This calculator is an educational scenario model. No weather API, municipal tariff feed or equipment catalogue is queried. The default R3.50/kWh rate, 4.5 sun hours and 20% losses are editable hypothetical inputs, not validated figures for a customer location.
Energy balance
- Monthly consumption = (bill − fixed charges) ÷ variable tariff. An average month has 365 ÷ 12 days.
- Daily consumption is split into daytime and night-time energy using the slider.
- Battery energy available for daily cycling = selected nominal capacity × 80%. Delivered stored energy is charging energy × 90%.
- PV target = daytime demand + the smaller of available battery capacity or night-time demand ÷ 90%.
- Panel count = target ÷ (0.55 kW × sun hours × (1 − loss fraction)), rounded up to a whole panel.
- Solar first meets daytime demand. Surplus charges the battery, limited by battery capacity and night-time demand. Remaining surplus has no financial value in this model.
- Monthly savings = displaced grid energy × tariff, limited to the variable part of the bill. Annual savings = monthly result × 12.
What it cannot establish
Average-day totals do not represent the timing of appliance demand, weather variability, shading, seasonal production, inverter clipping, charge-rate limits or battery reserve needed during outages. The model cannot size an inverter or guarantee backup time. Export compensation, finance, maintenance and equipment degradation are excluded.
Before choosing a system
Obtain location-specific production estimates, interval consumption data where possible, a roof and electrical assessment, and equipment compatibility checks. Review the scenario with the installer. A model such as NREL PVWatts can support a location-specific production assessment; this website does not use its outputs.
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