Methodology

Assumptions and sources

Every default in the Geostellar calculators, where it comes from, and why it's editable. If a number here goes stale, the calculators are wrong — corrections welcome.

Assumptions

Installed cost

$2.75 per watt (default, editable)

US residential average range of roughly $2.50–$3.00/W before incentives; market trackers such as EnergySage publish current quote medians.

Production

1,400 kWh per kW per year (default, editable)

Mid-US figure; actual range is roughly 1,100–1,700 kWh/kW/yr by region and roof. Model your address with NREL PVWatts.

Electricity rate

17¢/kWh (default, editable)

Approximate US residential average; EIA publishes current state-level rates. Your bill shows your real number — use it.

Federal incentive

30% for systems placed in service 2022–2025; 0% for homeowner purchases from 2026

Residential Clean Energy Credit (IRC §25D), claimed on IRS Form 5695; terminated for expenditures after Dec 31, 2025 by the One Big Beautiful Bill Act (July 2025).

Panel degradation

0.5% output loss per year

Typical warranty-backed degradation for modern modules.

Utility-rate escalation

2.5% per year

Long-run US average trend; actual escalation varies by utility.

System life

25 years

Standard panel warranty term. Inverter replacement (string inverters, years 10–15) is not modeled and adds $2,000–$3,000 to lifetime cost.

Home-value uplift

$4,000 per kW installed, owned systems only

Zillow (2019): ~4.1% sale premium; Lawrence Berkeley National Laboratory paired-sales research: ~$4/W. Leased systems excluded — they can reduce resale value.

What the calculators deliberately don't do

No address-level irradiance modeling — NREL PVWatts does that authoritatively and free. No state-incentive database — rules change too often to mirror; check DSIRE directly. No export-rate modeling per utility — read the net metering guide and ask your utility for the current tariff. And no lead capture: results stay in your browser and are never sent anywhere.