Size the solar array
The array must replace the energy you use each day in the worst month, after all losses. Use the Solar array size calculator on this page.
How
- Take your design month daily kWh (step power-3) and its PSH (step power-4).
- Apply losses. PVWatts uses 14% default system losses (soiling, shading, mismatch, wiring, connections, light induced degradation, nameplate and availability) and 96% inverter efficiency. Add battery round trip losses: about 92% for LiFePO4 and about 80% for average lead acid in one maker's data.
- Array kW = daily kWh divided by (PSH times combined efficiency). Then divide by the panel wattage to get panel count.
- Add margin. Under the Australian and New Zealand standard approach (AS/NZS 4509.2), cutting autonomy from 3 days to 2 needs roughly 10 to 20% more array, lead acid banks without a generator may need about 30% extra for equalisation, and panels lose about 0.5 to 1% per year.
- Cross check the result in the PVGIS off-grid tool to see how often the battery would run empty.
Sources
- PVWatts Version 5 Manual (NREL/TP-6A20-62641, archived copy). National Renewable Energy Laboratory, accessed 2026-11
- PVGIS 5 tools: off-grid PV systems. European Commission Joint Research Centre, accessed 2026-11
- Oversized PV array and battery days of autonomy in stand-alone power systems. GSES (Global Sustainable Energy Solutions), accessed 2026-11
- Datasheet: 12.8 Volt lithium iron phosphate batteries Smart. Victron Energy, accessed 2026-11