Electricity Calculator

The calculator above turns an appliance's power rating into what it actually costs to run, over a day, a week, a month and a year. Pick an appliance from the list or enter your own figures, set how hard it works and how long it runs, and put in the price you pay per kilowatt-hour.

The four numbers it needs

Appliance power is the rating on the label or in the manual, in watts unless you change the unit. Use/run at is the share of that rating the appliance really draws, which for most things is well under 100%. Usage is how long it runs, in whichever period is easiest to think in. Electricity price is the per kilowatt-hour rate from your bill.

Getting the capacity figure roughly right matters more than getting the wattage exactly right. A 3,500 W air conditioner does not pull 3,500 W all afternoon; it pulls that while the compressor runs and much less once the room is at temperature. Multiplying the label rating by the hours the unit is switched on gives a number that can be double the real one.

Watts, kilowatts and kilowatt-hours

A watt is a unit of power, the rate at which energy moves, defined as one joule per second. A kilowatt is 1,000 watts. A kilowatt-hour is a unit of energy: the amount used by running one kilowatt for one hour.

Power and energy are related through time, and the two formulas are all the arithmetic here really is:

P = E / t    and    E = P × t

Your meter counts energy, not power, which is why the bill is in kilowatt-hours. A 100 W bulb left on for ten hours uses 1 kWh, and so does a 2,000 W kettle run for half an hour. Utilities use kilowatt-hours rather than watt-hours purely for scale: a typical US home uses roughly 10,500 kWh a year, and writing that as 10,500,000 watt-hours would help nobody.

BTU and BTU per hour

A British thermal unit is a measure of heat, the amount needed to raise one pound of water by one degree Fahrenheit. Because heat is energy, it converts directly into kilowatt-hours.

1 BTU = 0.2931 watt-hours, so 1 kWh is about 3,412 BTU.

BTU/h is the rate, and it is where the confusion starts, because air conditioners are almost always labelled in plain BTU when they mean BTU per hour. A unit sold as 12,000 BTU is moving 12,000 BTU every hour. That is roughly 3,517 W of heat, which is not the same as the electricity it consumes, since an air conditioner moves several units of heat for every unit of electricity it draws.

Horsepower and tons

Horsepower turns up on motors and pumps. There are two definitions in common use and they differ by a little over 1%: mechanical horsepower is 745.7 W and metric horsepower is 735.5 W. The term came from James Watt, who compared his steam engines to the draft horses they replaced by measuring how many turns of a mill wheel a horse managed in an hour.

A ton, in this context, has nothing to do with weight. A ton of refrigeration is the rate of cooling needed to melt one short ton of ice at 0 degrees Celsius in 24 hours, which works out at about 3,517 W or 12,000 BTU/h. It is mostly a US convention for sizing air conditioning.

Unit conversions

Unit	In watts	Note
1 kilowatt (kW)	1,000 W	the everyday unit for appliance power
1 BTU/hour	0.2931 W	1 BTU = 0.2931 watt-hours
1 metric horsepower	735.5 W	used for motors outside the US
1 mechanical horsepower	745.7 W	the US definition
1 ton of refrigeration	3,517 W	the same as 12,000 BTU/h
1 kilowatt-hour (kWh)	3,412 BTU	a unit of energy, not power

Typical appliance wattages

These are label ratings, which is to say maximums. Real usage is usually well below them, so multiplying a figure from this table straight by the hours in a day will overstate the bill.

Common appliances	Estimated wattage (W)
Home appliances
Air conditioner (HVAC)	2,500 - 10,000
Air conditioner (window unit)	1,500 - 5,000
Heater (home)	5,000 - 20,000
Heater (portable)	750 - 2,000
Humidifier	25 - 350
Dehumidifier	200 - 750
Fan (ceiling, table)	15 - 200
Light bulb (LED)	3 - 25
Light bulb (incandescent)	15 - 200
Electric water heater	3,000 - 6,600
Kitchen appliances
Refrigerator	500 - 1,000
Electric range/oven	2,000 - 5,000
Electric cooktop/stove	750 - 5,000
Microwave oven	750 - 1,500
Dishwasher	1,200 - 2,000
Coffee maker	600 - 1,200
Toaster	750 - 1,500
Electric kettle	1,000 - 2,000
Electric cooker	160 - 1,500
Other appliances
Electric vehicle charger	1,500 - 20,000
Television	25 - 500
Clothes washer	400 - 1,500
Clothes dryer	1,800 - 5,000
Clothes iron	750 - 2,000
Hair dryer	750 - 2,000
Desktop computer	100 - 250
Laptop computer	35 - 150
Phone charger	5 - 25
Water pump/motor	750 - 2,000

Which appliances actually cost you money

Two things drive cost: how much power something draws and how many hours it runs. Neither alone tells you much.

A hair dryer rated at 1,500 W sounds expensive and is not, because it runs for ten minutes at half power: about 0.13 kWh a day, or 57 cents a month at 15 cents. A refrigerator draws far less at any moment but never stops, and at 750 W with the compressor cutting in a fifth of the time it uses 3.6 kWh a day, which over a year is nearly thirty times the hair dryer's total.

The reliable rule is that anything which makes heat or moves it is where the money goes. Heating, cooling, water heating and clothes drying dominate a typical bill. Electronics and lighting, which people worry about most, usually add up to a small share of it.

Standby power is the quiet exception. A device drawing 3 W does nothing noticeable per hour but runs 8,766 hours a year, which is about 26 kWh. One device is loose change; fifteen of them is a real line on the bill.

Six ways to cut the bill

1. Track before you change anything. Run the calculator on your five biggest appliances before assuming you know where the money goes. People routinely guess wrong, switching off a television that costs a few dollars a year while an old water heater quietly runs up hundreds.
2. Replace incandescent bulbs with LED. A 60 W incandescent and a 7 W LED put out about the same light. At eight hours a day and 15 cents that is $26.30 a year against $3.07 for one fitting, and the LED lasts many times longer.
3. Get the thermostat under control. Heating and cooling are usually the largest single share of a bill, and every degree of setback saves. A programmable thermostat helps, but so does the habit of setting it back manually before bed and before leaving the house.
4. Weigh running cost when you buy. A cheap appliance that runs daily for a decade can cost more over its life than an efficient one that costs more up front. Put both figures side by side before deciding.
5. Deal with windows and insulation. Heat escapes through glass in winter and pours in through it in summer. Curtains, blinds and draught-proofing are cheap; better glazing and loft insulation cost more and pay back over years rather than months.
6. Shift load to cheaper hours if your tariff has them. On a time of use plan, running the dishwasher, the washing machine or the car charger overnight can cut those items' cost substantially without changing anything about how you live.

Reading your result

The yearly figure is the one to act on. Daily costs on a single appliance are small enough to feel harmless, which is exactly why they are easy to ignore: 40 cents a day is $146 a year.

Compare the yearly kilowatt-hours against your own bill rather than against an average. If you add up the appliances you run and the total lands well under what the meter says, the gap is usually heating, hot water or something running that you have not thought of.

Be careful with the price field too. Many bills carry a fixed daily standing charge on top of the per-unit rate, and some have tiered rates that rise once you pass a threshold. The rate to enter here is the marginal one, meaning what the next kilowatt-hour costs you, since that is what changes when you run something for another hour.

To check the current a circuit draws before adding a large appliance to it, use the Ohms Law Calculator, and for cable sizing on a long run the Voltage Drop Calculator covers the losses in the wiring itself.