A watt-hour (Wh) is not a unit of time or length, but rather a unit of electrical energy. It quantifies the total amount of energy consumed or stored, similar to how a gallon measures a volume of liquid. The term represents the amount of energy used by a device drawing one watt of power continuously for one hour. The core concept is that a watt-hour measures a quantity of energy, while the “hour” component is simply a standard unit of time used in its calculation.
Defining the Watt-Hour: Energy vs. Power
Understanding the watt-hour requires first distinguishing between power and energy. Power, measured in watts (W), describes the rate at which energy is transferred or consumed at any single moment. Think of power as the speed at which a device operates, such as a 60-watt light bulb drawing energy instantly.
Energy, on the other hand, is the total quantity of power used over a specific duration. The relationship is expressed directly: Energy equals Power multiplied by Time. This means one watt-hour is the result of one watt of power being sustained for one hour of time.
To visualize this distinction, consider a car analogy where power (watts) is the car’s instantaneous speed. Energy (watt-hours) would then be the total distance traveled. A faster speed (higher wattage) means you cover more distance (use more watt-hours) in the same amount of time.
A practical example illustrates this relationship: a 100-watt television running for five hours consumes 500 watt-hours of energy. Conversely, a 5-watt LED lamp would need to run for 100 hours to consume the exact same 500 watt-hours.
Calculating Device Run Time
Consumers often ask how long a battery will last, which requires calculating run time based on watt-hours. This calculation is performed by dividing the total stored energy of a battery (Wh) by the power draw of the device (W). The result yields the estimated number of hours the device can operate continuously.
For example, a portable power station with a stored capacity of 500 Wh attempting to run a 50 W fan will theoretically last for ten hours. The basic formula is Duration in Hours = Stored Energy (Wh) / Power Draw (W). This method provides a quick estimate for how long a power source can sustain a specific load.
However, the actual run time is always an estimate because real-world usage involves conversion inefficiencies and variable power draw. The conversion from the battery’s direct current (DC) to the alternating current (AC) required by many household devices involves an inverter, which can lose 5% to 15% of the energy as heat. Furthermore, the power draw of most devices, like laptops or refrigerators, fluctuates based on usage and operational cycles.
For a more precise estimate, one must account for these losses by reducing the available watt-hours. For instance, a 100 Wh battery powering a 20 W device through an inverter with 90% efficiency will not run for exactly five hours. The usable energy is closer to 90 Wh, meaning the estimated run time is reduced to approximately 4.5 hours.
Common Applications of Watt-Hours and Kilowatt-Hours
The watt-hour and its larger counterpart, the kilowatt-hour, are the standard metrics used across two distinct areas: battery capacity and utility billing. Watt-hours are primarily used to rate the energy storage capacity of individual batteries and small power banks. A laptop battery, for example, might be rated at 60 Wh, indicating the total energy it can deliver before needing a recharge.
For larger energy storage systems, such as those found in electric vehicles (EVs) or home solar batteries, the kilowatt-hour (kWh) is the preferred unit. A kilowatt-hour is simply 1,000 watt-hours, making it a more manageable number for large capacities; an EV battery capacity can range from 40 kWh to over 100 kWh. This unit indicates the total energy available to power the vehicle’s motor and accessories.
The kilowatt-hour is also the unit used by electric utility companies to calculate monthly energy consumption and generate residential bills. The energy meter on a home monitors the cumulative kilowatt-hours consumed over a billing cycle, and consumers are charged per kWh used. While the fundamental unit of energy in physics is the Joule, one watt-hour is equal to 3,600 Joules.