What is a kilowatt hour?

You might be answering the title question with a big “Who cares?” But if you generate solar power to feed in to the grid, it is worth understanding how you will be paid. And you will be paid for kilowatt hours. I can give you the specs in Ontario; in other jurisdictions the numbers will vary, but the basics – especially the physics – will remain the same.

Here in Ontario, the government pays us 64 cents per kilowatt hour, and that rate is guaranteed for 20 years. But what is a kilowatt hour?

We have two posts that each have a capacity of five kilowatt hours, for a total production capacity of ten kilowatt hours (the maximum allowed under the provincial MicroFIT program). So if the meter on each inverter (that converts the sun’s DC power to the AC power required for the grid) reads 5000 watts (or five kilowatts), that means we are generating maximum power. It means that if it remains steady at 5000 for a full hour (five kilowatt hours), we will generate 5000 watts from each of our two posts.

This is a measure of velocity. A reading of 5000 does NOT mean we have generated that much electricity. It is a rate of generation, and we generate that much in one hour only if that rate does not fluctuate over that hour.

To help visualize this, imagine you are driving at 100 kilometres per hour (60 miles per hour). You will only drive 100 kilometres if you keep that pace up for an hour. If you stop for nature’s call or you hit the brakes or speed up to pass, at the end of an hour you will have driven 99 kilometres. Or 103. Or 82. And if you drive at 100 kilometres per hour only for 30 minutes, you will drive only 50 kilometres or 30 miles in total.

Make sense?

Back to our solar panels. If they are generating five kilowatt hours on one post (a reading of 5000), and after 30 minutes you cover them up with a thick, thick cloud, they will generate only 2.5 kilowatt hours.

Simple, right?

But wait. It gets simpler. Er…I mean complicateder. More complicated?

Kilowatts themselves are a measure of velocity. The actual measure of electricity you will generate with your solar panels is in joules. So how many joules do you generate each kilowatt? And So how many joules do you generate each kilowatt hour?

Now I wish I had stayed awake in high school chemistry class. All I remember is that a wheatstone bridge doesn’t cross a river. For that reason, I will refer you to Others who can better explain this whole kilowatt confusion for you.

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One Response to What is a kilowatt hour?
  1. Gustavo | Frugal Science
    December 15, 2011 | 10:34 pm

    Hi David!
    I liked the explanation very much and I liked even more the whole project. It is so sad a project like this could be so profitable here in Ecuador where (I believe) we have the higher sunlight exposure, but I am afraid local politics must still go a long way.

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