Methodology

Every WattSpend calculator uses public data and a formula printed on the page. This is the full reference for where the numbers come from and how each tool computes its answer.

Data sources

• Electricity rates, residential ¢/kWh in the style of U.S. Energy Information Administration (EIA) state averages. Refreshed from EIA monthly data and stamped with a “last updated” date.
• EV efficiency and battery size. EPA ratings (miles per kWh, measured from the wall, so ~10% charging losses are already included) and manufacturer usable-capacity specs.
• Gas baseline, 28 mpg at $3.20/gallon as a national example; editable in every comparison.
• Public / DC fast charging, about 42¢/kWh, a typical US networked rate.
• Heating fuels. DOE reference BTU values and 2026 example prices: natural gas ($1.55/therm, 92% furnace), heating oil ($3.80/gal, 85%), propane ($2.90/gal, 90%), electric resistance (100%).

The formulas

What these estimates don’t capture

Estimates are estimates. Weather, time-of-use plans, terrain, ductwork, and your specific utility tariff all move the real number. Where a factor is large, winter range loss, deregulated markets, install scope, we say so on the relevant page. Always check your own utility bill and get real quotes for real projects.