Watt Hour Calculator

What the watt hour calculator does

A watt-hour is a unit of energy, and it can be found two ways. This calculator does both: it computes energy from voltage and charge, which suits batteries, and it computes watt-hours from power and time, which suits estimating how much energy a device uses over a period.

Below is what a watt-hour is, the two equations behind it, how it relates to battery capacity, and a worked example.

How to use it

1. For battery energy, enter the voltage and the charge, such as a battery's ampere-hour rating.
2. For energy use over time, enter the power and the time.
3. Press Calculate for the energy in watt-hours, or Reset to clear it.

What a watt-hour is

A watt-hour is a unit of energy, the total amount of energy used or stored, as opposed to the rate of use. It represents the energy of one watt of power sustained for one hour. Where the watt measures how fast energy flows, the watt-hour measures how much flows in total over time. This makes it the natural unit for things that store or consume energy over a period: batteries, electricity bills, and the running of appliances. A device using a steady power over several hours racks up a number of watt-hours equal to the power times the time.

The distinction between power and energy is worth keeping clear. Two devices might draw the same power, but if one runs far longer, it uses far more energy and so more watt-hours. Equally, a high-power device used briefly might consume less total energy than a low-power one left on for days. The watt-hour captures this total, which is what actually drains a battery or shows up on a bill. This calculator finds watt-hours in the two situations that arise most often, from a battery's voltage and charge, and from a device's power and running time.

The two equations it uses

The calculator uses two simple relationships. For a battery, the stored energy is the voltage multiplied by the charge:

energy = voltage × charge

When the charge is given in ampere-hours and the voltage in volts, this comes out directly in watt-hours. For energy used over time, the relationship is power multiplied by time:

watt-hours = power × time

with power in watts and time in hours. Both are expressions of the same idea, that energy is power sustained over time, or equivalently voltage driving charge. The calculator handles each independently, so you can use whichever matches what you know, and it converts the units so the answer lands in watt-hours either way.

Battery capacity and runtime

Batteries are often rated in ampere-hours, a measure of how much charge they can deliver, but ampere-hours alone do not tell you the energy, because energy also depends on the voltage. A battery's energy in watt-hours is its charge in ampere-hours times its voltage, which is why two batteries with the same ampere-hour rating but different voltages store different amounts of energy. Converting to watt-hours puts batteries on a common footing and reveals their true energy content.

This matters for estimating runtime. If you know the energy a battery holds in watt-hours and the power a device draws in watts, dividing one by the other tells you roughly how many hours the device will run. A larger watt-hour figure means longer running time for the same load. This is the everyday calculation behind sizing a battery for a project, planning how long a power bank will keep a device alive, or comparing the real capacity of different cells. The calculator gives the watt-hour figure that makes these estimates possible.

The kilowatt-hour on your bill

The watt-hour's larger cousin, the kilowatt-hour, is the unit electricity companies use to bill for energy. One kilowatt-hour is a thousand watt-hours, the energy of a one-kilowatt appliance running for an hour. When you see your electricity usage measured in kilowatt-hours, it is simply the total energy your home has consumed, the sum of every appliance's power multiplied by how long it ran.

This is why both the power of your devices and how long you use them affect your bill. A powerful appliance used briefly and a modest one left on continuously can add up to similar amounts of energy and similar cost. Understanding watt-hours and kilowatt-hours lets you see where your energy goes and make sense of what you are paying for. The calculator works in watt-hours and kilowatt-hours alike, so you can move naturally between small electronics and household-scale energy, all in the same consistent unit of energy.

Units and precision

The calculator takes voltage from nanovolts to megavolts, charge in coulombs or ampere-hours, power in watts and its multiples, and time from milliseconds to years, returning energy in joules, watt-hours, kilowatt-hours, and beyond. The default units of volts, ampere-hours, watts, and hours suit most battery and appliance work. The relationships are exact, and the unit conversions handle the bookkeeping so the energy lands in the unit you choose.

A worked example

Suppose you have a 12-volt battery rated at 100 ampere-hours.

Its stored energy is voltage times charge, 12 × 100 = 1,200 watt-hours, or 1.2 kilowatt-hours. Separately, suppose a 60-watt device runs for 5 hours: the energy it uses is power times time, 60 × 5 = 300 watt-hours. Putting the two together, that 1,200-watt-hour battery could in principle run the 60-watt device for about 20 hours, since 1,200 divided by 60 is 20.

Questions people ask

How do you calculate watt-hours?

Two ways: multiply power by time (watts times hours), or for a battery, multiply voltage by charge (volts times ampere-hours). Both give the energy in watt-hours.

How do I find a battery's energy in watt-hours?

Multiply its voltage by its charge in ampere-hours. A 12-volt, 100-ampere-hour battery holds 1,200 watt-hours, because ampere-hours alone ignore the voltage.

How long will a battery run a device?

Divide the battery's watt-hours by the device's power in watts. A 1,200-watt-hour battery running a 60-watt device lasts roughly 20 hours.

What is a kilowatt-hour?

A thousand watt-hours, the unit on your electricity bill. It is the energy of a one-kilowatt appliance running for an hour, the total energy consumed.

References

A quick note on where the physics comes from. Electrical energy, the watt-hour, and battery capacity are standard physics, set out in OpenStax's University Physics and in Georgia State University's HyperPhysics. The units follow NIST. The HyperPhysics link is worth a quick click to confirm it lands where you expect.

1. OpenStax, University Physics Volume 2, Section 9.5, Electrical Energy and Power. https://openstax.org/books/university-physics-volume-2/pages/9-5-electrical-energy-and-power
2. HyperPhysics, Electrical Energy. http://hyperphysics.phy-astr.gsu.edu/hbase/electric/elepow.html
3. National Institute of Standards and Technology (NIST), SP 811, Guide for the Use of the International System of Units. https://www.nist.gov/pml/special-publication-811