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Divide your daily energy use in watt-hours by your location's peak sun hours, add about 25% for losses, and divide by the panel wattage. A house using 30 kWh a day in a 4.5-sun-hour location needs roughly 21 panels of 400W; a small off-grid cabin using 3 kWh a day needs about 4 panels of 200W; a weekend RV using 1 kWh needs two 200W panels. Off-grid systems then size the battery for two to three days of autonomy.
The Four Numbers That Decide It
Panel count is not a guess. It comes from four figures, and if you know them the arithmetic takes a minute.
1. Daily energy use (Wh or kWh per day). For a grid-tied house, take a year of bills, add up the kWh and divide by 365. For an off-grid build, list every load, multiply its watts by the hours it runs, and add them up. This is the number that matters most, and the one people most often get wrong by forgetting the fridge runs 24 hours and the well pump draws 1,000 watts.
2. Peak sun hours. Not daylight hours — the equivalent number of hours per day of full-strength sun. Most of the continental US sits between 3.5 and 6; Arizona and New Mexico at the top, the Pacific Northwest and the Great Lakes at the bottom. For an off-grid system, use the worst month you plan to operate in, not the annual average.
3. System losses. Panels are rated under lab conditions. Heat, dust, wiring, the charge controller and the inverter each take a few percent. A conservative planning figure is 20–25% total, so multiply your daily energy by 1.25 before dividing.
4. Panel wattage. Residential panels are 400–450W; the 12V-class panels used on RVs and small cabins are 100–200W. The panel count changes with the panel you pick, so decide that first.
The formula: Panels needed = (daily Wh × 1.25) ÷ peak sun hours ÷ panel watts. Round up.
Worked Example 1: A Grid-Tied House
A typical American home uses about 10,500 kWh a year — roughly 29 kWh per day. Say it is in North Carolina, where a south-facing roof averages about 4.5 peak sun hours across the year.
29,000 Wh × 1.25 = 36,250 Wh of panel output needed per day.
36,250 ÷ 4.5 sun hours = 8,055 W of panels.
8,055 ÷ 400 W = 20.1 — call it 21 panels of 400W, an 8.4 kW array.
A grid-tied system can use the annual average sun figure because the grid covers winter shortfalls and absorbs summer surplus. That is the single biggest reason a grid-tied array is smaller than an off-grid one for the same house.
Worked Example 2: An Off-Grid Cabin
Off-grid, you size for the worst month you intend to live there, and you size the battery as well. Take a cabin used year-round with these loads:
| Load | Watts | Hours/day | Wh/day |
|---|---|---|---|
| 12V compressor fridge | 45 (avg) | 24 | 1,080 |
| LED lighting | 40 | 5 | 200 |
| Laptop + phone charging | 60 | 4 | 240 |
| Water pump (12V) | 90 | 0.5 | 45 |
| Wi-Fi / small electronics | 15 | 24 | 360 |
| Occasional tools, blender, etc. | — | — | 600 |
| Total | ~2,500 Wh |
Round up to 3,000 Wh a day for headroom. In a northern location the December figure might be only 2.5 peak sun hours.
3,000 × 1.25 = 3,750 Wh ÷ 2.5 = 1,500 W of panels ÷ 200 W = 7.5 → 8 panels of 200W for year-round use. If the cabin is only used May–October, at 5 sun hours it drops to 4 panels. That is the winter penalty, and it is why off-grid arrays look oversized in summer.
Battery. Off-grid, plan for two to three days without sun. 3,000 Wh × 2 days = 6,000 Wh of usable storage. Lithium (LiFePO4) can be drawn to 80–90%, so a 12V system needs about 600 Ah of LiFePO4 — for instance six 100Ah batteries, or better, a 24V or 48V bank of the same energy. Lead-acid should only be drawn to 50%, so it would need double the nameplate capacity.
Worked Example 3: An RV or Van
A weekend RV with a 12V fridge, lights, a fan and device charging uses about 1,000 Wh a day. Assume 4 sun hours because the roof is flat and sometimes shaded.
1,000 × 1.25 = 1,250 ÷ 4 = 312 W of panels → two 200W panels (400W), which also gives margin for a cloudy day. A 200Ah LiFePO4 battery covers two days of autonomy. Full-timers running a rooftop air conditioner or an induction cooktop are in a different league — 800–1,200W of panels and 400Ah+ of lithium is common, and many pair it with a DC-to-DC charger so the alternator tops up the bank while driving.
