Solar Savings Calculator

How it works

Solar used at home = the smaller of monthly household use and solar production × self-use percentage. Exported solar = production − self-used solar. Remaining grid purchases = household use − self-used solar. Monthly solar value = self-used kWh × import rate + exported kWh × export rate. The yearly figure multiplies by 12 and the 25-year figure keeps every input unchanged for 300 months.

Worked example: 750 kWh of solar in one month

A home uses 900 kWh and the solar system produces 750 kWh. At 60% direct self-use, 450 kWh supplies the home and 300 kWh is exported. The home therefore still purchases about 450 kWh from the grid.

At a $0.17/kWh import rate, the 450 self-used kWh avoids $76.50 of grid purchases. At a $0.05/kWh export rate, 300 exported kWh adds $15 of bill credit or income. Total monthly solar value is $91.50, or $1,098 for 12 identical months.

Why self-used solar and exported solar have different value

A self-used kWh replaces one kWh that the home would otherwise buy at the retail import rate. An exported kWh receives the value defined by the utility, supplier, or solar agreement. Those rates can be equal under some net-metering structures, but they are often different and the credit may not be paid as cash.

Use the effective values in your actual agreement. If exported energy creates bill credits, enter the per-kWh credit value and interpret the export result as bill value. If exports receive nothing, enter zero.

Get production from a location-based estimate or real meter data

Roof direction, slope, shade, weather, system size, equipment, temperature, and losses affect production. The U.S. Department of Energy points homeowners to PVWatts for a location-based estimate of grid-connected PV output, then recommends a custom installer estimate for the specific property.

For an existing system, use a representative inverter, utility, or monitoring total. Production is not the same as export: production first serves coincident household loads, and only the surplus is exported. Monthly values also vary by season, so adding 12 real monthly estimates is more accurate than repeating one month.

This is gross energy value, not profit or payback

The result does not subtract the purchase price, loan interest, lease or power-purchase payments, maintenance, insurance, taxes, inverter replacement, panel degradation, or battery losses. It also keeps electricity and export rates unchanged for 25 years.

Use the solar panel payback calculator after estimating first-year energy value, and compare its result with written installer production assumptions, utility rules, financing terms, warranties, and multiple quotes before making a purchase decision.

Sources used for assumptions: U.S. Department of Energy — Homeowner’s Guide to Solar, PVWatts, net metering, and savings factors; National Laboratory of the Rockies — PVWatts solar production calculator; Ofgem — Smart Export Guarantee supplier tariffs and export payments.

Frequently asked questions