Dan Way for Carolina Journal has an important piece today about what solar energy output really means.

In its application to build a solar facility on Gov. Roy Cooper’s Nash County property, Durham-based Strata Solar said its generating capacity would be about 5 megawatts. Enough energy to power continuously about 3,750 homes.

But the plant won’t generate 5 MW of energy 24 hours a day, seven days a week. Much of the time it won’t produce anything.

“Solar is an intermittent energy source, and therefore the maximum dependable capacity is 0 MW,” the application notes.

The “enough to power ___ homes” is a recurring theme in promoting and reporting on renewable energy output. Duke Energy’s Twitter account provided two recent examples:

It’s a completely worthless notion. If any of those homes were exclusively powered by solar (or wind), they would be powerless and dark for much of the time, especially when people need power the most. Hence my responses to those two Duke tweets (wind, solar) — and hence Strata Solar’s disclosure that their “maximum dependable capacity is 0 MW.”

Because they’re so unreliable, because they’re intermittent and not dispatchable, they need a traditional, dispatchable power plant constantly cycling for backup generation at a moment’s notice. That’s why even though their “fuel” sources are “free,” solar and wind are the most expensive energy sources (even if you’re trying to reduce greenhouse emissions).

Way’s article has a fantastic graphic illustrating these differences. Not just between solar’s expected capacity and delivered capacity during the day, but also delivered capacity among differing weather conditions, and then as compared with traditional energy sources that are dispatchable (not hostage to the whims of weather).