Methodology
How all BuildCalc calculators work. Every calculator on BuildCalc shows the formula it uses. Enter your own numbers, and the page calculates from those numbers, using the units shown on the page. Where a result depends on a rate, price, coverage or other assumption, that assumption is shown as an editable starting point, not as a current market rate. Results are estimates for planning only, not financial, legal, tax or medical advice. Sources and starting points. Standard math, finance, construction and science formulas are used, and are shown on each page. Typical rates, such as material coverage, seeding rates and electricity rates, are typical US starting points. For a current rate, use your bill, your supplier, your utility website, or the US Energy Information Administration (EIA) for state electricity averages. No individual professional credential is claimed for any calculator. How the home energy calculators on BuildCalc work, and what their numbers do and do not mean.
Formulas
Electricity: kWh = watts ÷ 1,000 × hours. Cost = kWh × electricity rate.
Heat pump: electricity needed = heating demand ÷ COP. Furnace and gas water heater: fuel therms = (demand ÷ efficiency) ÷ 29.307, because 1 therm = 29.307 kWh.
EV: monthly kWh = miles × kWh per 100 miles ÷ 100. Solar payback: annual generation = system size × production per kW, payback years = system cost ÷ (generation × rate).
Data sources
Default values are typical US figures used as editable starting points. They are not a claim that a rate is current for your state or utility. For real rates, use your electricity or gas bill, your utility website, or the US Energy Information Administration (EIA) for state averages. For vehicle fuel economy, use fueleconomy.gov. For a real solar or heat pump quote, use a local installer.
What is not included
Running-cost calculators do not include purchase price, installation, maintenance, taxes, delivery charges or fixed fees, unless a calculator has a separate fixed-fee field. Payback and saving results are estimates, not guarantees.
Other categories
Calculators and tools outside the home energy group use standard formulas that are shown on each page. All results are estimates for planning only. They are not financial, legal, tax or medical advice.