times more solar energy in June than in December. Your own logs will give a better answer for your rig.
What is a good tool for estimating winter solar output?
NREL's PVWatts calculator gives location-based estimates. It is built for grid-connected systems, so use it to compare months rather than as a promise for a moving RV.
Should I add more panels for winter?
Maybe, but first check shade, tilt, loads and backup charging. More panels help at the margin when days are short, so measure your own data before spending.
What is the best backup charging for winter?
It depends on how you travel: shore power at hookups, DC-DC charging while driving, a generator used per its manual, or a move to a sunnier site.
Can I run a heater from my solar in December?
Usually not a space heater. Measure your furnace draw first and look at lower-draw options such as insulation and 12V blankets.
Sources and verification
- US Department of Energy, Solar Radiation Basics: seasonal variation at mid-latitudes, the Denver June-versus-December comparison, and the effect of weather on direct sunshine.
- NREL, PVWatts Calculator: location-based production estimates for grid-connected systems, and version 8's monthly loss input for soiling or snow.
No sun-hour figures, panel wattages, amp-hour totals or sizing formulas are given, because they depend on location and equipment. Use the controller log, your equipment manuals and a qualified RV technician or electrician. Lithium cold-charging limits are the battery maker's to state, and generator use belongs to the generator manual.