Electricity Usage Calculator

Electric meters accumulate kilowatt-hours. Subtracting an earlier reading from a later one gives whole-home energy use over that interval, which can then be normalized for comparison with another month or season.

The starting and ending readings must come from the same meter register and the ending reading must be later. Smart-meter portals may already show interval use, while analog dials require careful reading from right to left.

Daily average divides period kWh by elapsed days. Multiplying that rate by 30.437 gives an average calendar month and by 365 gives a simple annualized value.

Annualizing a short sample does not capture seasons. Air conditioning, electric heat, pool pumps, school schedules, and vacations can move use substantially.

Assumptions and edge cases

The subtraction assumes both numbers come from the same register of the same meter, in the same units, with the ending reading taken later. Three edge cases break that. A mechanical register that rolls over from 99,999 to zero returns a negative difference, so the register capacity must be added before subtracting. Some meters carry a faceplate multiplier that the displayed difference has to be multiplied by. A replaced meter restarts near zero, so readings spanning a meter change cannot be subtracted at all.

Worked example

A meter moving from 24,150 to 24,570 kWh in 28 days used 420 kWh, or 15 kWh/day. That pace is about 457 kWh in an average month.

Second worked example

Take a longer sample for contrast. A meter reading 41,208 kWh on March 1 and 45,096 kWh on June 1 spans 92 days and 3,888 kWh, an average of 42.3 kWh per day. That works out to about 1,287 kWh in an average 30.437-day month and 15,430 kWh annualized. Against the 15 kWh per day of the 28-day winter sample above, the spring period runs nearly three times higher, which points at a specific seasonal load such as cooling, a pool pump, or an electric water heater rather than at general household drift.

How to read the result

Use at least a full billing cycle for a stable baseline. Compare like seasons and note weather when evaluating an efficiency change.

Three thresholds turn the number into a decision. A daily average below 15 kWh is low for a US home and usually means gas heating and gas water heating; 20 to 35 kWh per day is typical for an all-electric or heavily air-conditioned house; above 50 kWh per day something large runs close to continuously and deserves a submeter. Between two periods, a change under 5 percent is weather and occupancy noise, while more than 25 percent almost always has one identifiable cause.

Planning inputs are deliberately editable because two homes in the same state can have different tariffs, roof conditions, equipment, and operating schedules. Save a scenario with “Copy link,” then change one assumption at a time. This makes the result useful for comparing decisions without implying false precision.

Formulas and defaults are documented in our data method. The principal references for this page are:

1. U.S. Department of Energy — Estimating Appliance and Home Electronic Energy Use (2024) — Watts-to-kWh cost method.
2. EIA — Electric Power Monthly, Table 5.6.A (2024 annual) — State residential rate benchmarks.
3. EIA — Residential Energy Consumption Survey (2020, released 2023) — Household end-use context.
4. U.S. EIA — Electricity explained: Measuring electricity (2025) — Definition of the kilowatt-hour and the registers a residential meter accumulates.
5. U.S. EIA — Form EIA-861 residential sales and customers (2023) — Typical household monthly consumption used for the comparison thresholds.

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