Formulas
Monthly kWh = watts ÷ 1,000 × hours/day × days/month × quantity. Explicit monthly schedules are multiplied by 12 for annual results.
Weekly annual kWh = watts ÷ 1,000 × hours/day × days/week × (365.2425 ÷ 7) × quantity. Monthly average = annual ÷ 12.
EnergyGuide annual kWh = label annual kWh × quantity.
Energy cost = kWh × rate in cents ÷ 100. Household costs are summed before display rounding.
Scope, assumptions, and limitations
Example heater and EnergyGuide values come from the product’s worked examples. They are not measured device specifications or a catalog of typical appliance wattages.
State rates are EIA residential averages for June 2026, released August 26, 2026. They are averages, not your utility tariff. Prefer a current bill.
A 30-day monthly schedule annualizes to 360 operating days. A seven-day weekly schedule uses 365.2425 days. These are intentionally different conventions.
Comparing an alternative shows modeled energy savings only. Purchase price, disposal, installation, service life, and manufacturing impacts are excluded.
Confidence and precision
Calculations retain precision until display formatting. The input-quality indicator is based on your classification of the source groups, not whether a field happens to contain a value. It does not measure the probability that a future outcome will match the estimate.
Example inputs are for learning the tool. Replace them with your own values, and compare a less favorable scenario to understand sensitivity.
Primary sources
U.S. Energy Information Administration: Electric Power Monthly, Table 5.6.A ↗
June 2026 · released August 26, 2026 · Checked September 5, 2026
U.S. Energy Information Administration: Measuring electricity ↗
Undated reference · Checked September 5, 2026
Federal Trade Commission: How to use the EnergyGuide label ↗
August 2022 · Checked September 5, 2026
Methodology version 1.0 · Reviewed September 5, 2026.