How We Calculate
Every estimate on CleanEnergyCalc traces back to a named, public dataset and a transparent formula. No black boxes, no inflated numbers designed to close a sale. This page explains the data and assumptions behind each category of calculator.
Data foundation
We rely exclusively on U.S. government and national-laboratory datasets. Where a number can change month to month (electricity rates, incentives), we cite the release it came from and review it against new publications.
Data Sources
Data current as of February 2026. Rates and incentives are subject to change.
Formulas & assumptions
- •Annual production is modeled on NREL PVWatts v8 using state-level peak sun-hour data and a standard system derate (~14%) for inverter, wiring, and soiling losses.
- •Electricity offset value uses EIA residential rates for the selected state (¢/kWh), not a national average.
- •Payback and ROI compare net upfront cost (after any modeled incentives) against cumulative bill savings, with optional panel degradation (~0.5%/yr).
- •Charging cost = vehicle efficiency (kWh/100 mi) × annual miles × electricity rate, with optional time-of-use off-peak pricing.
- •EV-vs-gas comparisons use EIA electricity rates and current fuel-price inputs; maintenance deltas follow DOE estimates.
- •Battery-health and road-trip tools use manufacturer warranty baselines (typically 8 yr / 100k mi) and published consumption figures.
- •Backup sizing is based on user-entered essential loads and stated usable capacity from manufacturer spec sheets.
- •Self-consumption and solar-plus-storage savings are derived from the same EIA rate and NREL production models used elsewhere.
- •Heat-pump operating cost compares modeled annual energy use against gas at user-entered or default fuel prices.
- •IRA rebate eligibility (HOMES / HEAR) follows DOE program rules and income thresholds; amounts are estimates pending state program rollout.
How often we update
Electricity rates are checked against EIA's monthly releases. Incentive amounts are updated when programs change. Each calculator and guide shows a “last reviewed” date so you know how current the inputs are. Found something out of date? Tell us and we'll fix it.
Estimates, not quotes
Our calculators provide estimates for informational purposes only. Real-world results depend on site-specific conditions — local weather, roof and home characteristics, equipment selected, installer pricing, and utility rate plans. Always obtain multiple professional quotes and consult a tax professional for incentive eligibility before making a decision.