Methodology & data sources
Last updated June 2026
Our principles
We are an independent calculator site, not an installer or lead broker's marketing page. That shapes three commitments. First, honesty over optimism: where cheap natural gas or weak sunlight makes a switch uneconomic, our tools say so. Second, current rules: figures reflect 2026 law, including the expiry of the federal residential solar/battery credit (Section 25D) and the heat pump credit (Section 25C) on December 31, 2025. Third, no false precision: we present estimates with realistic ranges rather than a fabricated confidence score.
Data sources
Electricity and fuel prices come from the U.S. Energy Information Administration (EIA), applied at the state level. Rooftop solar production is modeled with NREL PVWatts (version 8), the U.S. Department of Energy's standard performance model, which pulls location-specific irradiance from the National Solar Radiation Database. Grid carbon intensity uses a U.S. average of about 0.39 kg CO₂ per kWh.
Solar calculator
We query PVWatts for your ZIP's latitude, longitude, and weather station, then model hourly output for your system size, tilt, azimuth, and shading. Savings value the share you use on-site (default 50%) at your retail electricity rate, and the exported share at a conservative net-metering rate (default 9¢/kWh). Installed cost defaults to $3.00/watt. Payback is net cost divided by annual savings; 25-year totals apply about 0.5%/year panel degradation. No federal tax credit is applied in 2026.
Heat pump calculator
Annual heating energy is estimated from home size and a climate-intensity factor (in MMBtu), then converted: your current system's fuel use is priced at your state's fuel rate and efficiency, while the heat pump's electricity use is heating demand divided by its seasonal efficiency (COP, typically 2.3–3.6 depending on climate). Annual savings is the difference. Net upfront cost is the heat pump install minus the system you'd otherwise buy (furnace plus AC) minus any rebate — which is why replacing both heating and cooling at once dramatically improves payback.
Battery calculator
We model only the two benefits that reliably save money: self-consumption of your own stored solar (valued at the retail rate you avoid) and time-of-use arbitrage (the peak minus off-peak spread), each after about 10% round-trip efficiency loss. We deliberately exclude speculative "sell power to the grid" income, which is rarely profitable in the US. Backup-power value is real but not monetized.
Accuracy and limits
Solar production modeling is typically within about 10–15% of real output; heat pump and battery figures depend heavily on your exact utility rate plan, home condition, and usage. Treat every result as a well-grounded estimate for education, not a quote or financial advice. Prices and incentives change frequently — always confirm current figures with your utility and a licensed installer before purchasing.