How to Size an Off-Grid Solar System

1. Calculate your daily energy consumption

List every device you expect to use, its power in watts and the number of hours it runs per day. Multiply watts by hours to get watt-hours (Wh), then add the results.

Real appliances are not always as simple as their nameplate rating: refrigerators cycle, pumps have starting surges, and AC loads incur inverter losses. Use measured consumption where possible and add a sensible margin rather than assuming ideal conditions.

2. Size the LiFePO₄ battery bank

Battery capacity is driven by daily energy use and the number of low-sun or no-charge days you want to cover. For planning, work in watt-hours or kilowatt-hours first; convert to amp-hours only after choosing the system voltage.

If the example system uses 2,190 Wh/day and you want two days of autonomy, the loads require 4,380 Wh before allowing for conversion losses and reserve. If you plan around 90% usable capacity, that alone points to roughly 4.87 kWh of nominal storage. Manufacturer limits, temperature, BMS limits and inverter losses can justify more margin.

3. Estimate the solar array

Solar panels are rated under test conditions, not by the energy they will produce every day. A useful first estimate divides daily energy by local peak sun hours, then increases array size to account for real-world losses, seasonal variation and recharge requirements.

For 2,190 Wh/day, 4 peak sun hours and an illustrative 80% overall solar-to-load planning efficiency, the starting point is about 684 W. In a real off-grid design you may choose substantially more solar so the system can recover the battery after cloudy weather rather than merely cover one average day.

4. Choose the inverter from simultaneous loads

The inverter is not sized from daily kWh. It is sized primarily from the AC power that may be demanded at the same time. Add the wattage of likely simultaneous AC loads, then account for starting or surge loads such as pumps, compressors and some power tools.

Also verify that the inverter DC input voltage matches the battery bank, and that the battery/BMS can safely supply the inverter's required current.

5. 12V vs 24V vs 48V: why voltage matters

For the same power, increasing voltage reduces current. That matters because high DC current demands larger conductors and puts more stress on connections and protection devices.

Those are idealized currents before losses. There is no universal voltage that is best for every installation: 12V remains practical for many small mobile systems, while 24V and 48V become attractive as power rises. The battery, inverter, charge controller, DC loads and protection scheme all need to be compatible.

6. Worked example: a small off-grid cabin

Suppose a cabin consumes 4 kWh per day, should survive 2 days with little or no solar, and uses LiFePO₄ storage. For a planning example using 90% usable battery capacity:

1. Energy: 4 kWh/day.
2. Storage before extra loss margin: 4 × 2 ÷ 0.90 ≈ 8.9 kWh nominal battery.
3. Solar: with 4 peak sun hours and 80% planning efficiency, 4,000 ÷ 4 ÷ 0.8 ≈ 1.25 kW minimum starting estimate. More array capacity can improve recovery after poor weather.
4. Inverter: determine the highest realistic simultaneous AC load. If it can reach 2.3 kW, choose a compatible inverter with continuous output above that figure and adequate surge capability.
5. Voltage: at this scale, compare 24V and 48V architectures rather than choosing solely from battery Ah.

7. Common sizing mistakes

Final check before you buy

A calculator or article can produce a useful first estimate, but a real installation also depends on location, panel orientation and shading, temperature, cable length, over-current protection, charge-controller limits, battery BMS limits, local electrical requirements and the exact equipment combination.