Off-Grid Solar & Battery Sizing Calculator
Size an off-grid solar array and battery bank from your daily energy use, local sun hours and days of autonomy — the two numbers that make or break a standalone system.
How it works
Array kW = daily kWh ÷ sun hours ÷ 0.75 (system losses) · Battery kWh = daily kWh × days autonomy ÷ (depth of discharge %)
An off-grid system has two independent jobs: the solar array must generate a full day's energy during the limited peak-sun hours, and the battery must store enough to run the loads through the night and several cloudy days. Array size is your daily kWh divided by local sun hours, then divided again by about 0.75 to cover charge-controller, inverter and wiring losses. Battery capacity is daily use times the days of autonomy you want, divided by the usable depth of discharge — because draining a battery fully shortens its life, lithium is sized to ~80% and lead-acid to ~50%.
Worked example
A cabin using 10 kWh/day with 5 peak sun hours needs 10 ÷ 5 ÷ 0.75 ≈ 2.67 kW of panels. For 2 days of autonomy on a lithium bank at 80% depth of discharge, the battery is 10 × 2 ÷ 0.80 = 25 kWh — enough to ride out two sunless days without over-draining the cells.
Frequently asked questions
What are "peak sun hours"?
It is the number of hours per day the sun delivers 1,000 W/m² equivalent — a way of collapsing a whole day of varying sunlight into a single figure. Most locations average 4–6, higher in deserts and lower in cloudy northern climates; use a local solar map for your real number, as it drives the entire array size.
Why not size the battery to 100% depth of discharge?
Fully discharging a battery every cycle dramatically shortens its life. Lithium (LiFePO4) tolerates deep cycling and is commonly used to about 80–90%, while lead-acid should stay above 50% to last. Sizing to usable depth means the bank holds enough real energy without being abused.
Do I need days of autonomy if I have solar?
Yes — solar produces nothing at night and little on heavily overcast days, so the battery must carry the load through them. Two to three days of autonomy is typical; more if your climate has long cloudy stretches or the system is critical and you have no generator backup.
Related calculators
Educational estimate only — have an installer verify sizing, code and safety for a real system.