Measuring What an Appliance Really Uses

A label tells you what the maker printed. It doesn't tell you what your fridge does on a hot afternoon or what your laptop does while it's charging. For a budget that holds up, you need the real figure.

The wattage on a label is a starting point. Real draw depends on the setting you use at the time, such as a fan speed¹. And labels often list one number for something that changes all day.

Two ways to measure

Start with the plug-in meter. A wattmeter measures electric power in watts, and a meter that also adds up energy over time is essentially a wattmeter that integrates the power². That's the number you want, because your budget is in watt-hours.

Using a plug-in meter

• Plug the meter into the outlet, then plug the appliance into the meter.
• Run the appliance the way you normally do. A kettle full, a fan on its usual speed.
• Note the watts shown while it's working.
• Leave it in place for a full day if it cycles on and off, like a fridge. Read the watt-hours at the end.
• Do the same with it switched off but still plugged in.

That last step matters. Some appliances draw power while "off". Older devices with large power adapters that feel warm to the touch use several watts³. A quick check with the meter shows you which ones.

One caution: cheap meters can't see tiny loads. In one inexpensive model's manual, the lowest measurable current is 0.02 A, which works out to about 5 W at 230 V AC². A phone charger idling may read zero even though it's drawing something. Check your own meter's manual for its limits.

Using a clamp-on ammeter

If an appliance has no wattage on it, you can estimate watts by finding the current in amps and multiplying by the voltage the appliance uses¹. The extension guide describes a clamp-on ammeter as an electrician's tool, with a reading taken while the device is running¹. It also says to make sure you know how to use the ammeter safely and to follow the maker's recommendations¹.

A clamp ammeter only works on a single conductor. On an ordinary appliance cord you can't isolate one conductor safely without a purpose-made adapter or opening the cord, so leave that to a licensed electrician. For appliances, use the plug-in meter instead.

Don't cut, strip or open any wiring to get a wire to clamp. Don't open the casing of a mains appliance either. And fixed wiring in a building, such as a switchboard or a circuit behind a wall, is licensed electrical work in Australia. Leave it to a licensed electrician.

Motors start hard

Fridges, pumps and power tools have motors, and a motor pulls more when it starts than when it runs. In the guide's words, the meter will show about three times more current in starting the motor than when it is running smoothly¹.

That matters in two ways. A plug-in meter's top watt reading can look scary, but the number to put in your budget is the running average over time. The short surge only matters when you choose an inverter that has to handle it.

Measuring on the battery side

You can also measure what the battery delivers, using a battery monitor with a shunt (a small, precise resistor that the monitor reads to count current). Fitting one means connecting or disconnecting battery cables. Isolate the battery first. If the monitor's instructions leave you unsure, hire a licensed electrician. This isn't a routine DIY job.

A battery-side reading also catches the inverter's own use, which a plug-in meter on the appliance side can't see. The page on losses inside the inverter explains the gap.

What to write down

For each appliance, record:

• watts while running
• watts on standby or "off"
• hours used on a normal day
• watt-hours over a full day, if you can get them

Then go back to your budget and swap the label figures for these.