The Off-Grid Sizing Calculator Master Guide (2026)
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An off-grid sizing calculator turns four inputs into a complete equipment specification: daily consumption, days of autonomy, worst-month sun, and peak load. This master guide walks each one with the real arithmetic, so you can size battery, solar and inverter together rather than guessing at any single piece.
The four calculations
1. Battery = daily Wh × days of autonomy ÷ usable depth. 2. Solar = daily Wh ÷ worst-month peak sun hours ÷ 0.7. 3. Inverter = largest simultaneous load + surge. 4. System voltage from total scale. Do all four; a system is only as good as its weakest number.
Off-grid sizing calculator input 1: daily consumption
List every load, its average watts and hours per day, and total the watt-hours. Use average draw, not label peak, and never forget the always-on loads — a fridge control board, standby electronics, the inverter’s own idle draw — which together often exceed your lighting. The full method is in watt-hours explained.
Calculation 1: Battery bank
Multiply daily watt-hours by your days of autonomy, then divide by usable depth. For LiFePO4 you can use about 0.9; for lead-acid, 0.5. So 3,000 Wh a day × 2 days ÷ 0.9 = about 6,700 Wh of LiFePO4 capacity. Add 20% for cold-weather loss and growth. Size against 100Ah vs 200Ah vs 300Ah.
Calculation 2: Solar array
Divide daily watt-hours by your worst-month peak sun hours, then by 0.7 for real-world losses. At 3,000 Wh and 4 peak sun hours: 3,000 ÷ 4 ÷ 0.7 = about 1,070W of panel. At 2 winter hours that doubles — which is exactly why sizing for the worst month matters. Location data is at the National Renewable Energy Laboratory.
Calculation 3: Inverter
Add up everything that might run at once at its worst moment, and that continuous figure sets your inverter. Then check the surge rating covers motor and compressor startups, which spike two to three times running wattage. Crucially, confirm your battery’s BMS can supply the current the inverter draws — the trap covered in inverter sizing.
Calculation 4: System voltage
The total scale of the system sets the voltage. Under 2 kWh daily suits 12V, 2–5 kWh suits 24V, and 5 kWh-plus should be 48V to keep current and cable cost sane. The full logic is in 12V vs 24V vs 48V.
Off-grid sizing calculator: a worked example
| Input | Value | Result |
|---|---|---|
| Daily use | 3,000 Wh | — |
| Battery (2 days, LiFePO4) | ÷ 0.9 | ~6.7 kWh + margin |
| Solar (4 sun hrs) | ÷ 4 ÷ 0.7 | ~1,070 W |
| Voltage | >5 kWh battery | 48V |
Run your own numbers the same way, then take the specification to the complete off-grid guide to choose components.
Frequently asked questions
Why size solar for the worst month?
Because a system that only works in summer is not off-grid, it is seasonal. Winter peak sun hours can be a third of summer, so sizing for the darkest month you will live through is what keeps the lights on year-round — or tells you a generator is needed.
What is the 0.7 factor in the solar calculation?
It accounts for real-world losses — charge controller inefficiency, heat, wiring, dust and imperfect panel angle. Panels rarely hit rated output outside the lab, so dividing by 0.7 sizes the array to deliver what you actually need rather than the nameplate figure.
Can I use less battery if I have more solar?
Up to a point. Solar reduces how often you draw the battery down, but you still need enough storage to ride out sunless stretches. Battery covers autonomy; solar covers daily replacement. Both matter, and one cannot fully substitute for the other.
Why check the BMS current when sizing?
Because capacity and output are different specs. A battery with ample energy can still be unable to supply the current a large inverter demands. Sizing must confirm the battery’s continuous discharge rating meets the inverter’s draw, not just that the capacity is sufficient.
Last updated: July 24, 2026. Figures are calculated estimates; size against your own measured consumption and local sun data.
