Decide how many kilowatt-hours you need to store first. The array size falls out of that number, not the other way round.
Start with battery sizing →Sizing solar panels for off-grid backup is one calculation done honestly, and most people get it wrong in the same place: they treat the sticker watts as if the panel produces that all day. A 400-watt panel does not make 400 watts for eight hours. On a December morning in Vermont it might average a fifth of that. The gap between those two numbers is the whole article.
The only equation you need
Daily energy in watt-hours equals array watts multiplied by peak sun hours multiplied by a derate factor. That is it. Peak sun hours is a normalized figure: it converts a whole day of varying sunlight into the equivalent number of hours at full-strength noon sun. The derate factor absorbs everything that steals output between the glass and the battery.
Use 0.75 for a well-installed system with a good MPPT controller and short cable runs. Use 0.65 for portable panels leaned against a fence at whatever angle the ground allows, which is how most backup arrays actually get deployed.
Peak sun hours are not daylight hours
A December day in Maine has nine hours of daylight and maybe two peak sun hours. The sun is low, the air path is long, and half the days are overcast. Approximate figures, and the last column is what one 400-watt panel returns on an average winter day at a 0.75 derate.
| Region | Summer PSH | Winter PSH | 400 W, winter day |
|---|---|---|---|
| Southwest | ~6.5 | ~5.0 | ~1,500 Wh |
| Southeast | ~5.5 | ~3.3 | ~990 Wh |
| Midwest | ~5.5 | ~2.8 | ~840 Wh |
| Northeast | ~5.0 | ~2.3 | ~690 Wh |
| Pacific NW | ~5.0 | ~1.8 | ~540 Wh |
Look at the Northeast row. That single panel returns under 700 watt-hours on an average winter day, and on a genuinely overcast one it may return 150. Your refrigerator alone wants 1 to 2 kWh a day. This is the number that decides whether solar is your backup or your supplement.
Where the missing 25 percent goes
Panels are rated at 25 degrees C cell temperature. A panel in direct summer sun runs 20 to 30 degrees hotter than the air, and silicon loses roughly 0.3 to 0.4 percent of its output per degree above that. Then the charge controller takes a few percent, the wiring takes a percent or two, dust and pollen take a few more, and the battery itself loses energy to heat on the way in and out. None of these are faults. They are the physics, and they add up to about a quarter of the sticker rating on a good day.
A worked example: refilling a 2 kWh power station
Say you have a 2 kWh power station running a refrigerator, LED lights, phones and a modem, and you are drawing it down about 1.5 kWh a day. You want the panels to put that back before sunset so you start each morning full.
- Northeast winter, 2.3 PSH, 0.75 derate: 1,500 ÷ (2.3 × 0.75) = about 870 watts of panel. Call it 800 to 1,000 W, or three 400-watt panels to have margin on a cloudy day.
- Same house in July at 5.0 PSH: about 400 watts does the job. One panel.
- Southwest winter at 5.0 PSH: also about 400 watts.
That factor-of-two spread between summer and winter, and between regions, is why generic advice like “get 800 watts of solar” is useless. Two identical houses need different arrays.
Design for the worst week, not the average
The averages above hide three-day stretches of dense overcast where an array returns 10 to 20 percent of a clear-day figure. If your plan depends on daily solar refill and you get four grey days in a row, you are out of energy on day two.
There are two honest answers. Oversize the array so that even a poor day covers the essentials, which gets expensive fast in winter. Or accept solar as the thing that stretches your battery and keep an engine for the days it cannot, which is the pattern most people on a well and a septic tank end up with. If you go that route, how long a generator runs on a tank tells you how much fuel that costs.
The input limit nobody reads
Every battery and power station has a maximum solar input, given as a wattage and as a voltage window. A unit that accepts 800 W of solar input will not take more, and panels wired in series can exceed its voltage ceiling on a cold, bright morning when open-circuit voltage climbs above the rated figure. That is not a slow degradation, it is a way to damage the input stage.
Check the maximum open-circuit voltage of your string against the unit’s limit at the coldest temperature you get, not at room temperature. And be aware that grid-tied rooftop solar is a completely different animal that shuts down in an outage unless it has been specifically built not to. That surprise is covered in why solar panels die in a power outage.
Portable, ground rack, or roof
Folding portable panels are convenient and forgiving, but they sit at whatever angle you can prop them at and they are the reason to use a 0.65 derate. A fixed ground rack at a steep winter angle, roughly your latitude plus 15 degrees, will beat portable panels of the same wattage by a noticeable margin in December, and you can sweep snow off it standing up. Roof mounting is the tidiest and the least practical to clear or re-angle.
What we’d actually do
Measure your real daily draw for a week, take the worst day, divide by winter peak sun hours for your area, divide again by 0.75, and buy that many watts of panel. Then be honest that in the northern half of the country a winter-proof solar array for a whole house is a large and expensive thing, and a modest array plus a battery plus an engine is the cheaper path to the same result. The two weak points to say out loud: snow on a flat-mounted array produces exactly zero until someone clears it, and lithium batteries will refuse to accept a charge below freezing unless they have internal heating.
Frequently Asked Questions
How many solar panels to charge a 5 kWh battery in a day?
In summer at 5 peak sun hours, roughly 1,300 watts of panel. In a northern winter at 2.3 hours, closer to 2,900 watts. Same battery, very different array.
Do solar panels work on cloudy days?
Yes, at roughly 10 to 25 percent of clear-sky output depending on how heavy the cloud is. Enough to keep phones and lights going, not enough to refill a battery you drained overnight.
Should panels be wired in series or parallel?
Series raises voltage and suits long cable runs and MPPT controllers, but partial shade on one panel drags the whole string down. Parallel tolerates shade better and keeps voltage low. Most backup setups do a mix.
What angle should winter panels sit at?
Roughly your latitude plus 15 degrees, which is steeper than it looks. The steep angle also sheds snow rather than collecting it.