Off-grid solar calculator: direct answer
An off-grid solar system is sized from energy and power separately. First calculate daily energy in kWh. Then estimate battery energy from daily use, autonomy, usable depth of discharge and delivery efficiency. Estimate PV power from daily energy, conservative peak-sun-hours and system efficiency. Finally, check the inverter against maximum simultaneous load and motor or compressor surge.
Open the TORCHN off-grid solar calculator to enter the load list, runtime and backup target.
Inputs required before calculating
- Load name, quantity, rated watts, surge watts and hours per day.
- Critical loads that must remain on during low-solar periods.
- Required hours or days of autonomy and whether grid or generator backup exists.
- Project location, conservative seasonal solar resource, temperature and shading.
- Battery model, nominal kWh, permitted depth of discharge, discharge current and BMS parallel limit.
Core sizing formulas
| Calculation | Preliminary formula | Required check |
|---|---|---|
| Daily energy | Σ(load W × quantity × hours/day) ÷ 1000 | Standby use, cycling loads and simultaneous operation |
| Nominal battery energy | Daily kWh × autonomy ÷ usable DoD ÷ delivery efficiency | Battery datasheet, temperature, aging and discharge current |
| Battery quantity | Required nominal kWh ÷ kWh per battery, rounded up | Series/parallel rules, BMS communication and DC protection |
| PV array power | Daily kWh ÷ conservative peak-sun-hours ÷ system efficiency | Seasonal weather, orientation, shading, Voc and MPPT range |
| Inverter power | Maximum simultaneous running watts plus design margin | Surge duration, output phase, battery current and generator input |
Illustrative calculation, not a fixed package
Assume a load table totals 12 kWh/day, the design target is one day of autonomy, usable depth of discharge is 80%, and end-to-end delivery efficiency is 90%. The preliminary nominal battery energy is 12 ÷ 0.80 ÷ 0.90 = 16.7 kWh. With 5.12 kWh modules, 16.7 ÷ 5.12 = 3.26, so the arithmetic result rounds up to four modules. The final design still depends on the selected battery's BMS, current limit, parallel rules, temperature range and the inverter's DC demand.
If conservative peak-sun-hours are 4 and the assumed PV-system efficiency is 75%, the same 12 kWh/day gives an initial PV estimate of 12 ÷ 4 ÷ 0.75 = 4 kW. This is only an energy-balance starting point; winter recovery, consecutive cloudy days, array voltage and charge-current limits may require a different array.
What the calculator cannot confirm
A web calculator cannot confirm final cable size, fault current, protection coordination, grounding, certification, local permits, battery-to-inverter communication, actual backup duration, price or delivery. Use model-level datasheets and a qualified local designer before installation.
Official references and next reading
- NREL PVWatts estimates grid-connected PV production and provides a solar-resource reference; NREL states that results contain assumptions and uncertainty.
- Santa Cruz County off-grid requirements show a government load-table and battery-autonomy sizing example.
- Read the house battery quantity guide.