Methodology & data sources

The formula

1. We turn your monthly bill into annual kWh using your state’s electricity rate.
2. We size a system to cover the share of usage you choose, using your state’s solar production factor.
3. Gross cost = system size × your installed $/W.
4. Net cost = gross cost − any state/utility incentive you enter. (No federal credit — it ended 31 Dec 2025 for purchased systems.)
5. Annual saving = the power you generate, valued realistically: the share you use as it’s made is worth your full retail rate, and the surplus you export is worth your state’s export rate. We assume a typical no-battery home self-consumes ~40% of production; in full-net-metering states the two rates are equal so this makes no difference, but in below-retail export states (like California under NEM 3.0) it correctly lowers the saving. A battery raises the share you keep.
6. Payback = the year your cumulative savings overtake that net cost, with power prices rising ~2.5%/yr and panels losing ~0.5%/yr — which is why payback is shorter than simply dividing net cost by year-one savings.

Where the data comes from

• Electricity rates — the average residential price per state from the U.S. Energy Information Administration (EIA), Electric Power Monthly (~2026). National average in our data: 18.4¢/kWh. This is a state average — total residential revenue ÷ total kWh sold — so it bakes in fixed charges and won't exactly match any one plan. Rates vary by utility and (in deregulated states like Texas) by the retailer you choose, so every calculator lets you type your own rate straight off your bill for a precise figure. We pull each state's number from the EIA Open Data API and use a 12-month trailing average (not a single month, which can be preliminary and volatile), discarding obvious data anomalies. Each state page shows the underlying EIA price trend so you can see the source data for yourself.
• Solar production — annual kWh per kW of panels, a south-facing rooftop ballpark by climate from NREL’s PVWatts. National average: 1330 kWh/kW/yr.
• Federal tax credit — the 30% Residential Clean Energy Credit (IRC §25D) was ended by the One Big Beautiful Bill Act for any system placed in service after 31 December 2025. We apply no federal credit to a purchased system. It can still reach you indirectly through a lease or PPA, where the third-party owner claims the separate commercial credit. See our guide on what changed.
• State & utility incentives — these change constantly and vary by provider, so we don’t hard-code dollar figures. We link each state page to DSIRE, the authoritative US incentives database, and let you enter any rebate.

EV charging cost

The EV charging calculator uses the same state electricity rates. We convert your annual miles into kWh using your EV’s real-world efficiency (miles per kWh measured “from the wall,” so charging losses are already included), price the home-charged share at your state rate and the public-charged share at the fast-charge price you enter, then compare against a gas car at the MPG and pump price you choose. Defaults — 13,500 miles/yr (FHWA average), 3.3 mi/kWh, 85% home charging, $0.48/kWh public, 27 MPG, $3.30/gal — are all editable, and we don’t hard-code per-state gas prices (you enter your local one).

Heat pump vs gas furnace

The heat pump calculator compares heating only — cooling costs are similar either way, since a heat pump cools at a SEER2 comparable to a standalone AC. We take the therms your furnace burns in a year, convert them to delivered heat at its AFUE efficiency (1 therm = 29.3 kWh of heat energy), and price the same delivered heat from a heat pump at its seasonal COP and your state electricity rate. Defaults — 600 therms/yr (near the EIA average of ~566 for a gas-heated home), $1.50/therm gas (~the national residential average; we don't hard-code per-state gas prices, you enter your local one), 95% AFUE, COP 2.6 (a ducted HSPF2-9 unit derated for real-world ducts; use less in hard winters), and a $3,500 install premium vs a like-for-like furnace + AC — are all editable. We also show the breakeven electricity price at your gas price, so you can see which side of the line you're on. The §25C federal heat-pump credit ended 31 December 2025, so we apply no federal credit; state and utility rebates can be entered via the premium field.

AC running cost

The air-conditioner calculator uses the same state electricity rates. We work out the unit’s electrical draw in watts (cooling capacity in BTU ÷ its EER efficiency rating), convert that to kilowatt-hours over the hours you run it, and price it at your state rate — then extend it to a day, a 30-day month, and a cooling season of the length you set. Defaults — 12,000 BTU, EER 11, 8 hours/day, 5-month season — are all editable. Real-world cost varies with thermostat setting, insulation, humidity and how hard the unit works on hot days, so treat it as a guide.

What this is — and isn’t

This is a fast, transparent estimate to help you decide whether solar is worth pursuing where you live. It is not a quote, a guarantee, or financial/tax advice. Real costs depend on your roof, shading, equipment and installer, and tax rules change — the federal credit ending in 2025 is a case in point. Always get itemised quotes and talk to a tax professional before buying.

Honesty policy

We use state averages and label them as such. We never invent precise rebates, fake “average savings,” or hide the answer behind a lead-capture form. Nothing you type leaves your browser. If a number here is wrong or out of date, that’s a bug — tell us.