Slide 1
Power measures how quickly energy is transferred
Power measures the rate of energy transfer. Two machines can do the same amount of work while taking different lengths of time.
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Lifting the same load to the same height requires the same ideal gravitational work. Completing the lift faster needs greater average mechanical power, even though the final stored potential energy is unchanged.
Slide 2
A watt is one joule per second
One watt means one joule per second. Energy accumulates over time, while power describes the rate at a particular time or over an interval.
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A device transferring energy steadily at 100 watts for 10 seconds transfers 1,000 joules. If its power varies, the total comes from adding the transfer during each part of the interval.
Slide 3
A kilowatt-hour measures energy, not power
A kilowatt-hour is an energy unit. It means one kilowatt sustained for one hour, not power divided by time.
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Because a kilowatt is 1,000 joules per second and an hour is 3,600 seconds, one kilowatt-hour equals 3.6 million joules. The name retains both factors in the multiplication.
Slide 4
Input power also supplies heat, sound and other transfers
A machine’s input power can exceed its useful output power. Heating, sound and other transfers account for the difference.
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Efficiency compares useful output with input for a stated task. The meaning of useful must be defined: heat wasted by a motor may be the intended product of an electric heater.
Slide 5
A short peak can use less energy than a long modest demand
A short burst of high power need not use much total energy. A modest power demand sustained for hours can use more.
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An electrical supply must handle demanding moments, while an energy store must contain enough energy for the duration. Those are different design constraints, explaining why both power and capacity appear in specifications.
Sources
- Energy Basics
US Department of Energy / NREL
- STEMonstrations: Kinetic and Potential Energy
NASA