How to find out how many watts your appliances use
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Every backup power plan starts with a list of appliances and how much power each one uses. Typical figures are a starting point, but your own appliances can be very different. Here is how to get the real numbers.
1. Read the label
Most appliances have a rating plate or sticker on the back, underneath, inside the door or on the power adapter. Look for:
- W or kW: the power, ready to use.
- V and A (volts and amps) without watts: multiply them. A 230 V appliance rated 0.5 A uses up to about 115 W. A 120 V appliance rated 2 A uses up to about 240 W.
- A range of voltages such as “100–240 V” on chargers and laptops: the watts on the adapter are what count.
The label shows the maximum draw. Many appliances use less most of the time, so the label gives you a safe, slightly cautious figure.
2. Measure it
A plug-in power meter (sometimes called an energy monitor or kill-a-watt style meter) sits between the socket and the appliance and shows the watts it is using right now, and the energy it uses over a day. It is the best way to measure fridges, freezers and anything whose use changes over time.
Leave it on a fridge for a full 24 hours and read the kWh figure: that is the fridge’s real daily energy use, which you can enter in the calculator as watts × hours.
3. Fridges, freezers and other cycling appliances
A fridge’s compressor switches on and off to hold the temperature. It might draw 150 W when running, but only run for 8 to 12 hours across a whole day, depending on the room temperature, how full it is and how often the door opens.
That’s why the calculator lists a fridge at 150 W for about 10 hours, not 24. In a hot kitchen, or with an old fridge, it may run more.
4. Starting surges
Anything with a motor or compressor draws far more power for a moment when it starts:
- fridges and freezers
- air conditioners
- water pumps, well pumps and sump pumps
- washing machines
- some power tools
The surge can be three times the running power or more, for a fraction of a second to a few seconds. Your inverter must cope with it, which is why the calculator adds a starting surge for the largest motor. “Inverter” type air conditioners and fridges, with variable-speed compressors, start much more gently.
5. Standby power
Many devices draw a few watts even when “off”: TVs, decoders, set-top boxes, chargers left plugged in, microwaves with clocks. Individually small, together they can add 20 to 50 W running all day, which adds up on a battery. Switch them off at the socket during outages.
6. The big users to watch
These appliances use a lot of power and can make a backup system far bigger and more expensive:
- electric kettles, irons, hair dryers and toasters (1,000–2,500 W, but only for minutes)
- space heaters and electric water heaters (1,500 W and up, often for hours)
- air conditioners (roughly 900–1,500 W each for common home sizes)
- electric cookers and ovens
Short bursts from a kettle or iron mostly affect inverter size. Long running from heaters and air conditioners mostly affects battery and solar size. Deciding which of these you truly need during an outage is the single most effective way to cut the cost of a system.
Put your real numbers into the calculator: every watt figure can be edited.