Defaults and assumptions

The defaults, and where they come from

Electricity price: 17 cents per kWh. That is close to the United States residential average. Your own bill is the only number that matters here; the default exists so the calculator shows something sensible before you type.

Duty cycle: the preset for each device. Heaters cycle on a thermostat, fridges cycle on a compressor, kettles and dryers run flat out. The preset is a starting point for a device in normal use, not a measurement of your device.

Hours: a typical day of use. Not the maximum, not the minimum. A household that runs a dryer once a day enters one hour; a household that runs it twice enters two.

What is measured and what is assumed

Measured: nothing. Every number on the page comes from the three inputs. The wattages in the device list are typical label figures for that class of appliance, and a label beats a typical figure every time.

Assumed: that the appliance draws its rated power whenever it is on duty, that the duty cycle is steady across the month, and that the price you enter covers the whole period. All three assumptions fail in specific circumstances, and each device page names the ones that matter for that device.

Accuracy

The arithmetic is exact. The inputs are estimates. A wall meter reading for one day, multiplied out, beats any preset on this site, and takes about as long as reading the label.