Mega-what?

So just what is a megawatt (MW) and how many homes can one MW of electricity really power?

In order to answer that question, we must be sure we’re on the same page about the vernacular of energy-speak. A Watt (W) is the basic measure of power (not energy). As such, a 60 watt light bulb is rated to consume sixty watts of power when you flick the light switch on. Here’s another common term: watt hours (Wh). Say you leave the lightbulb on for four hours. In that case, we say you’ve consumed 240 watt-hours (Wh) of energy. So watts are a measure of power and watt-hours measure the amount of energy consumed over any period of time. Power (watts) is a measure of instantaneous possibility; energy (watt hours) is the measure of power over a period of time. So far so good.

Some more important stuff: A megawatt (MW) is one million watts and a kilowatt (kW) is one thousand watts. These terms are used in the power business when describing generation or load consumption. A 180 MW rated wind farm has a capacity of 180 MW during optimal winds. Such a farm, however, will produce less than that during typical weather. (In that sense, where you put a wind farm is an essential consideration). Consequently, the total energy produced on this farm may range from year to year from 50 MW to 60 MW, for example.

Different forms of power come with inherent risks and problems associated with capacity. A 1,000 MW nuclear plant may average 800 MW of production annually due to maintenance closures and other such tests. Actually, a nuclear power plant will generally see less down time than most other power plants. Actual figures may vary from one geographical location to another, but for what it’s worth, I’ve lifted the following stats for effect: Nuclear power plants operate at 90% capacity, coal at 73% (more down time for maintenance and failures), natural gas plants (depending on type) from 16 to 38%, hydroelectric at 29%, wind at 27%, solar at 19%, and geothermal at 76%.

These percentages are known as a power plant’s capacity factor. The net capacity factor of a power plant is the ratio of its real output over a period of time to its output at full capacity. This can be calculated simply by dividing the actual total energy produced by a power plant (say, 80 MW) by its rated energy capacity (say, 100 MW) and then multiplying by 100 (for a percentage). Thus, in this instance, the power plant’s capacity factor is 80%.

Average capacity factor by energy type (give it a click).

Since energy is power multiplied by time (IE. Watts x hours), electricity is often measured in kilowatt hours (kWh), as mentioned above. We can determine the capacity factor this way, too. A wind farm with a capacity of 1,000 MW might produce 265,000 megawatt-hours in a 30-day month. To determine capacity factor we first need to determine the total maximum capacity over 30 days: Multiply the max capacity (1000 MW) by 30 days. Then multiply that by the number of hours in a day (24) for a total maximum capacity of 720,000 megawatt-hours in a month. Just as we did in the previous example, we determine the capacity factor by dividing the actual output by the maximum possible output. Therefore, in this case the capacity factor is 0.368 or 36.8%.

So how many homes is a 1000 MW power plant good for?

You’re probably not going to like the answer: That depends. While a lot of media reports try to simplify, for example by saying that 1 MW is enough to power 750 homes, that’s not always going to be the case, and sometimes not even near it.

First of all, different communities have very different energy needs for each season of the year. Homes in Florida consume a lot of energy in cooling homes and heating swimming pools, whereas homes in Nunavut may have heating requirements 10 months of the year. Temperate climates such as most of the west coast my forgo heating and air-conditioning for most of the year.

Also, the number of homes that may be serviced by a power plant with capacity for 1000 MW also depends on the kind of power plant. A coal power plant may have a capacity factor of 73%, while a 1000 MW wind power plant in the same area may generate 25% of its capacity in a year. Obviously, each of these plants is going to service a different number of homes, depending on the geographical location and the capacity factor.