SnapEnergyLab
Electricity

How to calculate your electricity usage from the meter

Two readings and a date give you a number no appliance estimate can argue with: your actual consumption.

Published 23 August 2026 · Published by SnapEnergyLab. Methodology and default assumptions are documented separately.

A man crouching in a utility cupboard checking a residential electricity meter beside the consumer unit

Quick answer

Take a meter reading, wait a known number of days, take another, and subtract. The difference is the kilowatt-hours you actually consumed — no appliance estimate can contradict it.

Divide by the number of days for a daily average, and multiply by your effective price per kWh for a cost. Two readings taken overnight, when nothing deliberate is running, give you something even more useful: your baseline load.

How to read the numbersYour inputCalculated result
Reading difference
142 kWh
Your input
Period
14 days
Your input
Daily average
10.1 kWh/day
Calculated result

The method

A domestic electricity meter is a cumulative counter in kilowatt-hours. It only ever increases, so any two readings define the consumption between them. This makes it the one measurement in household energy that needs no assumptions at all.

1. Record the first reading with a timestamp

Photograph the display. Note the date and the time — the time matters more than people expect for short measurement windows. On a digital meter, read the register showing total kWh; ignore the instantaneous kW display, which is a rate, not a total.

2. Wait a full number of days

A week is the shortest period that averages out the difference between weekdays and weekends. Two weeks is better. A single day tells you about that day only.

3. Subtract and divide

Consumption divided by elapsed days is your daily average.

  1. consumption (kWh) = reading₂ − reading₁
  2. daily average (kWh/day) = consumption ÷ days elapsed
  3. monthly estimate (kWh) = daily average × 30.44
  4. cost = consumption × effective price per kWh

30.44 is the average length of a calendar month (365.2425 ÷ 12). Using 30 understates a monthly figure by roughly 1.5%.

Worked example: a two-week reading

  1. Reading on 1 March, 08:0048,213 kWhYour input
  2. Reading on 15 March, 08:0048,355 kWhYour input
  3. Consumption48,355 − 48,213 = 142 kWhCalculated result
  4. Elapsed time14 daysYour input
  5. Daily average142 ÷ 14 = 10.14 kWh/dayCalculated result
  6. Monthly estimate10.14 × 30.44 = 308.7 kWhCalculated result
Estimated monthly consumption≈ 309 kWh

Finding your baseline load

The most valuable measurement you can take is the one where you deliberately change nothing. Read the meter late at night, then again first thing in the morning, before anything is switched on.

Divide by the hours elapsed and you have your average continuous draw in kilowatts — the fridge, freezer, router, standby devices, ventilation and anything else that never turns off. That figure multiplied by 8,765.8 hours is a genuine measurement of your annual always-on consumption, and it is frequently a quarter to a third of the whole bill.

Worked example: overnight baseline

  1. Reading at 23:3048,355.0 kWhYour input
  2. Reading at 06:3048,357.8 kWhYour input
  3. Overnight consumption2.8 kWh over 7 hCalculated result
  4. Average continuous draw2.8 ÷ 7 = 0.40 kW = 400 WCalculated result
  5. Annualised0.40 × 8,765.8 = 3,506 kWh/yrCalculated result
Always-on load≈ 400 W

Where meter readings go wrong

Multiple registers. Time-of-use tariffs record day and night consumption separately. Read both registers and add them, or you will silently omit half of your usage.

Export and generation meters. A home with solar may have a separate generation meter and an export register. Neither of those is your import consumption.

Estimated bills. If your bill says “E” or “estimated” next to the reading, comparing it against a real meter reading is comparing a measurement with a guess.

Decimal digits. Many meters show tenths of a kWh in a separate, differently coloured window. Include the decimals for short measurement windows; over two weeks they are noise.

Convert your measured usage into cost

Once you know your daily kWh, the calculator turns it into cost per day, month and year at your own rate — and lets you test what a change in price does to the total.

Open the electricity cost calculator

Assumptions and limitations

  • A two-week average captures the weather of those two weeks. In a home with electric heating or cooling, seasonal variation is larger than any measurement error.
  • Extrapolating a monthly figure to a year assumes every month resembles the measured one. It rarely does.
  • The cost figure depends on your effective price per kWh, including standing charges, network fees and taxes — not the headline unit rate.

Sources

  1. Units and calculators explained — electricity — U.S. Energy Information Administration

    Supports: The kilowatt-hour as the unit registered by domestic electricity meters.

  2. Smart metering equipment technical specifications — UK Department for Energy Security and Net Zero

    Supports: Smart meters record cumulative kWh registers and interval consumption data.

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