Every payback number on this site comes from the same small model. Nothing here is hidden inside the arithmetic, and the figures in the table below are read directly from the code that produces the estimates — not retyped, so this page cannot drift away from what the calculator actually does.
The modeled system
| System size | 7 kW DC |
|---|---|
| Installed cost | $3.20 per watt — $22,400 before incentives |
| Performance ratio | 78% — losses from inverters, wiring, soiling, heat and shading |
| Panel degradation | 0.5% per year |
| Electricity price escalation | 3% per year |
| Federal tax credit | None applied |
| Value of exported power | The full retail rate |
Why two payback numbers, and which one to trust
The state tables print two figures. The first applies the 3% annual escalation above; the second holds electricity prices flat forever. Divide the installed cost by the first-year saving and you get the flat-price answer, which is always the longer one.
Both are shown because the honest answer depends on something nobody can know: what your utility charges in fifteen years. If prices rise faster than 3% the real payback is shorter than either figure. If your utility freezes rates, the flat number is the one that happens.
The largest assumption, stated plainly
This model values every kilowatt-hour you export at the full retail rate. In a growing number of states that is simply not what you are paid. Where a state has moved to net billing, avoided-cost export, or has no statewide net-metering mandate at all, the payback shown here is optimistic — sometimes by years. Each state page with a written analysis says so in its own terms and names the tariff or commission order that decides it. Read that qualifier before you use the number.
Where the inputs come from
- Electricity rates — average residential price by state, from EIA Electric Power Monthly, Table 5.6.A. Calendar-year 2024 values.
- Sunlight — annual average daily global horizontal irradiance from NREL's National Solar Radiation Database.
- Incentives — state programs summarised from DSIRE, with the governing statute, commission order or tariff cited on the state page wherever a specific claim is made.
- Federal credit — the 30% Residential Clean Energy Credit under IRC §25D expired for expenditures made after 31 December 2025. See the federal credit guide.
What this model does not do
- It does not model your roof — no azimuth, pitch, or shading from your trees.
- It does not model your usage. A household that exports most of its production in a net-billing state will do worse than shown.
- It does not include batteries, financing cost, maintenance, inverter replacement, or the effect of a sale of the home.
- It does not apply state or utility incentives to the payback figure. Where those exist, your real payback is shorter than shown.
- It is not a quote. It is a starting point for reading one.
Corrections
Tariffs and incentive programs change, and some of the ones cited here will be stale before they are noticed. If you find a figure that is wrong, please tell us — corrections are made on the page, and the review date above is updated when they are.