Solar panel calculator: size, savings and payback

This solar panel calculator starts from your yearly kWh and works out the system size that would cover it, the number of panels and the rough roof area. It adjusts production for roof direction and tilt, then compares your bill before and after solar to give first-year savings, a solar payback year and a 25-year table with production, self-use, exports, savings and cumulative balance. You can add panel degradation, yearly rate increases and an inverter replacement.

The sizing formula is simple:

system size (kW) = yearly use (kWh) / (production per kW × orientation factor)

With 10,000 kWh a year and 1,300 kWh per kW, you need about 7.7 kW. With 400 W panels, a 7 kW system is 18 panels (7.2 kW). Each panel needs about 20 sq ft of roof.

Production per kW depends heavily on where you live, so replace the example value with a figure for your location. Export to PDF or Excel is free, no sign-up, and everything runs in your browser. This is an informational estimate, not financial advice.

How your utility credits the power you send to the grid matters as much as how much you produce.

• Full net metering: every exported kWh offsets a kWh you buy at the full retail rate. Solar is worth the same whether you use it at noon or not.
• Net billing or an export rate: exported kWh earn a lower credit than retail. Only the power you use as it is produced saves you the full rate, so the share of self-use drives the payback.

The calculator has a billing switch: net metering credits exports at the retail rate, net billing credits them at the export rate you enter. Your utility's tariff sheet or interconnection page tells you which rule applies.

Federal tax credit: according to the IRS, the Residential Clean Energy Credit (Section 25D) is not available for any property placed in service after December 31, 2025. For a system installed in 2026, do not count that federal credit; enter any state or utility incentive you qualify for in the incentive field.

Inputs: 10,000 kWh a year, 7 kW facing south at a 30° tilt (factor 1.00), 1,300 kWh per kW, 30% used on site, $0.17 per kWh, installed cost $19,600, no incentive, no degradation or rate increase.

1. Production: 7 × 1,300 = 9,100 kWh. On site: 2,730 kWh. Exported: 6,370 kWh.
2. Bill before solar: 10,000 × $0.17 = $1,700.
3. With full net metering, the 6,370 exported kWh offset purchases. You still buy 10,000 − 2,730 − 6,370 = 900 kWh, or $153.
4. First-year savings: $1,547. Simple payback: $19,600 / $1,547 = about 12.7 years.

Same system with an export rate instead of net metering:

*The calculator starts with 0.5% panel degradation a year, 2% yearly rate increases and a $2,000 inverter replacement in year 13. Rising rates shorten the payback more than degradation and the inverter lengthen it, so the default result is a little shorter than the simple figure. Set the rate increase to 0 for a conservative view.

Under an export rate you would look at a smaller system, a battery or shifting loads (laundry, water heating, EV charging) into daylight hours.

Orientation factors used by the calculator (1.00 = south at 30°). The table is an approximation built around 47° N latitude; at lower latitudes the losses from east or west roofs tend to be smaller.

A 6/12 roof is about 27°, a 4/12 roof about 18°. Values in between are interpolated.

• Assuming net metering. Many utilities have moved to export rates. Check your tariff before trusting a payback number.
• Ignoring shade. Trees, chimneys and dormers cut output; the factor assumes an unshaded roof.
• Counting an expired credit. Quotes that still subtract a 25D federal credit for a 2026 install are out of date.
• Forgetting the inverter. Enter an inverter replacement in the year you expect it.
• Sizing for today only. A heat pump or EV adds load; estimate it with the electricity cost calculator first.