How to size a rooftop solar system
Work out how many panels you need from your electricity bill and your roof.
Start with your electricity bill, not the roof. The bill tells you how many units you use, and that decides the size of the system.
Step 1: find your daily use
Take the units on a recent bill and divide by the days. A home that uses 300 units a month uses about 10 units a day.
Step 2: estimate the system size
As a rough guide, each 1 kW of panels makes about 4 units a day in good sun, though this varies by city and season. So 10 units a day needs a system of around 2.5 to 3 kW.
Step 3: count the panels
Divide the system size by the panel wattage. A 3 kW system is about six 540 W panels, or seven 440 W panels.
Step 4: check the roof
Plan on about 100 square feet of shade-free roof for each 1 kW. Panels should face south where possible, with no shadow from tanks, walls or trees during the middle of the day.
Final sizing, wiring and net-metering rules depend on your site and your electricity board, so have a qualified installer check the plan.