The watt-hour (Wh) is a measurement of electrical energy, quantifying the total energy consumed or work performed by an electrical device over time. This unit is important for consumers because it relates directly to the capacity of portable device batteries and the charges found on monthly utility bills. The watt-hour provides a standardized metric for assessing energy use and storage.
Defining the Watt-Hour (Wh)
A watt-hour is defined as the amount of energy consumed by a device operating at a constant power of one watt for one continuous hour. This unit is a product of power and time, represented by the physical relationship: Energy = Power × Time. The power component, measured in watts (W), indicates the rate at which energy is being used at any given moment. Multiplying this rate by the duration of use in hours (h) yields the total energy consumed in watt-hours (Wh). This calculation is a clear measure of accumulated usage. The watt-hour is a universally accepted unit for measuring electrical energy, though it is not a standard unit in the International System of Units (SI). One watt-hour is equivalent to 3,600 joules of energy, the SI unit for work and energy.
Distinguishing Watt-Hours from Watts
The distinction between a watt (W) and a watt-hour (Wh) is the difference between a rate and a total amount. A watt is a unit of power, measuring the rate of energy consumption at a single point in time. The watt rating on an appliance is similar to a car’s speedometer reading, indicating how fast energy is flowing to the device. In contrast, the watt-hour is a unit of energy, representing the cumulative total of that consumption rate over a duration. Using the car analogy, the watt-hour is comparable to the distance traveled, or the reading on the odometer. A device’s wattage determines how quickly it uses energy, but the watt-hour measures the final volume of energy expended.
Real-World Measurement and Use
The watt-hour unit is routinely scaled up for practical purposes, most commonly into the kilowatt-hour (kWh). Utility companies use the kilowatt-hour as the standard unit for measuring the total electricity consumed by a household or business over a month. The electric meter installed at a property tracks this cumulative consumption, and the resulting kWh total forms the basis for the monthly electric bill.
Battery Capacity
Watt-hours are prominently used to describe the capacity of batteries in portable devices and electric vehicles. The capacity rating, expressed in watt-hours, indicates the total amount of energy the battery can store. This metric is particularly useful for comparing different batteries because it accounts for both the voltage and the current capacity, providing a complete picture of the total energy available. A higher Wh rating means the battery can power a device for a longer period of time. For small electronics, battery capacity may sometimes be listed in milliamp-hours (mAh), but watt-hours offer a more accurate comparison across devices with different internal voltages. Knowing the watt-hour capacity allows a user to estimate run-time by dividing the battery’s Wh rating by the device’s average power consumption in watts.