What a device costs to run, once duty cycle is in it

The arithmetic for running cost looks settled. Watts, divided by a thousand, times hours, times the rate per kilowatt hour.

For a lamp that is exactly right. For the appliances that actually drive a bill, it is wrong by a factor that depends on something the formula never asks about.

Thermostats mean the thing is off half the time

A 2 kW space heater does not draw 2 kW for every hour it is in the room. It draws 2 kW while the element is energised, then the thermostat satisfies and it stops, then it starts again.

The share of the time it is actually drawing power is its duty cycle. For a heater maintaining a comfortable room, something around 60% is typical, so it runs roughly 36 minutes in each hour.

Put that through the arithmetic. Two kilowatts, at 60%, is 1.2 kW of effective draw. Over eight hours that is 9.6 kWh. At 15 cents a unit it is $1.44 a day, or about $43 over a month. Ignore the duty cycle and the same calculation says $2.40 a day and $72 a month, which overstates it by two thirds.

Which things cycle, and roughly how much

Anything with a thermostat or a compressor. Fridges and freezers sit near 40% in a normal room. Air conditioning and heating land around 60% while maintaining a temperature, rising toward continuous on an extreme day. Sump pumps might run 5% of the time, which is why a large motor costs almost nothing to own until the day it matters.

Anything without one runs at 100%. Lighting, televisions, computers, routers, anything that stays on because you left it on. For those the simple formula is correct and the bill is predictable.

The watts calculator converts a nameplate into kilowatt hours and a running cost at your own rate, which is the quickest way to compare two appliances properly. Apply the duty cycle to the hours rather than to the wattage and the answer comes out the same, if that is easier to hold in your head.

Cycling is also why ratings mislead

This is the part that confuses people comparing a 1.5 kW heater with a 2 kW one.

Both are trying to hold the same room at the same temperature, which takes a fixed amount of heat. The larger one gets there faster and then spends more of each hour switched off. Over a day their energy use is far closer than their ratings suggest, because the room’s heat loss sets the total, not the element.

The same logic applies to air conditioning, and it is why an oversized unit is not simply a more expensive way to be comfortable. It cycles in short bursts, which is less efficient and removes less humidity than a longer, steadier run.

Counting the whole house

Once there are more than a handful of appliances, the interesting number stops being any one of them and becomes the total.

Building that list is worth doing once, properly. The home wattage calculator carries typical running figures for common appliances, which saves hunting for nameplates, and the same list doubles as the starting point for generator sizing if you ever need backup power.

A single plug-in energy meter moved around the house for a week will beat every estimate in this article. It measures actual consumption over time, duty cycle included, which is the thing that cannot be looked up because it depends on your room, your habits and your climate.

The rate is rarely the number on the tariff

One last correction that applies to every calculation here. The headline rate per kilowatt hour is usually not what you pay per kilowatt hour.

Standing charges, tiered rates that step up past a threshold, and time of use bands all sit on top of it. Dividing a recent bill’s total by the units consumed gives an effective rate that includes all of it, and using that figure makes these estimates match reality considerably better.