What is a kilowatt-hour? The unit on your electric bill, explained

Watts versus watt-hours

The confusion is between a rate and a quantity. A watt is how fast something uses energy right now — a speed. A watt-hour is how much it used over time — a distance. A kettle draws 1,500 watts but runs for three minutes, so it uses 75 watt-hours: a lot of power, not much energy. A refrigerator draws 150 watts but runs eight hours a day, so it uses 1,200 watt-hours: little power, a fair amount of energy. The bill only sees the second number. Nothing on it cares how fast you used electricity, only how much — with one exception, demand charges, which a separate guide covers.

A kilowatt is a thousand watts. A kilowatt-hour is a thousand watt-hours. The prefix is there because household quantities would be awkward otherwise: a typical American home uses around 10,000 kWh a year, which would be ten million watt-hours.

What things use

Rough figures, because every appliance and every household differs, but the order of magnitude is what matters:

Two things stand out. The first is that anything that makes heat or cold — water heater, air conditioner, dryer, space heater, oven — uses ten to a hundred times more than anything that makes light or information. Turning off lights is good manners and bad economics. The second is that the car, if there is one, is the single largest movable load in the house, which is why it is the load time-of-use plans are designed around.

Turning kilowatt-hours into dollars

Take the rate from your plan and multiply. At 15¢ per kWh, the dryer load above costs about 35¢, the water heater around $1 a day, and the air conditioner on a hot day about $4. At 35¢ — a peak rate on a steep time-of-use plan — the same hot day is nearly $10. That is the whole argument for reading the rate rather than the average: the kilowatt-hours are the same, and the price is not.

Every rate page on this site shows an effective price per kilowatt-hour for three household sizes, which is the bill divided by the use — fixed charge included. It is a better number than the headline rate because it is the one you actually pay per unit. The median fixed charge in our index is about $205 a year, and on a small apartment that alone can add several cents to every kilowatt-hour.

Reading it off the bill

The bill shows two meter readings and the difference between them, in kWh. That difference is your use for the period. Divide it by the number of days for a daily figure, which is the only version worth comparing month to month, since billing periods vary in length. A one-bedroom apartment might see 8 to 15 kWh a day; a family house with central air, 25 to 40 in summer; a house with electric heat, 60 or more on a cold day. If your daily figure is well outside those, something is either very efficient or very wrong, and the high-bill checklist is the next stop.