Duty cycle: the number that makes every appliance estimate wrong

What the number on the label means

The rating on an appliance is its power draw while it is drawing — the load the circuit must be able to supply. It is an electrical safety figure, and for sizing a breaker it is exactly the right one. It was never intended to describe average consumption, and for anything thermostatic it does not.

Duty cycle is the fraction of its running time that the appliance is actually pulling that load. A refrigerator whose compressor runs 20 minutes in every hour has a duty cycle of about 0.33. A resistance heater holding a room at temperature might sit around 0.5. An electric kettle, which draws full power until it boils and then stops, is 1.0 — and for a kettle, nameplate arithmetic is correct.

The size of the error

Note the last two rows. A kettle and a television have no thermostat to cycle them, so both methods agree. The error is not universal — it is concentrated in exactly the appliances that dominate a bill, which is what makes it expensive.

Every appliance on this site, both ways

The full table. “Rated” is the plate; “real average” is the draw averaged over the hours the appliance is in service; “overstated by” is how far nameplate × hours misses. Sorted by the size of the error, because that is the interesting axis.

Why so many sites get it wrong

Because nameplate times hours is a calculation you can do without knowing anything about the appliance, and it produces a large, alarming number that reads as authoritative. Correcting it requires knowing how the appliance behaves, which is work. The result is a genre of article confidently telling people their refrigerator costs three hundred dollars a year to run.

Measuring your own

You do not have to trust anyone’s duty cycle, including ours. Two methods, in increasing order of effort:

1. Watch it. Stand by the appliance with a timer and record how long the compressor or element runs in a ten-minute window. Repeat a few times through the day. That ratio is your duty cycle, and for a fridge it will surprise you.
2. Meter it. A plug-in energy monitor logs actual kilowatt-hours. Leave it on the appliance for a fortnight and multiply by 26. This is the only method that captures defrost cycles, seasonal variation and a failing door seal.
3. Read the whole house. A circuit-level monitor in the panel gives you every circuit at once and settles arguments no single-appliance meter can.

What we use, and why

The duty cycles on this site are engineering values for a stated operating assumption, and each appliance page states that assumption in plain language. Where an appliance carries a regulated annual figure — an FTC EnergyGuide label, or an ENERGY STAR specification — the inputs are set so the computed total falls inside that published range. That is a check anyone can repeat, and it is the reason the numbers here are lower than the ones you will find elsewhere and still not made up.

Questions

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