Our Methodology
How GreenCalcs builds its calculators, where every number comes from, and the editorial standards behind our guides. We show our work so you can trust — and check — the results.
Where our data comes from
We deliberately avoid installer marketing material, which tends to be optimistic. Instead, every input traces to a primary, authoritative source, updated on the cadence below:
| Input | Source | Updated |
|---|---|---|
| Installed cost per watt | NREL benchmarks + market reports | Quarterly |
| State incentives, SRECs & net metering | DSIRE (NC Clean Energy Tech Center) | Quarterly |
| Residential electricity rates | EIA Electric Power Monthly | Annually |
| Federal tax credits (25D & 25C, ended after 2025) | IRS guidance & Form 5695 | On change |
| Heat pump efficiency & sizing | DOE / ENERGY STAR | As published |
| Panel degradation & lifespan | NREL PV reliability research | As published |
How our calculators work
We believe a calculator you can't audit is just a black box. So every tool on GreenCalcs shows its full formula in plain English on the page, along with the assumptions baked in — panel degradation, electricity-rate inflation, depth of discharge, and so on. You can change any assumption and watch the result update live. If our math and an installer's quote disagree, you'll know exactly which assumption to question.
Key default assumptions we use (all editable in the tools):
- Installed solar cost: $3.00/W national average (2026), adjustable.
- Panel degradation: 0.5% per year.
- Electricity-rate inflation: 3% per year (the long-run US average), adjustable.
- Federal solar/storage credit: $0 for 2026 installs. The 30% 25D credit ended for systems installed after December 31, 2025 under the One Big Beautiful Bill Act; tools that check past installs still apply 30% to 2025 and earlier.
- Heat pump credit: $0 for 2026 installs. The 25C credit (30% capped at $2,000) and the geothermal 25D credit ended after 2025; we model state-run HEAR rebates (up to $8,000 for households at or below 80% of area median income) and utility rebates instead.
- Heat pump efficiency: COP 3–4 typical; cold-climate units hold COP>2 below freezing.
How our guides are written and reviewed
Our guides are written to inform, not to sell. Each one is researched against the primary sources above, structured for clarity, and reviewed for accuracy before publishing. Where figures are estimates, we say so; where rules vary by state or change over time, we flag it and link to the authoritative source so you can confirm current details.
We follow a few firm editorial rules: cite primary government sources, distinguish facts from estimates, keep a named, credentialed reviewer on every page, and update content when the underlying rules change. We'd rather be accurate and useful than sensational.
Our independence
GreenCalcs is an independent publisher. We are not a solar installer, a lead-generation broker, or a financing company. We don't require your email or phone number to use a calculator, we don't store your inputs, and we never sell your data. We're supported by advertising, and in future may add clearly-labeled affiliate links to services we'd genuinely recommend — but that will always be disclosed and will never change the numbers a calculator shows you.
Corrections
Accuracy is the whole point of this site. If you believe a figure is out of date or wrong, please tell us via our contact page. We verify reported issues against the primary source and update promptly — usually within a week for confirmed corrections.