Sources and arithmetic
Methodology
Energy Cost Checker is a planning tool, not a bill predictor. Every calculator uses the same typed formula engine, keeps full precision until display, and labels the rate and assumptions used for its result.
Calculator formulas
1,500 W ÷ 1,000 × 3 hr × 365 d × $0.1834/kWh = $301.23/yrkWh × tiered rate + fixed charge = monthly totalΔcost = annualCostB − annualCostApayback years = upfront Δ ÷ annual savingscost share = appliance kWh ÷ home kWhwall kWh = miles × kWh/mi ÷ charger efficiencyRates and source provenance
A rate can come from a published utility plan, a regional fallback, or a value entered from your bill. The result panel distinguishes those cases and reports effective and retrieval dates when the source supplied them. Manual values are never presented as published data.
Precision and calendar rules
Energy is normalized to kilowatt-hours and currency to USD. Calculations use unrounded values internally; cents and display units are rounded only when rendered. Annualization uses the selected analysis year, including leap years where the contract requires it.
What estimates exclude
State and national EIA averages include revenue from fixed charges and taxes; they are not marginal tariff prices. Manual tariff estimates include only entered components. Demand charges, minimum bills, rebates, installation and maintenance are not automatically added. Do not add another fixed charge to an aggregate average. Estimates do not reproduce every utility bill.
Data and privacy
Published datasets are ingested into versioned Supabase tables before they are served; calculator requests never scrape a third-party source live. Inputs are not tied to an account. A scenario is stored only when you create an anonymous, expiring share link.