Understanding kilowatt hours

March 3, 2010
Archive
Solar and wind systems are great, but before you buy one, it’s good to know what you’re actually getting, how much power you need, and how much you’ll save. There’s some math involved, but a few examples should make it easier to understand.

What exactly is a kilowatt hour (kWh)?

First, you need to know about kilowatt hours. A kilowatt hour is simply a thousand watt hours (Wh). A watt hour is the power of 1 watt operating for 1 hour’s time.

How many watt hours will something require?

Let’s say we wanted to calculate watt hours needed to power a 60W lightbulb for a certain amount of time:
  • 60 watts * 0.5hr (30 min) = 30 Wh (0.03 kWh)
  • 60 watts * 1 hr = 60 Wh (0.06 kWh)
  • 60 watts * 7 hrs = 420 Wh (0.42 kWh)

How many hours will something run?

Now let’s say we wanted to calculate how long we could run something at a certain wattage with 1000 Wh (1 kWh):
  • 1000 Wh / 25 watts = 40 hrs
  • 1000 Wh / 100 watts = 10 hrs
  • 1000 Wh / 4000 watts = 0.25 hr (15 min)

How many watts was something using?

Also, if something ran for 5 hours and you know how many watt hours it consumed, you can determine how many watts it was using. Generally, you may never have to calculate this:
  • 2 Wh / 5 hrs = 0.4 watts
  • 600 Wh / 5 hrs = 120 watts
  • 5000 Wh (5 kWh) / 5 hrs = 1000 watts (1 kW)

Do I need to power my whole house?

Generally people wonder how much they need to power their home. Running your house entirely on renewable energy is a good lifetime goal, not a short term goal. The best (and most practical) way is to start slow, adding as you can afford. However, unless energy consumption is simply reduced, it could be a bit pricey. One way to reduce energy consumption is to insulate your home better.

What do I need to power my house?

You probably have a good idea of how much power you need in kWhs per month from your electric bill. But what do you need to get that from solar panels or wind generators? Fortunately, if you buy a complete system (or consider it and then buy parts separately), they usually have a chart or calculator. This will shows you average monthly kWh production for a given area or wind speed. Here’s some examples: If you look at the Whisper 500’s spec. sheet, notice the curve on the bottom right. If you have an annual average wind speed of 11.25 mph, one of these generators will give you an average of 450 kWh per month. If your average monthly usage is 2000 kWh, you would need 2000 kWh / 450 kWh = 4.4 (or 5) of these units. That would cost about ~$6000 * 5 = ~$30,000 for the generators, not including the towers, wiring, and labor! However, if you shop around, you might find something better or cheaper to suit your needs.