Megawatt

How much power is this?

It’s a lot. But it gets complicated, because energy exists in two dimensions:

  1. As a rate at any particular instant in time (“megawatt”)
  2. As a sustained, aggregated amount delivered over a period of time (“megawatt hour”)

So, “a megawatt” is an amount of power being delivered at a singular moment. If you deliver that same amount of power for an hour, you have delivered “a megawatt hour.”

It’s like driving. Going “60 mph” doesn’t move you any distance at any particular instant. But if you keep that up for an hour, you’ll have moved 60 miles.

The average American home consumes about 10,800 kilowatt hours per year. The numbers vary based on location (homes in the south run more air conditioning) and the type of home (mobile homes are often electrically heated).

Converting to megawatts (1,000 kilowatts in a megawatt), this means the average American home consumes 11 megawatt hours per year. Or, at any single moment in time, the average American home is consuming .001 megawatts.

There are 8,766 hours in a year. This means that a power source consistently generating 1 megawatt will generate 8,766 megawatt hours per year, which could theoretically power about 800 homes over the course of a year.

Here are the potential outputs of various sources:

Now, it’s important to note that these are potential outputs, which is how much these sources can theoretically output in any given instant. If we switch to megawatt hours, which is what they actually produce over time, the numbers change dramatically

The key point: you can’t really store electricity on a large scale. You can store it for small localized output (a car battery, for instance), but there are no “grid scale” batteries that can store enough electricity to power a entire electrical grid. At our current state of technology, you can’t “bank” electricity – you either output it and put it to use at the exact same time, or it’s lost.

The percentage of power a source can realize over time is its “efficiency rating.”

Obviously, solar panels depend on the sun. They will generate basically nothing at night, and less when the sun is weaker, like during the winter months. Windmills require the wind to blow. On calm days, they don’t turn or turn slowly. Wind turbines normally have efficiency ratings of 25-33%.

This means the sustained power production of solar and wind over time is much lower than their potential output, because they’re only outputting that for a fraction of the time.

Nuclear power plants are the steadiest option. They mostly run 24/7, except when they have to be taken down for maintenance. I saw a statistic that an average nuclear power plant has efficiency rating 93.5%. Coal power plants are about the same. They run wide-open, all the time.

Doing the math on a nuclear plant: producing 1,000 megawatts for 8,766 hours times 93.5% is 81,962,210 megawatt hours every year, which would power 745,000 homes, or a mid-size American city.

(Also of note – the output capacity of nuclear varies greatly. The smallest nuclear plant in the United States can output 582 Mw; the largest, 2,340 Mw.)

As a final piece of trivia, the average bicyclist generates .11 kilowatts. This means they would generate .96 megawatt hours per year if they cycled 24 hours a day, 365 days a year. So, 11 of those could power your house.

Why I Looked It Up

I’ve often wondered how many homes a windmill can power, so I dug through the calculations.

Postscript

Added on July 9, 2022

In the first chapter of Vaclav Smil’s book “How the World Really Works,” he discusses the difference between energy and power:

Another common mistake is to confuse energy with power […]

Energy is a scalar, which in physics is a quantity described only by its magnitude […] power measures energy per unit of time, hence its rate […]

Unfortunately, even engineering publications often write about a “power station generating 1,000 MW of electricity,” but that is impossible. A generating station may have installed (rated) power of 1,000 megawatts – that is, it can product electricity at that rate – but when doing so it would generate 1,000 megawatt-hours […] in an hour.

He also discusses efficiency ratings:

[…] large nuclear reactors are the most reliable producers of electricity; some of them now generate it 90-95 percent of the time, compared to about 45 percent for the best offshore wind turbines and 25 for photovolactic cells in even the sunniest of climates – while Germany’s solar panels produce electricity only about 12 percent of the time.

Postscript

Added on July 9, 2022

A company called NuScale is about to get approval on a “mini” nuclear reactor that produces 77 Mw. Using our same math from above, this would power about 60,000 homes.

The reactor is small enough to be constructed in a factory then shipped for installation on-site. It is nine feet in diameter and 65 feet tall, making it about 1% the size of a traditional reactor.

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