How solar system size is calculated
The required size comes from one formula: kW = daily kWh ÷ (sun-hours × performance ratio). Daily kWh is your monthly units divided by 30. The performance ratio (we use 0.8) accounts for inverter efficiency, temperature, dust and wiring losses — a system never delivers its full nameplate rating.
Why location changes everything
The same 800-unit household needs about 7 kW in Karachi but nearly 12 kW in Berlin, purely because of sun-hours. This calculator uses curated city data where available and a latitude-based model everywhere else, so it works worldwide.
Roof area rule of thumb
Traditional mounting needs roughly 60–70 square feet per kW, including maintenance walkways. Tight roofs can use higher-efficiency panels to fit more power in less space.
How many kW of solar do I need per 100 monthly units?
As a rule of thumb, roughly 0.8–1 kW per 100 monthly kWh in sunny regions, and more in cloudier climates. This calculator uses your local sun-hours for an exact figure.
Why is the recommended size bigger than the calculated size?
A 10% headroom covers future load growth and the slow, normal degradation of panels over their life.
What are peak sun hours?
The equivalent number of hours per day at full-strength (1000 W/m²) sunshine. Karachi averages about 5.3, London about 2.8.