What is the difference between energy and power?
Power is a rate; energy is a quantity. Power is how fast energy is used or produced; energy is how much has been used over time. Confusing them is behind a great deal of muddled discussion about electricity.
The units:
Power is measured in watts (W) and kilowatts (kW) — joules per second. A kettle rated at 3kW draws energy at that rate whenever it is on.
Energy is measured in kilowatt-hours (kWh) — a kilowatt sustained for an hour. This is what you are billed for.
The relationship: energy = power × time. A 3kW kettle running for ten minutes uses 0.5kWh.
Why the distinction matters practically:
A high-power appliance is not necessarily expensive to run. A kettle draws a lot of power briefly; a much lower-power device left on continuously can use more energy over a month. Duration matters as much as rating, which is why hunting down standby loads yields less than people expect while heating dominates bills.
Generation capacity versus generation output. A wind farm described as "100MW" is stating power — its maximum instantaneous output. What it actually produces over a year is energy, and it is far less, because the wind does not always blow. Comparing a solar farm's capacity with a nuclear plant's capacity is meaningless without the capacity factor — the proportion of theoretical maximum actually achieved, which differs enormously between technologies.
This is the single most common error in energy comparisons, and it makes intermittent sources look larger than they are relative to continuously running plant.
Battery specifications carry both, and both matter: capacity in kWh determines how long it can supply; power rating in kW determines how fast it can charge or discharge. A large battery with a low power rating cannot run a shower.
Electric vehicle charging likewise: a 7kW home charger and a 150kW rapid charger deliver the same energy eventually, at very different rates.
Other units: MW and GW for larger power; MWh, GWh and TWh for larger energy; and kVA, apparent power, used in electrical supply ratings.