How do I work out what an appliance costs to run?

Power is a rate, energy is a quantity

Almost every confusion about electricity bills comes from treating these two as the same thing, and separating them makes the rest straightforward.

A watt is a rate of energy use — energy per second. It describes how hard something is working at a given moment, not how much it has consumed. A 2,000 watt kettle draws 2,000 watts whenever it is boiling and nothing when it is not.

A kilowatt-hour is a quantity of energy: one kilowatt sustained for one hour. It is what a meter counts and what a bill charges for. The unit is a rate multiplied by a time, which is why the name contains both.

The calculation follows directly. Energy in kilowatt-hours equals power in kilowatts multiplied by hours of use. Cost is that multiplied by the price per kilowatt-hour. A 2,000 watt kettle is 2 kilowatts; boiling for three minutes is 0.05 hours, so 0.1 kilowatt-hours, which at 30 cents a unit is three cents.

The useful consequence is that wattage alone tells you almost nothing about cost. The kettle is one of the highest-wattage things in a house and costs very little to use, because it runs for minutes. A 100 watt device left on permanently uses 2.4 kilowatt-hours a day — twenty-four times the kettle boil — while drawing a twentieth of the power. Time is the multiplier people leave out.

What actually dominates a bill

Attention tends to land on the wrong appliances, because visible power is more salient than sustained use.

Anything that heats or cools dominates, essentially without exception. Space heating, water heating, air conditioning, tumble dryers and ovens convert electricity to heat, and heat is expensive because it takes a great deal of energy to change the temperature of anything substantial. In most homes with electric heating these categories together account for the majority of consumption.

Anything that runs continuously comes second, and this is where the surprises live. A refrigerator draws modestly but never stops, and its annual consumption exceeds most occasional appliances by a wide margin. So does an aquarium heater, a dehumidifier, or a desktop computer left running.

Everything else is usually a rounding error. Lighting was once significant and largely stopped being so when LEDs replaced incandescent bulbs — a change from 60 watts to 8 for the same light is nearly a ninety percent reduction on a category that ran many hours a day. Phone chargers, which attract a disproportionate share of the anxiety, draw well under a watt when idle and cost a few cents a year.

Standby power is worth quantifying rather than worrying about. Modern regulation caps standby draw for most equipment at around half a watt to a watt, so a device left on standby costs a small number of dollars a year at most. The exceptions are older equipment and anything that is not truly idle — a set-top box recording, a games console downloading, or a networked device staying awake can draw ten or twenty watts continuously, which is a real cost.

The practical method is to rank by watts multiplied by hours rather than by watts. That ordering is frequently the reverse of intuition, and it points at the few changes worth making.

Reading the numbers on the appliance

Rated wattage is printed on a label or in a manual, and it is a maximum rather than an average, which matters for anything that cycles.

A refrigerator rated at 150 watts does not draw 150 watts continuously. Its compressor runs in cycles, perhaps a third of the time depending on ambient temperature and how often the door opens, so actual consumption is far below the rated figure multiplied by twenty-four hours. The same is true of anything thermostatically controlled: heaters, ovens, water heaters and air conditioners all cycle, and the rated wattage describes the draw while running.

For these, the annual consumption figure on an energy label is the more useful number, because it is measured over a standardised cycle rather than derived from the rating. Where a label gives kilowatt-hours per year, dividing by 365 gives a daily figure directly.

Where no reliable figure exists, a plug-in energy monitor measures actual consumption over time and settles the question. They are inexpensive, and for anything suspected of being a large sustained load the measurement is worth more than any estimate.

One further wrinkle applies to motors and to power factor. Devices with large motors draw a brief surge on startup that can be several times the running current, which matters for sizing circuits and generators but not for energy bills, since the surge lasts a fraction of a second. Domestic meters bill real power, so power factor generally does not appear on a household bill, though it does on commercial tariffs.

Horsepower, and why it still appears

Motor and engine ratings frequently arrive in horsepower, and converting is straightforward once the variants are pinned down.

Mechanical horsepower, the one used in English-speaking countries, is defined as 550 foot-pounds per second and works out to about 745.7 watts. So one horsepower is roughly three quarters of a kilowatt, and a kilowatt is about 1.34 horsepower. A 3 horsepower pump is about 2.24 kilowatts.

Metric horsepower is a slightly different unit, about 735.5 watts, defined as the power to lift 75 kilograms one metre per second. It appears in European vehicle specifications under names like PS or CV, and the roughly one and a half percent difference from mechanical horsepower is why the same car is sometimes quoted at two slightly different horsepower figures.

The unit exists because James Watt needed to sell steam engines to people who owned horses, and expressing an engine in the number of animals it replaced was the persuasive comparison. That it survives in an industry now measuring everything else in metric units is a matter of familiarity rather than utility.

For running costs, the conversion matters when a motor is rated in horsepower and the electricity is billed in kilowatt-hours. Note that the horsepower figure for a motor is usually its *output* power, while the electricity consumed is the input — and a motor is not perfectly efficient. A 1 horsepower motor at eighty percent efficiency draws about 0.93 kilowatts, not 0.75, so estimating consumption from the output rating alone understates it.

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