Short answer
Energy describes an amount that can be stored, transferred, or transformed. Power describes how quickly energy is transferred or transformed. They answer different questions: how much, and how much per unit time. One watt of power corresponds to one joule of energy per second. 1 2
This is why a kilowatt and a kilowatt-hour are different units. A kilowatt measures power. A kilowatt-hour measures energy: the amount transferred by one kilowatt sustained for one hour. 3
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At a glance
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| Question | Energy | Power |
|---|---|---|
| What does it describe? | An amount stored, transferred, or transformed. | The rate of energy transfer or transformation. |
| SI unit | Joule, J. | Watt, W; equivalent to J/s. |
| Familiar electricity unit | Kilowatt-hour, kWh. | Kilowatt, kW. |
| Does duration matter? | The transferred total depends on both power and duration. | A stated rate does not by itself give the total energy. |
| Can it remain stored? | Yes; for example, chemical energy in a battery. | Power is a rate, not a stored inventory. |
| Typical question | How much energy did the device use? | How rapidly is it using energy? |
Sources: energy forms and conservation 1; rate and units 2; electricity measurement 3.
What each thing is
Energy
Energy is a physical quantity that allows us to account for changes such as motion, heating, and electrical or chemical processes. A moving object has kinetic energy; a battery stores chemical energy that can be converted and transferred through a circuit. Energy can change form while the total accounting remains conserved. 1
An individual object can gain or lose energy through interactions with its surroundings. Conservation does not mean that every object’s energy stays fixed. It means that changes must be accounted for across the relevant system and transfers. 1
Power
Power expresses the rate of an energy transfer or transformation. For a specified interval, average power = energy transferred ÷ elapsed time. Transferring 600 joules in 3 seconds corresponds to an average power of 200 watts. This is an illustrative calculation from the definition. 2
Power may change from moment to moment. An average over an hour and a brief peak are different descriptions, even when both are expressed in watts. An energy total alone does not show when the transfer occurred. 2
Key differences
Duration connects them. At constant power, energy equals power multiplied by time. A device drawing 100 watts for 10 hours transfers the same 1,000 watt-hours as one drawing 1,000 watts for 1 hour. The rates differ; the totals match. 2 3
A capacity is different from a rate limit. A stated store of energy tells you an amount. It does not, by itself, establish how quickly a device can deliver it. Conversely, a power rating alone does not establish how long a device can maintain that rate. These are direct consequences of the amount-versus-rate distinction. 1 2
Input and useful output need separate labels. A motor’s electrical input and mechanical output describe different parts of the energy balance. Some input may become thermal energy rather than the useful output being counted. Comparing two watt figures without identifying their roles can be misleading. 1 2
How to tell them apart
Start with the unit. Joules and kilowatt-hours describe energy. Watts and kilowatts describe power. The “hour” in kWh is multiplication by time, not division: kW × h. A kilowatt-hour is therefore an energy quantity, not “kilowatts per hour.” 2 3
Next ask what the number describes. Is it an accumulated total, a stored amount, an instantaneous rate, or an average rate? A bare claim that something “uses 500 watts” cannot establish its daily energy use without information about duration and actual operation. 2
Where they overlap
Energy and power describe the same process at different levels. A heater can transfer energy over an hour while its power describes the transfer rate during that hour. Neither quantity is restricted to electricity: mechanical work and other energy transformations also have associated power. 1 2
Changing units does not change the physical distinction. A kilowatt-hour equals 3.6 million joules because a kilowatt is 1,000 joules per second and an hour contains 3,600 seconds. This conversion follows directly from the unit definitions. 2 3
Edge cases
Variable operation needs an actual average. Multiplying a momentary peak by an entire day treats the peak as continuous. To obtain total energy, add the energy transferred during the different operating intervals, or use average power for the whole period. 2
High power can accompany a modest total. A short, intense transfer can have high power without exceeding the energy of a much weaker transfer sustained for a long time. Both the rate and the duration are needed. 2
“Used energy” has not disappeared. Everyday language describes energy consumption, while physics tracks where that energy went. An electrically powered device transforms and transfers energy; conservation does not guarantee that all of it remains useful for the intended task. 1
Why the distinction exists
An amount alone cannot describe how quickly a process happens. Lifting the same load through the same height slowly or quickly can require the same change in gravitational potential energy, while the faster lift requires greater average useful power. The distinction lets us describe both the task and its pace. 1 2
Common misconceptions
- “More watts always means more energy used overall.” Duration also matters. 2
- “A kilowatt-hour is a power unit.” It is energy. 3
- “A battery stores power.” Its energy store and delivery rate are distinct quantities. 1 2
- “Conserved energy means perfect efficiency.” Total energy accounting and useful output are different questions. 1
Examples
Suppose an idealized device draws a constant 200 W for 3 hours. Its energy use is 600 Wh, or 0.6 kWh. A second device drawing 600 W for 1 hour uses the same energy at three times the power. These figures illustrate the definitions, rather than the behavior of a particular product. 2 3
Sources
Sources checked October 3, 2026.
- OpenStax, College Physics 2e — Conservation of Energy. Energy forms, transfers, conservation, and efficiency.
- OpenStax, College Physics 2e — Power. Power as a rate, watts, and energy calculations.
- U.S. Energy Information Administration — Measuring Electricity. Watts, kilowatts, and kilowatt-hours.