Find the two bill inputs
Look for electricity consumption in kilowatt-hours and the dates or day count covered by that consumption. Do not combine gas consumption with electricity. Check whether the reading is actual or estimated when the bill provides that status.
The U.S. Energy Information Administration's residential terminology defines a billing period as the time between bills and notes that consumption is usually calculated from start and end meter readings, although it can be estimated.
Use annual kWh or annualize one period
If a reliable annual electricity-consumption figure is available, enter it directly. Otherwise:
annualized bill kWh = billing-period kWh ÷ billing days × 365For example, 300 kWh over 30 days annualizes to 3,650 kWh. This does not predict the next year; it extends that period's daily average across 365 days.
Read the inventory comparison
difference = annualized bill kWh − inventory kWhcoverage = inventory kWh ÷ annualized bill kWh × 100Coverage is unavailable when annualized bill kWh is zero. Coverage above 100% is allowed and can indicate that the modeled schedules, quantities, or measurement periods do not match the bill period.
Why one billing period may not represent a year
Heating, cooling, humidity control, pumps, lighting, occupancy, holidays, and outdoor equipment can change with seasons. Changed habits or equipment, estimated meter readings, unusual weather, and a short billing period can also make annualization unrepresentative.
Why the inventory and bill can differ
The inventory may omit central heating or cooling, water heating, pumps, vehicle charging, outdoor or shared loads, and small devices. Entered watts may differ from cycling average power, and measured periods may not represent normal use. The result is a coverage check, not a label for waste, leakage, theft, or unexplained usage.
Why the final bill amount is not reproduced
Ofgem's bill guidance distinguishes unit rates from standing charges and taxes and lists additional network, operating, and policy components. Bills and tariffs vary by location and supplier. Appliance Cost Lab therefore applies only the entered flat energy rate to modeled kWh.
When to contact the utility or a professional
Contact the utility or supplier when bill dates, kWh, estimated-reading status, meter identifiers, tariff details, or apparent billing corrections are unclear. If a bill dispute persists, use the applicable local complaint or consumer-support process.
Use a qualified electrical professional when checking the issue would require opening a panel, accessing hardwired circuits, working near damaged electrical equipment, or performing a measurement beyond the rating and instructions of available equipment. Do not alter meters or utility equipment.
Open the whole-home inventory bill-reference check
Related: whole-home inventory guide · accuracy guide · methodology · sources
Last reviewed: August 14, 2026