How many homes can you power with a single megawatt?

In reporting on Idaho Power's effort to reduce the amount of solar energy it is required to buy, we ran into an interesting question: How many homes does a megawatt of power supply?

In another life, I was a business reporter, and one of my beats was energy. When writing those stories, I almost always used the handy formula 1 MW = 1,000 homes. I probably would have continued using it if not for a startling number from Idaho Power: If all of the solar projects currently planned in Idaho come to fruition, nearly 2,000 new MWs of energy would begin flowing onto the grid in 2016. According to my math, that would be enough to supply 2 million homes, or 400,000 more homes than there are people in the state of Idaho. It would keep the lights on in almost every U.S. Census-designated housing unit in Cook County, Illinois (which includes the Chicago Metro Area). It could supply Seattle residents six times over.

That couldn't be right.

Then we learned not all MWs are created equal. According to the Solar Energy Industries Association, the number of homes powered by a MW of solar energy depends on average sunshine, electricity consumption, temperature and wind. Nationally, that's 164 homes per MW—a far cry from the ratio of 1 MW:1,000. Website has a more precise—and circuitous—way of finding how many homes are supplied by a MW, with a formula that includes regional yearly average usage, the type of power plant and how efficient it is. The spread ranges from 400-900 homes per MW hour, with coal at 60 percent capacity or more, and solar at less than 25 percent. So 1 MW of solar could equate to 164 homes; 400-900 homes; or 1,000 homes.

We asked the Idaho Public Utilities Commission to settle the matter, and it came down somewhere in the middle: 1 MW hour can serve about 650 residential homes. Using that formula, Idaho's impending solar boom is still impressive: enough power to supply 1.3 million homes or more than twice the number of housing units in Phoenix, Ariz., as identified by the 2010 Census.

That sounds like an energy gold strike but not to Idaho Power. Find out why here.

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