Solar Panel System Size Calculator

How This Calculator Works

A solar system’s size is how much electricity your home uses, divided by how much sunlight your system can capture and convert, adjusted for real-world losses. Enter your average monthly electricity usage and your location’s peak sun hours per day, and this calculator estimates the system size (in kilowatts) needed to offset your usage, along with roughly how many panels that takes.

“Peak sun hours” isn’t the same as daylight hours — it’s the equivalent number of hours of maximum-strength sunlight your location gets, since solar irradiance varies throughout the day and by season. This varies significantly by region (a sunny desert climate gets meaningfully more peak sun hours than a cloudy northern one), so this calculator asks you to enter your own — check your utility or a solar mapping tool for your specific area’s typical figure.

If you don’t already know your monthly electricity usage, the Electricity Cost Calculator can help you work it out, and once you have a system size in mind, the Solar Panel Savings Calculator estimates the payback period once you get a quote.

The Formula

$$\vE{\text{Daily Usage}} = \frac{\vA{\text{Monthly Usage}}}{30}$$ $$\vF{\text{Required Daily Production}} = \vE{\text{Daily Usage}} \times (\vB{\text{Offset Target}} \div 100)$$ $$\vG{\text{System Size (kW)}} = \frac{\vF{\text{Required Daily Production}}}{\vC{\text{Peak Sun Hours}} \times (1 - \vD{\text{System Losses}} \div 100)}$$ $$\text{Panels Needed} = \frac{\vG{\text{System Size (W)}}}{\text{panel wattage}}$$

Rounded up.

System losses account for inverter inefficiency, wiring, dust, and other real-world factors that reduce a system’s output below its theoretical maximum — commonly estimated in the 15-25% range, adjustable above.

Worked Example

900 kWh/month usage, 5 peak sun hours/day, a 100% offset target, 20% system losses, and 400W panels:

1. Daily Usage: $\vA{900} \div 30 = \vE{30} \text{ kWh}$.
2. Required Daily Production: $\vE{30} \times 100\% = \vF{30} \text{ kWh}$.
3. System Size: $\vF{30} \div \vC{5} \div 0.8 = \vG{7.5} \text{ kW}$.
4. Panels Needed: $\vG{7{,}500}\text{W} \div 400\text{W} = 18.75$ → 19 panels (rounded up).
5. Estimated annual production: about 10,950 kWh.