Confusion of Kilowatt Hours and Kilowatts
The terms power and energy are frequently confused. Power is the rate at which energy is generated or consumed. Power therefore has the unit watts, which is joules per second. A unit of energy is kilowatt hour.
For a simple example, consider a solar photovoltaic panel. The power that it puts out at any moment would be measured in watts. The energy that it generates over a period of time, say one sunny day, would be measured in kilowatt hours. The power output (watts) will rise and fall throughout the day due to clouds, shading, etc. The energy generated (watt hours) will increase at varying rates while the sun is shining.
For example, when a light bulb with a power rating of 100W is turned on for one hour, the energy used is 100 watt hours (W·h), 0.1 kilowatt hour, or 360 kJ. This same amount of energy would light a 40-watt bulb for 2.5 hours, or a 50-watt bulb for 2 hours. A power station would be rated in multiples of watts, but its annual energy sales would be in multiples of watt hours. A kilowatt hour is the amount of energy equivalent to a steady power of 1 kilowatt running for 1 hour, or 3.6 MJ.
Power units measure the rate of energy per unit time. Many compound units for rates explicitly mention units of time, for example, miles per hour, kilometers per hour, dollars per hour. Kilowatt hours are a product of power and time, not a rate of change of power with time. Terms such as watts per hour are often misused. Watts per hour (W/h) is a unit of a change of power per hour. It might be used to characterize the ramp-up behavior of power plants. For example, a power plant that reaches a power output of 1 MW from 0 MW in 15 minutes has a ramp-up rate of 4 MW/h. Hydroelectric power plants have a very high ramp-up rate, which makes them particularly useful in peak load and emergency situations.
Major energy production or consumption is often expressed as terawatt hours(TWh) for a given period that is often a calendar year or financial year. One terawatt hour is equal to a sustained power of approximately 114 megawatts for a period of one year.
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