Methodology
Methodology & sources
We show our work. Every calculator publishes its formula, assumptions and limitations — and every default value is tied to a dated, authoritative source.
How to read our methodology
Each calculator has its own methodology page with the exact formula, a worked example, known limitations and a “last reviewed” date. The defaults below are shared across calculators. They are representative figures, not exact: prices and grid intensity vary by region and over time, so every default is editable in the calculator. Always confirm against your own utility and suppliers.
Shared default assumptions (United States)
v1 defaults to United States (USD, imperial units). The figures below feed multiple calculators.
| Assumption | Default | Source |
|---|---|---|
| Residential electricity priceEditable — match your own utility rate. | 0.17 $/kWh | U.S. Energy Information Administration (EIA) |
| Regular gasoline priceVolatile — match your local pump price. | 3.30 $/gal | U.S. Energy Information Administration (EIA) |
| Grid carbon intensity (US average) | 0.39 kg CO₂/kWh | U.S. Environmental Protection Agency (EPA), eGRID |
| Gasoline combustion emissions | 8.887 kg CO₂/gal | U.S. Environmental Protection Agency (EPA) |
| Peak sun hours (US representative) | 4.5 h/day | National Renewable Energy Laboratory (NREL) |
Per-calculator methodology
- Appliance & electricity bill cost — methodologyThe exact formulas, assumptions, default values and limitations behind the appliance and electricity bill running-cost calculator.
- EV charging cost — methodologyThe exact formulas, assumptions, default values and limitations behind the EV charging cost calculator.
- EV charging time — methodologyThe exact formulas, assumptions, default values and limitations behind the EV charging time calculator.
- EV vs gas cost — methodologyThe exact formulas, assumptions, default values and limitations behind the EV vs gas total-cost-of-ownership calculator.
- Heat pump savings — methodologyThe exact formulas, assumptions, default values and limitations behind the heat pump savings, payback and CO₂ calculator.
- Home battery payback — methodologyThe exact formulas, assumptions, default values and limitations behind the home battery payback calculator.
- Solar panel savings & ROI — methodologyThe exact formulas, assumptions, default values and limitations behind the rooftop solar savings, payback, ROI and LCOE calculator.
- Solar system size — methodologyThe exact formulas, assumptions, default values and limitations behind the solar system size (how many panels) calculator.
All sources
The authoritative datasets behind our defaults, each with the date we last retrieved it.
- Average price of electricity to ultimate customers (residential)U.S. Energy Information Administration (EIA) · retrievedResidential prices vary by state and change monthly. Treat the default as representative and edit to your own rate.
- U.S. regular all-formulations retail gasoline pricesU.S. Energy Information Administration (EIA) · retrievedGasoline prices are volatile and regional. Default is a representative figure — edit to your local pump price.
- Greenhouse gas emissions from a typical passenger vehicle (8.887 kg CO₂ per gallon of gasoline)U.S. Environmental Protection Agency (EPA) · retrieved
- eGRID U.S. annual average CO₂ output emission rate for delivered electricityU.S. Environmental Protection Agency (EPA), eGRID · retrievedGrid carbon intensity varies widely by region; the default is the U.S. national average.
- PVWatts Calculator — system losses and methodologyNational Renewable Energy Laboratory (NREL) · retrievedPVWatts default system losses are ~14%, i.e. a derate factor near 0.86.
- Solar resource maps and data (peak sun hours / daily GHI)National Renewable Energy Laboratory (NREL) · retrievedPeak sun hours depend strongly on location; default is a U.S.-wide representative value.
- Photovoltaic Degradation Rates — An Analytical Review (Jordan & Kurtz)National Renewable Energy Laboratory (NREL) · retrievedMedian module degradation ~0.5%/year.
- Heat pump systems — efficiency guidanceU.S. Department of Energy (energy.gov) · retrievedModern air-source heat pumps typically deliver a seasonal COP of roughly 2.5–4.0.
- Estimating appliance and home electronic energy useU.S. Department of Energy (energy.gov) · retrieved
- Electricity (EV) — vehicle efficiency and charging lossesU.S. DOE Alternative Fuels Data Center (AFDC) · retrievedAC charging losses commonly add ~10–15% to energy drawn from the wall (charge efficiency ~0.85–0.90).
- Fuel economy ratings (mpg) and EV efficiency (mi/kWh)U.S. DOE / EPA — fueleconomy.gov · retrieved
- Lithium-ion battery storage round-trip efficiencyNational Renewable Energy Laboratory (NREL) · retrievedResidential lithium-ion systems typically have ~85–95% round-trip efficiency.
- Natural gas prices delivered to residential consumersU.S. Energy Information Administration (EIA) · retrievedRoughly $0.04–0.06 per kWh of gas energy once converted from $/therm; varies by region and season.
- GHG Emission Factors Hub — stationary combustion (natural gas, fuel oil)U.S. Environmental Protection Agency (EPA) · retrievedNatural gas combustion ≈ 0.18 kg CO₂/kWh of fuel energy; heating oil is higher.
- U.S. Solar Photovoltaic System Cost BenchmarkNational Renewable Energy Laboratory (NREL) · retrievedResidential installed cost is commonly around $2.5–3.5 per watt before incentives; varies widely.
- Residential Clean Energy Credit (federal solar tax credit)U.S. Department of Energy / IRS (energy.gov) · retrievedA federal tax credit (30% at time of writing). Eligibility and rate can change — verify current rules.
- Residential Energy Consumption Survey (RECS) — household end-use breakdownU.S. Energy Information Administration (EIA) · retrievedSpace heating/cooling and water heating dominate typical household electricity use; shares vary by climate and home.