How to Size a Solar System for Your Home
Sizing is the single most important decision in a solar purchase. Undersize and the system never covers your bill; oversize and your payback stretches for years. The good news: correct sizing is four steps of simple arithmetic.
Step 1 — Find your real daily usage
Take your last 12 electricity bills and average the monthly units (kWh). Twelve months matters because summer and winter usage differ sharply — an average smooths seasonal swings. Divide by 30 for daily kWh. Example: 800 units/month → 26.7 kWh/day.
Step 2 — Find your peak sun hours
Peak sun hours (PSH) are the equivalent hours per day of full-strength sunshine your location receives, averaged over the year. Karachi gets about 5.3, Dubai 5.8, London just 2.8. This is why identical households need very different systems in different countries.
Step 3 — Apply the formula
System kW = daily kWh ÷ (PSH × performance ratio). The performance ratio (~0.8) covers what brochures leave out: inverter losses, heat, dust and wiring. Our example: 26.7 ÷ (5.3 × 0.8) = 6.3 kW.
Step 4 — Add headroom and round
Add ~10% for future appliances and the slow annual degradation of panels, then round up to the nearest half kilowatt: 6.3 × 1.1 ≈ 7 kW. With 585 W panels that is 12 panels, needing roughly 420–490 ft² of shade-free roof.
Common sizing mistakes
Sizing from one summer bill (oversizes the system), ignoring the performance ratio (undersizes it), and forgetting roof reality — panels need walkways and airflow, not just raw area. And if your grid pays little for exported units, prefer matching daytime usage over maximal size.
Skip the arithmetic — the system size calculator does all four steps with your local sun-hours, or get the complete system design in two minutes.