Solar Panel Calculator

Solar Panel Calculator

Estimate the solar array size, number of panels, and roof area needed to offset your electricity bill. Enter your annual consumption, local solar hours, panel specifications, and usable roof area to plan a residential or small commercial solar installation.

Solar array output

First, calculate the continuous power output needed to match annual consumption:

$$P_{\text{output}} = \frac{\text{consumption (kWh/yr)}}{365 \times \text{solar hours per day}}$$

Solar hours per day is the average peak-sun equivalent for your location. Higher values (closer to the equator, sunnier climates) reduce the required array size.

Solar array size

Adjust for how much of your bill you want to offset and real-world environmental losses:

$$P_{\text{array}} = P_{\text{output}} \times \frac{\text{bill offset \%}}{100} \div \frac{\text{environmental factor \%}}{100}$$

The environmental factor accounts for losses from humidity, pollution, snow, shading, and inverter efficiency. A factor of 70% means only 70% of theoretical output reaches your meter.

Number of panels and roof area

Convert the required kilowatts to a panel count:

$$\text{panels} = \frac{P_{\text{array (kW)}} \times 1000}{\text{panel wattage (W)}}$$

The total roof area occupied is:

$$\text{area occupied} = \text{panels} \times \text{width (m)} \times \text{length (m)}$$

Compare this to your usable roof area (excluding shaded or obstructed sections). If the occupied area exceeds available space, consider higher-efficiency panels or a lower bill offset.

Payback estimate

When you provide an electricity rate ($/kWh), the calculator estimates annual savings and payback period. Annual savings equal the portion of consumption offset multiplied by your rate:

$$\text{annual savings} = \text{consumption} \times \frac{\text{bill offset}}{100} \times \text{rate}$$

Payback years are estimated by dividing an approximate installation cost ($2,500/kW) by annual savings. Actual costs vary by region, incentives, and installer.

Example

A home using 7,500 kWh/yr with 4.5 solar hours/day, 100% bill offset, and 70% environmental factor:

$$P_{\text{output}} = \frac{7500}{365 \times 4.5} \approx 4.56 \text{ kW}$$

$$P_{\text{array}} = 4.56 \times 1.0 \div 0.7 \approx 6.52 \text{ kW}$$

$$\text{panels} = \frac{6.52 \times 1000}{300} \approx 22 \text{ panels}$$

For related renewable energy tools, see the Hydroelectric Power Calculator or the Rainwater Collection Calculator.