Solar guide
Solar calculator methodology
Transparent formulas for solar production, savings, payback, ROI, degradation and panel count.
See the formulas and assumptions used by the solar panel calculator.
Core formulas
- Annual production = system size kW × peak sun hours × 365 × performance ratio.
- Panel count = system size kW × 1000 ÷ panel wattage.
- Annual benefit = self-used solar savings + export tariff earnings.
- Payback period = installed cost ÷ annual benefit.
- ROI = ((lifetime benefit - installed cost) ÷ installed cost) × 100.
The default performance ratio is 80%. This is a practical allowance for inverter losses, heat, wiring, dirt and other real-world losses.