Watt-Hour Calculator

What a watt-hour measures

A watt-hour is energy: one watt of power held for one hour. A 60 watt laptop running for 4 hours uses 240 watt-hours, and the same laptop for 8 hours uses 480. Power tells you how fast energy moves; watt-hours tell you how much moved.

The unit steps in powers of 1,000. One kilowatt-hour is 1,000 watt-hours, which is the unit on an electric bill, and one megawatt-hour is 1,000 kilowatt-hours. A single watt-hour is small: a modern LED bulb at 10 watts takes 6 minutes to use one.

Because a watt is one joule per second, a watt-hour is 3,600 joules. That bridge matters when a spec sheet quotes joules, BTU or calories. The NIST guide to SI units defines the watt as a derived unit of power and lists the accepted non-SI energy units beside it.

The unit is also the one behind the meter. Electric meters count kilowatt-hours, utilities price in kilowatt-hours, and appliance labels quote watts. Converting one to the other is the whole job, and it is why a 1,500 watt heater for one hour and a 100 watt fan for 15 hours both show up as 1.5 kilowatt-hours on the bill.

From watts to watt-hours, and on to every other unit

Four relations cover almost every question a household asks. You get watt-hours by multiplying power in watts by time in hours. Kilowatt-hours are watt-hours divided by 1,000. Joules are watt-hours multiplied by 3,600. Cost is kilowatt-hours multiplied by the price per kilowatt-hour.

The same energy is also sold as milliamp-hours, which needs a voltage to become watt-hours. Multiply milliamp-hours by volts and divide by 1,000, so 20,000 mAh at 3.7 volts is 74 watt-hours. Skip the voltage and the number describes charge rather than energy, which is why two power banks with the same mAh label can hold different amounts of energy.

The BTU figure comes from the definition of the BTU as 1,055.06 joules, so one watt-hour is 3,600 / 1,055.06, which is 3.412 BTU. Calorie conversions work the same way: 1 watt-hour is 860 calories, and a food Calorie is a kilocalorie, so 240 watt-hours is 206 food Calories.

Watt-hours are the fair way to compare devices, because they fold power and time into one number. A 1,200 watt hair dryer for 10 minutes uses 200 watt-hours, which is less than a 60 watt laptop for 4 hours at 240 watt-hours, despite drawing twenty times the power while it runs.

Worked example: a laptop over a month

A 60 watt laptop used 4 hours a day is the default case in the calculator. Energy per day is 60 x 4 = 240 watt-hours. Over 30 days that is 240 x 30 = 7,200 watt-hours, or 7.2 kilowatt-hours.

Price turns energy into money. At the 18.19 cents per kilowatt-hour average that the Energy Information Administration reports for United States residential customers in the first seven months of 2026, 7.2 x 0.1819 = $1.31 for the month. A full year at the same daily use is 240 x 365 = 87,600 watt-hours, which is 87.6 kilowatt-hours and $15.93.

Hours move the answer more than watts in most homes, because small loads run for long stretches. Halving the laptop hours to 2 a day cuts the month from 7,200 to 3,600 watt-hours and the bill from $1.31 to $0.65. Doubling the wattage and halving the hours leaves the energy where it started, which is the tradeoff worth remembering when you shop for a replacement.

Worked example: a power bank in watt-hours

Power banks are labelled in milliamp-hours, and the label only becomes energy once you add the cell voltage. A 20,000 mAh bank built on 3.7 volt cells holds 20,000 x 3.7 / 1,000 = 74 watt-hours. A phone with a 4,000 mAh battery at 3.8 volts holds 15.2 watt-hours.

The useful number is smaller than the division suggests. A USB power bank converts its cell voltage to 5 volts, and that conversion plus cable loss throws away roughly 15 percent. So 74 x 0.85 = 62.9 watt-hours reaches the phone, and 62.9 / 15.2 = 4.14 full charges is the realistic count, not the five or more the label implies.

Runtime runs the same formula in reverse. A 500 watt-hour portable power station running a 60 watt laptop lasts 500 / 60 = 8.3 hours.

Laptop packs are labelled the same way. A 50 watt-hour pack at 11.1 volts works out to 4,505 mAh, since 50 x 1,000 / 11.1 = 4,505. The same 50 watt-hours at 3.7 volts is 13,514 mAh, because a lower voltage means more charge for the same energy. Read the watt-hour figure when you compare two laptops, and reach for mAh only when the voltages match.

Watt-hours on your electric bill

The meter records kilowatt-hours, and every appliance in the house adds to that number. The Energy Information Administration reports an average United States residential customer bought 10,491 kilowatt-hours in 2025, which is about 874 kilowatt-hours a month. At 18.19 cents, that month costs roughly 159 dollars before fixed charges.

One watt-hour is a tiny purchase. At 18.19 cents per kilowatt-hour, a watt-hour costs 0.01819 cents, so a 240 watt-hour laptop day costs 4.4 cents. Devices that run all day at low power still add up: a router drawing 10 watts uses 240 watt-hours a day, the same as the laptop in the example.

The price you pay includes delivery, taxes and sometimes a fixed customer charge, so the all in rate on your bill is the number to use. State averages in the EIA monthly price table run from about 13 cents to about 35 cents per kilowatt-hour, which is why a national average is only a starting point.

Time of use plans and tiered rates break the single price assumption. On a plan that charges more from 4 pm to 9 pm, the same 240 watt-hours costs more at dinner than at noon, and the calculator has no way to know your schedule. Use the average rate for a rough figure and the peak rate for the worst case.

Why the label and the meter disagree

A nameplate rating is the highest draw a device can pull, not what it pulls minute to minute. A refrigerator rated 700 watts may average 150 watts because the compressor cycles, and a laptop charger rated 65 watts may sit near 40 while the battery is full.

Batteries lose energy in three places: conversion, heat and the reserve a battery keeps to avoid damage. Lithium packs give up about 80 to 90 percent of their label before the voltage sags, lead-acid about half. Inverters lose another 5 to 15 percent when they turn direct current into household alternating current.

Standby loads are the other gap. A television that draws 0.5 watts off and 100 watts on, watched 4 hours a day, uses 480 watt-hours for the show and 10 watt-hours standing by. Small, but it is the reason a meter keeps moving when nothing seems to be on.

A plug in energy meter settles most arguments. It sits between the outlet and the appliance, logs watt-hours over a week, and reports the total. For a refrigerator or a window air conditioner, a week of logged watt-hours beats any nameplate figure, because both cycle on and off all day.

What the calculator returns

The default inputs are 60 watts, 4 hours a day, 30 days, 18.19 cents per kilowatt-hour, a 20 watt-hour battery and 3.7 volts.

Change the watts and every energy row moves with it, because energy is linear in power. Change the hours and the same happens. The battery rows depend on the pack, not the appliance, so a 100 watt-hour pack doubles the runtime of a 20 watt-hour one and leaves the energy rows alone.

Two habits keep the numbers useful. Use running watts rather than nameplate watts when you have both, since a cycling appliance never holds its peak. And keep the price field in cents per kilowatt-hour, the unit printed on a United States bill, so the cost rows match the number you can check.