Watts and Watt-hours

(UtiliPoint.com - May 4, 2007)

May 5, 2007 - PowerMarketers Industry Publications


May 4, 2007

By Bob Bellemare President and CEO

In the past week several IssueAlert subscribers have contacted me about how to interpret the difference between watts and watt-hours and the number of homes served by certain generation devices. As we will see, simple questions unfortunately do not always have simple answers in the power industry.

A Classic Question

People like to discuss electric generators in terms of how many equivalent homes the new generator will serve. For instance, a reader asked how many homes are served in the following example:

“Oregon's proposed 100-MW Coos County wave energy project…would have a generating capacity of 100 MW; the total annual generation from the project is estimated to be approximately 175 gigawatt-hours per year.”

The reader was having a difficult time understanding the difference between a mega-watt (MW) and a megawatt-hour (MWh) and how to relate those numbers to houses served. These concepts are discussed in a prior IssueAlert article (see http://www.utilipoint.com/issuealert/article.asp?id=1728) but I thought the topic was worth revisiting, given the recent interest.

So let's start with the basics. The power being used at an instant in time is measured in “watts,” or “W.” The amount of energy used over a certain period of time is known as “watt-hours” (Wh). For example, a 100 W light bulb consumes 200 Wh of energy over two hours time (100 W * 2 hours = 200 Wh).

Because power system issues range from the very small (a light bulb) to the very large (power plants), symbols are used to describe their quantity. “One thousand” of anything in this business is usually referred to as a “kilo,” or “k” for short. Similarly, a “million” we call “Mega” or “M” and “Giga.” or “G” is one thousand times one million.

For example, suppose the peak power demand in a region was 40 gigawatts (GW). A gigawatt is 1000 megawatts (MW), so said another way the peak demand was 40,000 MW. One MW is one thousand kilowatts (kW) so 40 GW = 40,000 MW = 40,000,000 kW = 40,000,000,000 W.

So How Many Houses Could be Served by the 100 MW Wave Generator?

If the100 MW wave generator were to operate at its rated capacity in all hours, it would produce 876,000 MWh, or 876 gigawatt-hours (876 GWh) of energy over the course of the year (100 MW *8760 hours in a year). But the developers are predicting it will produce a much smaller amount, 175 GWh per year. Essentially the generator will produce 20 percent (175/876) of its theoretical maximum production in a year. Said another way, this generator, while rated at 100 MW, will average 20 MW of power production in a year.

This 20 percent is called the capacity factor by the utility industry. It's the ratio of the actual energy produced by a generator compared to the maximum possible production from the generator. Coal plants, for example, typically have a 70 to 90 percent capacity factor, while certain renewable resources such as wind, solar and wave, normally range from 20 to 35 percent. The reason these renewable resources have a “low” capacity factor is because of the varying amounts of energy available from the renewable resource itself. For instance, the wind only blows sometimes and with varying intensity, the sun only shines in certain hours and cloud cover lowers production, and waves vary in intensity.

So how many houses would the wave generator serve? If we assume the average house uses 8 MWh in a year (which is 0.008 GWh/year) then by this measure the wave generator would serve 22,000 houses (175 GWh/0.008 GWh). A more accurate way of saying this would be the 100 MW wave generator produces the equivalent amount of energy as 22,000 houses consume in a year.

Another calculation people sometimes do is how many households a generator serves at the time of its peak production (in this case 100 MW). To do that calculation you need to know the average demand per households at that moment in time. This number also varies greatly around the country and by time of day and season, but let's assumes here the average peak household demand is 2.5 kW, or 0.0025 MW. So when the generator is producing 100 MW it serves 40,000 households at that moment in time (100 MW/.0025 MW).

Some advocates and those opposed to certain technologies can use a variety of numbers to support their arguments. Unfortunately there is no one solid number that captures all the nuisances of the situation but the average number (22,000 households) is probably the most common way of discussing the issue.


A utility planner needs to add generation to the system to meet the future demands of the region. When looking out over the next 10 years, the utility needs to add 1000 MW (1 GW) of new generation capacity capable of producing at least 2628 GWh of energy in the year. .What would be the capacity factor for our new generation if it only generates 2628 GWh of energy in a year? How many “equivalent” homes would be served by the generator (based on average production) if the average home consumes 0.008 GWh of electricity in a year? Finally, if it cost the utility $1500 per kW ($1500/kW) to install the generator, how many dollars per average household served was invested?

Please send an e-mail to rbell@utilipoint.com with your answers.

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