M
Mei Lin Tan
1 Oct 2026 · 5 min read
Start from your bill, not your roof. A simple way to size a system that pays back fast without being clipped by the grid.
Most Perth homes are sold systems that fit the roof rather than the household. The better starting point is the bill.
Turn the bill into kWh Divide your average monthly bill by about 31c and multiply by twelve. A $250 monthly bill is roughly 9,700 kWh a year.
Aim for about 85% Each kilowatt of panels makes about 1,700 kWh a year in Perth. Covering 85% of 9,700 kWh needs about 4.8 kW, or 11 of our 440 W panels.
Then check the limits - Single-phase homes can connect up to a 5 kW inverter, which carries at most 15 panels (6.6 kW). - Three-phase homes can go to 30 panels on a 10 kW inverter. - Every panel needs about 2.2 m² and we only use 70% of the roof.
The calculator does all of this for you when you press Size it to my bill.
