What it does
This calculator estimates how large a solar photovoltaic (PV) system you need to cover your electricity use. Enter your average daily energy consumption, the peak sun hours for your location, a performance ratio that accounts for real-world losses, and the wattage of the panels you plan to install. It returns the required system size in kilowatts (kW) and how many panels that works out to.
How to use it
1. Daily Energy Use — your average daily consumption in kWh (divide a monthly bill by ~30). 2. Peak Sun Hours — the equivalent hours of full-strength sun per day for your area, typically 3–6. 3. Performance Ratio — a factor (commonly 0.75–0.85) for inverter losses, heat, wiring and soiling. 4. Panel Wattage — the rated output of a single panel, e.g. 400 W.
The formula explained
System kW = Daily kWh ÷ (Peak Sun Hours × Performance Ratio). Multiplying peak sun hours by the performance ratio gives the effective generation per kW installed each day, so dividing your demand by it gives the kW you must install. Panel count = ceil(System kW × 1000 ÷ Panel Watt), rounded up so capacity is never short.
$$ \text{Size}_{kW} = \frac{\text{Daily Use (kWh)}}{\text{Sun Hours} \times \text{Perf. Ratio}} $$$$ N = \left\lceil \frac{1000 \cdot \text{Size}_{kW}}{\text{Panel (W)}} \right\rceil $$
Worked example
Suppose you use 30 kWh per day, your location gets 5 peak sun hours, the performance ratio is 0.8, and you use 400 W panels. System size = \(30 \div (5 \times 0.8) = 30 \div 4 = 7.5\) kW. Panels = \(\lceil 7.5 \times 1000 \div 400 \rceil = \lceil 18.75 \rceil = 19\) panels, giving an installed capacity of \(19 \times 400 = 7{,}600\) W = 7.6 kW.
FAQ
Where do I find peak sun hours? National solar maps or PV tools list average daily insolation (kWh/m²/day), which equals peak sun hours.
What performance ratio should I use? 0.80 is a good default for a modern, well-installed system; use lower for hot climates or shading.
Does this size a battery? No — it sizes generation only. Battery storage is sized separately from your nighttime load and desired backup time.
