How it works
A panel rated 400 W only makes that power in full sun. Multiply its rating by the number of peak sun hours (the equivalent hours at full sun per day) and by the share that survives system losses to get the energy per day. Divide your daily load by that to see how big the array must be.
Batteries carry you through nights and cloudy stretches. Multiply your daily energy by the days you want to ride through, then divide by the fraction of the battery you can safely use. That nameplate capacity, divided by bank voltage, gives amp-hours.
Worked example
A cabin using 5 kWh per day, in a winter with 3.5 peak sun hours, 25% losses, 400 W panels, 2 days of autonomy, an 80% usable lithium bank at 48 V:
- Array = 5 kWh ÷ (3.5 sun-hours × (1 − 25%)) = 1.9 kW
- Panels = ⌈1,905 W ÷ 400 W⌉ = 5
- Battery = 5 kWh × 2 days ÷ 80% = 12.5 kWh
- Amp-hours at 48 V = 12,500 Wh ÷ 48 = 260 Ah
| Input | Value |
|---|---|
| Daily energy use | 5 kWh/day |
| Peak sun hours per day (worst month you must cover) | 3.5 h |
| System losses (wiring, inverter, temperature, dirt) | 25 % |
| Panel rating | 400 W |
| Days of autonomy (cloudy days with no sun) | 2 days |
| Usable depth of discharge | 80 % |
| Battery bank voltage | 48 V |
| Result | Value |
|---|---|
| Array size needed | 1.9 kW |
| Number of panels (rounded up) | 5 panels |
| Battery bank nameplate capacity | 12.5 kWh |
| Battery capacity at bank voltage | 260 Ah |
| Array current at bank voltage (before code safety factors) | 40 A |
Assumptions and limits
- Peak sun hours are an input, not a built-in number. Use the worst month you must cover; NREL solar resource maps and PVWatts give local values.
- The single loss percentage covers wiring, inverter, controller, temperature, and soiling. Typical values are 20–30%.
- The battery figure is nameplate capacity. It does not include generator backup, cold-weather derating, or battery aging.
- Does not size the inverter, controller, wiring, or overcurrent protection.
Common questions
How many solar panels do I need to run a cabin?
Work out your daily kWh, then divide by the peak sun hours and the loss factor. A 5 kWh/day cabin at 3.5 winter sun hours and 25% losses needs about 1.9 kW of panels, or five 400 W panels.
How many peak sun hours do I have?
Peak sun hours vary by location and season. Northern states can drop to 2 or 3 in winter and reach 5 to 6 in summer. Look up your site in NREL's solar resource data.
How many days of autonomy should I plan for?
Two to three days is common for off-grid homes. Backup systems for short outages may use less, and systems with a generator can use less as well.
Lead-acid or lithium?
Lead-acid should only be discharged to about 50%, so you need twice the nameplate capacity for the same usable energy. Lithium iron phosphate can typically use 80–90%.
Sources
- Standard PV system sizing method (energy-balance): array = load ÷ (peak sun hours × derate); battery = load × autonomy ÷ depth of discharge.
- Peak sun hours: NREL solar resource data and PVWatts (user input in this tool).
Updated 2026-09-30