Solar Panel Calculator

How to Use the Solar Panel Calculator

1. Enter your location and usage. Enter a location label, average monthly electricity use, and the bill offset you want the solar array to cover.
2. Add sunlight and roof details. Enter peak sun hours for your area, roof area, usable roof percentage, panel wattage, and panel footprint.
3. Set rate and lifetime assumptions. Enter your local electricity rate, expected rate increase, panel degradation, and savings window.
4. Review the solar plan. Click Calculate to see annual kWh, required system size, roof-limited capacity, monthly output, and long-term savings.

Solar Panel Calculator Formula

The performance ratio is 1 minus system losses. For example, 14% system losses means a performance ratio of 86%. Losses represent inverter conversion, wiring, temperature, soiling, panel mismatch, shade, and downtime.

What Peak Sun Hours Should I Use?

Peak sun hours are location-specific. The presets above are starting points, but a detailed design should use a local solar map or installer estimate for the exact roof tilt, azimuth, and shade pattern.

How to Interpret Roof-Limited Results

If the calculator says the project is roof-limited, the required system for your target offset is larger than the system that fits your usable roof area. You can improve the result by using higher-wattage panels, increasing the usable roof percentage, reducing the target offset, adding a ground mount, or improving household efficiency before sizing the solar array.

What the Savings Estimate Includes

The savings estimate is the value of electricity the solar array avoids buying from the utility. It models annual panel degradation and annual electricity rate increases over your selected savings window. It does not subtract installation cost, financing cost, maintenance, tax credits, rebates, or fixed utility charges.

FAQ

How many solar panels do I need?

The number of panels depends on your annual electricity use, desired bill offset, local peak sun hours, system losses, and each panel's wattage. The calculator first estimates the kW system required for your target offset, then converts that kW size into a panel count and checks whether it fits your usable roof area.

What are peak sun hours?

Peak sun hours are the average daily hours when sunlight equals 1,000 watts per square meter. A location with 5 peak sun hours can produce roughly the same solar energy as 5 hours of full-strength sun, even though daylight lasts longer.

How accurate is this solar panel calculator?

It is a planning estimate, not an engineering design. It accounts for sunlight, roof area, panel wattage, system losses, degradation, and rate growth, but it does not model exact roof azimuth, tilt, shading by hour, local permitting rules, inverter clipping, or utility net-metering policy.

What system losses should I use?

A default loss of 14 percent is a practical starting point for wiring, inverter, temperature, soiling, mismatch, and availability losses. Use a lower value for a clean, cool, well-designed roof and a higher value if the roof has heat, shade, older equipment, or complex wiring.

Does the calculator include installation cost or incentives?

No. This calculator focuses on energy output, roof capacity, required system size, and avoided electricity purchases. Installation cost, tax credits, rebates, financing, and payback period belong in a solar ROI calculation.