How the numbers are made
The calculator combines PVGIS production data with an hourly model of your home’s electricity use. PVGIS comes from the European Commission’s Joint Research Center: US estimates use PVGIS-NSRDB for 2005 to 2015 through API 5.2, with PVGIS-ERA5 for Alaska. Monthly totals are matched to your ZIP prefix, with an hourly profile for your state and assumed system losses of 14%.
The hourly model accounts for home base load, the inverter’s AC output limit and battery storage. Base load defaults to 164 W, from NRDC’s 2015 Home Idle Load report; this is a study average, not a measurement of your home. When a battery spec sheet gives no round-trip efficiency, the assumed value is 0.9.
Default electricity rates come from EIA Electric Power Monthly’s state residential price table, using prices for January to July 2026. The calculator names its assumptions and lets you change the model inputs, including panel size and position, inverter limit, base load, battery settings and electricity rates. Panel power in W DC and inverter output in W AC are separate inputs.
How to read the result
Electricity used at home is the main source of savings, not total production. Produced energy is the solar output; used-at-home energy replaces purchased electricity; exports go to the grid. Energy lost to the inverter limit is output the inverter cannot deliver when available panel power exceeds its AC limit.
For California ZIP codes starting 933, the calculator gives 811 kWh in a typical year from south-facing, vertical panels rated at 800 W DC with an 800 W AC inverter. With a 164 W base load, your home uses 471 kWh (58%) and exports 340 kWh. At 33.25 cents/kWh, that means $157 a year in savings when exports earn nothing.
Exports are valued at zero unless you enter an export rate; do not assume your utility pays for them. Payback means the entered cost divided by estimated annual savings, with results above 25 years shown as “more than 25 years.” The California example gives $1,566 over 10 years without tariff growth or panel aging; that is accumulated savings, not profit after equipment costs.
What changes the result most
Panel position, daytime electricity use, storage and your electricity rate change the result. The calculator examples below show why more production does not always mean proportionally more savings.
Use the Typical year by state table for a consistent starting comparison, then the city orientation table to compare direction and tilt. Neither table measures your home’s base load; change that input to reflect your electricity use.
- Direction: in New York City, the calculator gives 513 kWh/year facing east versus 732 kWh/year facing south for vertical 800 W panels.
- Tilt: the California example rises from 811 kWh/year vertically to 1,354 kWh/year at 30°, while annual savings rise from $157 to $189.
- Base load: in Los Angeles without a battery, increasing base load from 100 W to 300 W changes calculated savings from $105 to $226 a year.
- Battery: at a 164 W base load in Los Angeles, calculated savings rise from $156 a year without storage to $225 with a 1 kWh battery.
- Rate: the California example uses 33.25 cents/kWh and saves $157 a year; Texas uses 16.06 cents/kWh and saves $68. Production and self-use also differ between these locations.