Methodology and sources
Last reviewed 19 August 2026
This page exists so you can check our work. It lists every default figure our calculators use, its source, and the formulas behind each result. If you disagree with an assumption, every input on every calculator can be overridden with your own numbers.
Default assumptions
| Assumption | Value | Source |
|---|---|---|
| Residential electricity, US average | $0.1745/kWh | U.S. Energy Information Administration |
| Off-peak / EV tariff, typical | $0.09/kWh | Composite of major US utility EV plans |
| Public Level 2 charging | $0.30/kWh | Composite of major US charging networks |
| DC fast charging | $0.48/kWh | Composite of major US charging networks |
| Regular gasoline | $3.15/gal | EIA / AAA national average |
| Residential natural gas | $1.55/therm | U.S. Energy Information Administration |
| Average annual mileage | 13,500 mi | Federal Highway Administration |
| Peak sun hours, US average | 4.5 h/day | NREL solar resource data |
| Solar system derate factor | 0.86 | NREL PVWatts default |
National averages are, by definition, wrong for almost everyone. Electricity in Louisiana costs roughly a third of what it costs in Hawaii. Always substitute the rate from your own bill — it is the single change that most improves the accuracy of any result here.
Charging efficiency
Energy drawn from the wall always exceeds energy stored in the battery. We apply these wall-to-battery efficiencies:
| Charging method | Efficiency | Why |
|---|---|---|
| DC fast charging | 94% | Bypasses the onboard AC/DC converter |
| Level 2 (240V) home | 90% | Onboard converter plus thermal management |
| Level 1 (120V) outlet | 80% | Fixed overheads run far longer at low power |
Core formulas
EV energy and cost
kWh at battery = miles ÷ efficiency (mi/kWh)
kWh at meter = kWh at battery ÷ charging efficiency
cost = kWh at meter × rate
Combustion vehicle cost
cost = (miles ÷ mpg) × price per gallon
Appliance running cost
kWh = (watts ÷ 1000) × hours/day × days/year × duty cycle
Duty cycle matters enormously for anything thermostatically controlled. A refrigerator plugged in continuously runs its compressor only 30–40% of the time; ignoring this overstates its cost roughly threefold.
Solar production and payback
annual kWh = system kW × peak sun hours × 365 × derate
Payback modelling assumes 2.5% annual utility rate escalation and 0.5% annual panel degradation, then finds the year cumulative savings exceed net installed cost.
Heating systems
heat pump kWh = BTU needed ÷ HSPF ÷ 1000
furnace therms = BTU needed ÷ AFUE ÷ 100,000
How we verify the maths
Every formula lives in one shared library covered by an automated test suite. Those tests check results against figures computed by hand and against published reference values, and they include deliberate awkward cases — divide-by-zero inputs, and scenarios where the "green" option loses.
The browser code is generated directly from those same tested functions rather than rewritten by hand. The code you run is the code the tests verify, so the two cannot drift apart. If a test fails, the site does not build.
What we deliberately do not model
Being clear about limits matters as much as being accurate:
- Standing charges and fixed fees. We calculate marginal energy cost. Fixed monthly charges apply whether or not you use the appliance.
- Tiered and demand tariffs. Some utilities raise rates above a usage threshold or bill on peak demand. We assume a flat rate.
- Local incentives. Rebates vary by state, utility and month. We model the federal picture and let you enter local amounts.
- Maintenance, insurance and depreciation. Running-cost tools only.
- Battery and equipment degradation over time, except in solar payback, where it is material.
Review schedule
Energy prices move. Defaults are reviewed quarterly against their primary sources, and this page's review date is updated when they change. Spotted something stale? Tell us — corrections are made promptly.