What is a Megawatt?

Jun 24, 2003 02:00 AM

Mega-what? The term is tossed around a lot. Megawatts are basic to understanding electricity planning concepts, but what are they?
News stories covering electric generation topics often try to illustrate the worth of a megawatt in terms of how many homes a particular amount of generation could serve. A June 11, 2003 article describing the potential sale of AEP's Texas generation facilities states that AEP is offering to sell "29 generating units with a total net generation capacity of 4,497 MW, or roughly enough electricity to power 4.5 mm average homes." A May 21, 2003 article describes an agreement with Sempra that "involves 1,900 MW, enough to supply 1.9 mm homes."
Such articles give the impression that one MW is enough electricity to supply 1,000 homes. Yet, occasionally, an article will illustrate a different conversion such as an April 17, 2003 article which states "Tucson Electric Power expanded its solar capacity to 2.4 MW, enough to power 420 homes."
So what really is a MW and how many homes can one MW of generation really serve?

The answer starts with understanding the basic definition of energy terms. Watts (W) are the yardstick for measuring power. A one hundred Watt light bulb, for example, is rated to consume one hundred Watts of power when turned on. If such a light bulb were on for four hours it would consume a total of 400 Watt-hours (Wh) of energy. Watts, therefore, measure instantaneous power while Watt-hours measure the total amount of energy consumed over a period of time.
A MW is 1 mm Watts and a kW is one thousand Watts. Both terms are commonly used in the power business when describing generation or load consumption. For instance, a 100 MW rated wind farm is capable of producing 100 MW during peak winds, but will produce much less than its rated amount when winds are light. As a result of these varying wind speeds, over the course of a year a wind farm may only average 30 MW of power production.
Similarly, a 1,000 MW coal plant may average 750 MW of production over the course of a year because the plant will shut down for maintenance from time-to-time and the plant operates at less than its rated capability when other power plants can produce power less expensively.

The ratio of a power plant's average production to its rated capability is known as capacity factor. In the previous example, the wind farm would have a 30 % capacity factor (30 MW average production divided by 100 MW rated capability) and the coal plant would have a 75 % capacity factor (750 MW average divided by 1,000 MW rated capability).
Load factor generally, on the other hand, is calculated by dividing the average load by the peak load over a certain period of time. If the residential load at a utility averaged 5,000 MW over the course of a year and the peak load was 10,000 MW, then the residential customers would be said to have a load factor of 50 % (5,000 MW average divided by 10,000 MW peak).

Knowing the peak and average demand of a power system is critical to proper planning. The power system must be designed to serve the peak load, in this example 10,000 MW. But the actual load will vary. The load might be 10,000 MW at noon, but only 4,000 MW at midnight, when fewer appliances are operating. The capacity or load factor gives utility planners a sense of this variation.
A 40 % load factor would indicate large variations occur in load, while a 90 % load factor would indicate little variation. Residential homes tend to have low load factors because people are home and using appliances only during certain hours of the day, while certain industrial customer will have very high load factors because they operate 24 hours a day, 7 days a week.

Residential electricity consumption
The amount of electricity consumed by a typical residential household varies dramatically by region of the country. According to 2001 Energy Information Administration (EIA) data, New England residential customers consume the least amount of electricity, averaging 653 kWh of load in a month, while the East South Central region, which includes states such as Georgia and Alabama and Tennessee, consumes nearly double that amount at 1,193 kWh per household.
The large disparity in electric consumption is driven by many factors including the heavier use of air conditioning in the South. So it stands to reason that a one MW generator in the Northeast would be capable of serving about twice as many households as a generator located in the South because households in the Northeast consume half the amount of electricity as those in the South.

Going through the math, a 1,000 MW rated coal generator with a 75 % capacity factor generates about 6.6 bn kWh in a year, equivalent to the amount of power consumed by about 900,000 homes in the Northeast but only 460,000 homes in the South. In other words, each MW of rated capacity for a coal plant in the Northeast generates the equivalent amount of electricity consumed by 900 homes in the Northeast but only about 460 homes in the South.
By comparison, a 30 % capacity factor, 100 MW wind farm would generate the equivalent amount of power consumed by about 35,000 homes in the Northeast and 18,000 homes in the South. In other words, each MW of rated capacity for a wind farm in the Northeast generates the equivalent amount of electricity consumed by 350 homes in the Northeast and 180 homes in the South.

So what is a Megawatt worth?
The examples demonstrate that there are two very important aspects to knowing what a MW of generation capacity is worth in terms of how many equivalent homes it represents.
The first factor is how much electricity the power plant itself produces, which can be calculated by knowing the plant's rating and capacity factor.
Second, the location of the plant is very important as the amount of electricity consumed in a typical household can vary dramatically across the country.

The numbers used in the examples were typical representations of coal and wind power plants. A low-cost coal plant typically operates at capacity factors of 60 % or higher. High quality wind sites will generate at about 30 to 40 % of their rated capability on average because of wind speed variations. Solar generators average even less production, typically under 25 % capacity factor, because the generators do not produce electricity during the night time or during cloudy days.
The commonly used "one MW of generation equates to 1,000 homes" is a myth that likely originated years ago when households were smaller and air conditioning wasn't as common.

For conventional generators, such as a coal plant, a MW of capacity will produce electricity that equates to about the same amount of electricity consumed by 400 to 900 homes in a year. For renewable energy such as wind or solar, the equivalent is even less because they typically produce less energy than conventional generators since their "fuel source" is intermittent.
Of course, no one generator is normally considered sufficient by itself to supply an individual customer. All generators must be taken out of service for maintenance and some types of generators, such as nuclear, wind, and solar, are not normally able to "follow" changes in load. For these reasons power systems require the use of backup generation sources and occasionally electric energy storage, such as batteries, to ensure the amount of power generated always matches the load demand, every second.

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