Electricity basics
How to Measure Appliance kWh with a Plug-In Meter
Use a suitable plug-in electricity monitor to measure cumulative kWh. Record elapsed time, operating conditions, safety limits, and a useful daily average.
Use a compatible monitor within its rated limits. Record cumulative kWh and elapsed time over normal use. Divide kWh by days for a daily average.
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A plug-in electricity monitor can show both power at a moment and energy over time. For a running-cost question, cumulative kWh is usually the more useful reading.
The P3 monitor manual describes one meter's kWh and elapsed-time functions. Other monitors have different controls and electrical limits. Read the manual for the device you actually use.
Check compatibility before connecting
Use a monitor designed for your local supply and the appliance's plug and load. Check its voltage, current, power, and load-type limits. A matching plug shape alone does not establish compatibility.
Do not use a consumer plug-in meter on hardwired equipment or a dryer connection it does not support. Do not improvise adapters. For a portable heater, follow CPSC guidance to plug directly into the wall; this guide does not recommend an intervening meter.
If a socket, cable, or plug is damaged or becomes hot, stop using it. Ask a qualified professional. Do not open a panel to obtain a measurement.
Choose a measurement period
A single watt reading can miss compressor cycling or a defrost heater. The DOE standby measurement page explains why varying power calls for energy measured over time.
For a fridge, several normal days are more useful than a few minutes. For a dishwasher, record a complete cycle. For a device used on different speeds, measure the speed you intend to use.
Avoid measuring an unusual event and presenting it as normal. A newly filled freezer or a dehumidifier drying a recently wet room may use different energy from its later routine.
Keep a simple measurement record
- Record the appliance, mode, room conditions, and start time.
- Note the starting cumulative kWh or reset according to the manual.
- Use the appliance normally for the chosen period.
- Record final kWh and the actual elapsed time.
- Subtract the starting reading if you did not reset.
- Calculate average daily energy and multiply by your tariff.
Do not reset the monitor halfway through the period. Check whether it retains data after power loss. Follow the manufacturer's instructions for moving or unplugging it.
Convert the result correctly
Suppose an illustrative record shows 7.2 kWh over exactly six days. The daily average is 7.2 ÷ 6 = 1.2 kWh. At an example $0.20/kWh, that is $0.24 per day.
Thirty days at that same average would cost $7.20. This is a projection from the stated period. It does not prove that summer, winter, or a holiday month will match.
If elapsed time is in hours, divide by 24 to get days. A 48-hour record spans two days. Avoid rounding a short duration to a full day before dividing.
Know what the monitor misses
A monitor measures the electricity passing through it. It does not include hot water heated elsewhere, gas burned by the appliance, or the cost of a changed room temperature.
Very small standby readings may also be limited by the meter's accuracy or resolution. Check the specification before claiming a tiny difference.
Enter the average in the calculator's measured daily kWh mode. Keep the measurement conditions with your result so another reader can understand its limits.
Sources
Source pages checked during research on 7 October 2026. Examples are Handy Daily calculations unless stated otherwise.