It was a phrase repeated a thousand times last week: The newly dedicated Ivanpah Solar Electric Generating System will generate enough electricity “to power 140,000 homes,” the stories went. But what exactly does that mean?

This is, after all, a concentrating solar thermal power plant that produces no power at night and might reach peak output for just a few hours or so a day. Clearly there are periods when Ivanpah will power zero California homes. Likewise, the amount of power California homes use is constantly rising and falling.

So the 140,000 figure is an attempt to step past ever-shifting generation and load. It’s a number arrived at by estimating Ivanpah’s annual production of electricity and dividing it by the annual household usage in California. Let’s go through the calculations.

Ivanpah’s 173,500 heliostats and its three boiler-topped towers are capable of producing 392 megawatts of power. But because the plant requires operating power – the “parasitic” energy needed for things like circulating the water up the towers, powering cooling fans to condense steam, and keeping the heliostats following the sun – Ivanpah actually sends at most 372 megawatts of power onto the grid.

But as we mentioned, Ivanpah isn’t constantly producing at peak output, and this is where the term *capacity factor* comes into play.

Capacity factor is the ratio of actual output to possible output over a period of time.

Sticking with a year, if Ivanpah were operating at full capacity 24 hours a day, 365 days a year – that is, if it put out 372 megawatts for all 8,760 hours of the year – it would produce 3,258,720 megwatt-hours of electricity. But Ivanpah has a capacity factor of about 29 percent (or so goes the forecast).

Thus, to determine Ivanpah’s annual generation, we have the equation 3,258,720 MWh x 0.29, giving us a figure of 945,028 MWh.

With the average California home using about 6.7 MWh of electricity per year, that means Ivanpah’s annual output is equal to the amount of power used by 141,108 homes (945,028 MWh/6.7 MWh).

Now, as NRG points out in a fact sheet [PDF], at a peak output – at that moment in time – Ivanpah will actually be powering far more than 140,000 homes, “more than twice that number,” NRG says. What’s that about?

Well, according to the California Independent System Operator, the state’s major grid overseer, 1 MW is “roughly enough electricity for the instantaneous demand of 750 homes at once.” Some houses might be using nearly zero; some might be at 2 kilowatts; but across a wide area, the average load is around 1.33 kW.” Thus: 372 MW x 750 = 279,000 homes.

All of this is based on forecasts and expectations. In time, data will tell an even more interesting story.