Quick Sizing Table
Using the formula with 25% losses and rounded up. Pick the sun-hour column that matches your worst operating month for off-grid, or the annual average for grid-tied.
| Daily use | 3 sun hrs | 4 sun hrs | 5 sun hrs | Typical setup |
|---|---|---|---|---|
| 1 kWh | 3 × 200W | 2 × 200W | 2 × 200W | Weekend RV, van |
| 3 kWh | 7 × 200W | 5 × 200W | 4 × 200W | Small off-grid cabin |
| 6 kWh | 7 × 400W | 5 × 400W | 4 × 400W | Efficient off-grid home |
| 15 kWh | 16 × 400W | 12 × 400W | 10 × 400W | Small family home |
| 30 kWh | 32 × 400W | 24 × 400W | 19 × 400W | Average US home |
The Mistakes That Undersize a System
Using summer sun hours for an off-grid system. December sun in Michigan is under half of July. An off-grid array sized on the annual average will run the generator all winter.
Forgetting continuous loads. A fridge, a router, a propane furnace fan and a water pump on a pressure switch all run when you are not thinking about them. Meter them for a day with a plug-in watt meter rather than guessing.
Ignoring the inverter's idle draw. A 3,000W inverter can burn 20–40 W doing nothing — that is up to 1 kWh a day on its own. Size the inverter to the loads, not to the biggest number you can afford.
Panel tilt and shading. Flat-mounted panels lose 10–20% in winter versus tilted ones, and a single shaded panel in a series string drags the whole string down. Tilt for winter if the array is ground-mounted, and use an MPPT controller — see our MPPT vs PWM comparison.
No margin. Every figure above is a planning estimate. Add 20% to the panel count if you can; nobody has ever regretted an oversized array, and panels are the cheapest part of the system.
Sizing the Battery Bank to Match
Panels decide how much energy you harvest; the battery decides how many cloudy days you survive. The two are sized together. Multiply daily use by the days of autonomy you want (two is the usual minimum, three is comfortable), divide by the usable fraction of the chemistry — about 0.85 for LiFePO4, 0.5 for lead-acid — and you have the watt-hours of nameplate storage to buy. A 3 kWh-a-day cabin with two days of autonomy needs about 7 kWh of LiFePO4, which at 48V is about 150 Ah.
Then check the charge rate: the array should be able to recharge the bank from empty in one to two good days, which for LiFePO4 means the panels can deliver at least 0.2C — a 7 kWh bank wants at least 1,400 W of panels behind it. If the numbers do not meet, the array is undersized for the bank, not the other way round.
Gear We Recommend
Once the numbers are settled, these guides cover the hardware for each part of the system.
- Best Solar Panels for Off-Grid (2026) — the panels themselves, including 200W and 400W-class options
- Best Solar Panel Kits for Off-Grid (2026) — complete kits for cabins and RVs that bundle panels, controller and wiring
- Best LiFePO4 Batteries for Off-Grid (2026) — LiFePO4 batteries for the bank
- Best Solar Charge Controllers (MPPT) for Off-Grid (2026) — MPPT controllers sized to the array
- Best Pure Sine Wave Inverters for Off-Grid (2026) — inverters, and why idle draw matters
- Best Off-Grid Solar Systems (2026) — whole-home systems if you would rather buy the package
Frequently Asked Questions
How many solar panels do I need to power a house?
An average US home using about 29 kWh a day in a location with 4.5 peak sun hours needs roughly 20–21 panels of 400W — an 8–8.5 kW array — when grid-tied. An off-grid house has to size for its worst month and add batteries, so it typically needs 30–50% more panels for the same consumption.
How many solar panels do I need for an off-grid cabin?
A modest cabin using 3 kWh a day needs about 4 panels of 200W in a sunny season and 8 for year-round use in a northern winter, plus a battery bank sized for two to three days of autonomy — around 6–7 kWh of LiFePO4. The exact count depends on your loads and your December sun hours.
How many solar panels do I need for an RV?
A weekend RV using about 1 kWh a day is well served by two 200W panels and a 200Ah lithium battery. Full-time living with air conditioning or induction cooking pushes that to 800–1,200W of panels and 400Ah or more of battery.
What are peak sun hours?
The number of hours per day at which the sun's energy equals full-strength sunlight of 1,000 watts per square metre. A location might have 14 hours of daylight but only 5 peak sun hours. It is the figure solar sizing uses, and it varies by season — use the worst month for off-grid systems.
How many solar panels do I need to charge a 100Ah battery?
A 12V 100Ah battery holds about 1,200 Wh. To recharge it from empty in one day at 4 sun hours with losses, you need roughly 375 W of panels — two 200W panels. One 200W panel will do it over two sunny days.
Is it better to have more panels or more batteries?
Panels first. Panels are cheaper per watt-hour harvested than batteries are per watt-hour stored, and an undersized array leaves the batteries chronically undercharged, which shortens their life. Size the array to fully recharge the bank in one to two good days, then add battery capacity for autonomy.
Related Guides
How We Research
Picks are based on manufacturer specifications, current Amazon availability, and patterns across verified owner reviews. Specs, capacities, and included accessories vary by listing — confirm the current details at the retailer before buying.
Note: Sun-hour figures vary by location, tilt and season. For a final design, look up the peak sun hours for your ZIP code and worst operating month, and meter your actual loads for at least a day.