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The method
- Put an energy-logging plug meter between the socket and the appliance. You want one that accumulates kilowatt-hours, not one that only shows instantaneous watts — the instantaneous reading on a cycling appliance is meaningless because it depends entirely on when you look.
- Leave it for fourteen days. Not a day. A fridge has defrost cycles; a heater follows the weather; a dehumidifier follows the humidity. A single day can be off by half in either direction.
- Read the total kilowatt-hours and divide by 14. That is your daily figure.
- Multiply by 365 for a year — unless the appliance is seasonal, in which case multiply by the days you actually use it and say so.
The four mistakes that ruin a reading
- Measuring for a day. The single most common error. Anything with a thermostat needs at least a week to average out.
- Measuring in the wrong season. A freezer in a garage in February tells you nothing about the same freezer in August. Say which season your figure is from.
- Using a meter rated below the load. Most plug meters top out around 15 A. A space heater or a portable air conditioner is close to that limit, and an electric kettle can exceed it. Check the rating on the meter before you plug anything with a heating element into it.
- Trusting the watts display. On a cycling appliance the live wattage swings between zero and full. Only the accumulated kilowatt-hour total means anything.
Three worked examples
What a fortnight of metering typically tells you, and how to read it against the model on this site.
| Appliance | 14-day meter reading | Implied year | Cost a year | This site models | Reading it |
|---|---|---|---|---|---|
| Refrigerator | 16.1 kWh | 420 kWh | $77 | 420 kWh | Close to the model. A normal, healthy unit. |
| Garage refrigerator | 34.8 kWh | 907 kWh | $166 | 710 kWh | High even for a garage unit. Check the door seal and the coils before anything else. |
| Dehumidifier | 52 kWh | 1,356 kWh | $249 | 1,244 kWh | Running far more than the model assumes, which means the space is wetter than typical. Fix the water, not the air. |
The comparison against the model is the useful part. A reading close to it means the appliance is behaving normally and the way to spend less is to use it less. A reading well above it means something is wrong with that specific unit — a seal, a coil, a thermostat, a duty cycle driven by the room it sits in — and that is a repair, not a replacement.
What a usable meter has to do
Most of what is sold as an “energy meter” will not answer the question. Four requirements, in order of how often their absence ruins a measurement:
- It must accumulate kilowatt-hours. A display showing instantaneous watts is useless on anything that cycles — the reading depends entirely on whether the compressor happened to be running when you looked. You want a running total that survives being left alone for a fortnight.
- It must keep the total through a power cut. A meter that resets on interruption will silently discard a week of measurement and show you a plausible smaller number, which is worse than showing nothing.
- It must be rated above the load. Most plug meters stop around 15 A. A space heater sits close to that, and a kettle can exceed it. Check the meter’s rating before plugging in anything with a heating element.
- It should log over time, not just total. A total tells you the cost. A log tells you the pattern — and a pattern is what reveals a heater cycling at three in the morning, or a pump running when the house is empty.
The measurement that needs no equipment
Before buying anything, there is a free version that answers the biggest single question. Read your electricity meter last thing at night, then again first thing in the morning, having changed nothing. Divide the difference by the hours elapsed and you have your average overnight draw in kilowatts.
Multiply that by 8,760 and you have the annual consumption of everything in your house that never switches off. At 18.34¢ per kWh, each watt of it is $1.61 a year. If the figure is small, no meter is going to find you anything and you can stop here — which is the outcome this site would rather you reach cheaply than expensively.
What you cannot meter this way
Anything hard-wired: your water heater, central air conditioning, baseboard heaters, a well pump, an EV charger, and most electric ovens and cooktops. Between them those are the majority of a large bill, which is the honest argument for a panel-level monitor over a plug meter.
What to buy, if anything
A plug-in energy meter that accumulates kilowatt-hours — not one that only shows instantaneous watts, which on a cycling appliance tells you nothing except what it happened to be doing when you looked. They are widely sold; plug-in energy meters and monitoring smart plugs covers both the dumb meters and the logging smart plugs. Check the current rating before metering anything with a heating element: most top out around 15 A and a space heater is close to that.
Turning the number into a decision
A measurement on its own is trivia. The question it should answer is whether replacing the appliance pays. Take the difference in annual running cost between what you have and what would replace it, and divide the purchase price by that difference. That is the payback in years, and if it is longer than the replacement will last, the answer is no.
Questions
How long should I really leave it on?
My meter shows a different wattage every time I look. Is it broken?
Can I use a smart plug instead of a dedicated meter?
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