Electric power is usually measured in watts or in watt-hours. The watt is a measure
of energy conversion or transfer, applicable not only to electricity but also to
heat, kinetic energy, or any other sort of energy.
The technical definition of the watt is a measure of energy transfer or conversion
equal to one joule per second. (A joule is defined as the energy derived from applying
a force of one newton over a distance of one meter, or from passing an electric current
of one ampere through a resistance of one ohm for one second.
A newton is the force required to accelerate a mass of one kilogram at a rate of
one meter per second squared. In terms of electric power, the watt is used to measure
the generation of electricity or the use of electricity at any one moment. For example,
we speak of appliances, such as light bulbs, as using so many watts of power.
We also speak of a source of power as generating so many watts, or so many kilowatts
or megawatts or whatever multiplier is most appropriate. All of the terms like kilowatt,
megawatt, terawatt, etc. refer to multiples of the watt (thousand watts, million
watts, billion watts, etc.).
In scientific research, other terms refer to fractions of a watt, such as milliwatts,
nanowatts, picowatts, and femtowatts, useful in some fields of physics involving
very small masses and very small amounts of energy.
All of these terms refer to energy generation or consumption at any given moment.
When you receive your electric bill, though, you may observe that your power consumption
is measured not in watts but in watt-hours (or, more commonly, in kilowatt-hours).
The reason for this is that you use electricity over time (bills are normally generated
once per month), and it is more useful and relevant to measure the amount of power
over time that you have used rather than the intensity of your power use at any one
A watt-hour is the amount of energy expended when one watt is used or generated constantly
for one hour. A kilowatt-hour is therefore the amount of energy expended when 1,000
watts are used or generated constantly for one hour. A megawatt-hour (to return to
the title of this article) is the amount of energy expended when a million watts
of current are used or generated constantly for one hour.
Megawatts Versus Megawatt Hours
Megawatts and megawatt-hours are measures of electricity appropriate to a medium-to-large
scale. A household does not use energy in the megawatt or megawatt-hour range on
a monthly basis; however, a thousand households would do so, and so the megawatt-hour
is a useful measure of energy consumption by sizeable communities, such as large
towns or small cities.
Megawatts Versus Megawatt Hours Megawatts and megawatt-hours are measures of electricity appropriate to a medium-to-large scale. A household does not use energy in the megawatt or megawatt-hour range on a monthly basis; however, a thousand households would do so, and so the megawatt-hour is a useful measure of energy consumption by sizeable communities, such as large towns or small cities. Power production by large-scale facilities such as a commercial power plant is also often measured in megawatts, so that production over time is measurable in megawatt-hours. The production of a power plant is normally measured in megawatts rather than megawatt-hours. A power facility that produces ten megawatts generates a current of that intensity at any given moment. There are some purposes for which megawatt-hours can be a more useful measure than megawatts in terms of large-scale power production, however. One of these involves the advantages of solar energy over fossil fuels (and disadvantages of same). The reason is that solar power, unlike fossil fuel electricity generation, is variable over a 24-hour period. Solar power is generated only during the day, and is strongest when sunlight strikes the solar panels or mirrors used to generate the power most directly. A 50-megawatt solar facility generates this amount of power only for part of the day; at night, it generates none at all. It may generate lower amounts in the early morning and late afternoon, and reaches its peak production capacity only once the sun is fully risen. (Batteries normally store excess power during production peaks to be used when production falls off.) To determine how many households a solar facility can supply, therefore, it's useful to think in terms of megawatt-hours rather than megawatts. A solar farm or solar thermal power plant has the exact opposite concern from a fossil-fuel plant in terms of efficiency. A fossil-fuel electric facility must have a production capacity in megawatts capable of meeting the peak needs of its users, and for maximum efficiency should be able to operate at lower production levels during times when electricity demand is lower. A solar facility, however, needs to be able to generate enough power over time to meet the needs of its users. The peak of power consumption is important mainly in determining the necessary battery capacity rather than the generation capacity, since that peak is likely to occur during the night when no power is being generated anyway. Both of these measures of power are useful for their respective applications and both should be employed for a full appreciation of energy needs and production capacity.