How to Calculate Energy Used

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When calculating the amount of energy used in a home, determine the cost of an appliance against the kilowatt hours that it uses. Calculate energy, and weight it against what an electric company charges, with help from a science teacher in this free video on energy and physics.

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Video Transcript

Hi, I'm Steve Jones and I'm going to be telling you how to calculate energy used. By that I mean energy used in the home, which is where it's quite interesting to find out how much power you're using. Now, power, as such, is measured in watts or kilowatts. You'll find on most devices there will be a rating in watts or kilowatts. We know the voltage is always two hundred and fifty volts. Or there abouts, two forty, two fifty, unless you're in the United States or Japan, and then I think it's a hundred and ten. The time is hours. We always measure the time in hours, so for this calculation, it's fairly simple. What I've done is I've created a table here, and I'm going to estimate how much energy I actually use. First of all I need to know which appliance I'm looking at and here are three typical ones, a lamp of some kind, a light bulb that is, an iron, for ironing your clothes, and a kettle for boiling water. If you're in England, this is for making cups of tea, and if you're in America, for making cups of coffee. Okay. Now you have to do a little bit of an estimate. How long each day do you use that device for? Now be very careful. For example, you might have a refrigerator, and it might say it's three hundred watts. The point is it's only on for a very short time, because the motor is only working for a short time, as it switches on and off. So if it's on all day, you don't calculate twenty four hours, you may calculate something like an hour, maybe, because for an hour, you know, for ten minutes here, five minutes there, it will be on. This is exactly the same with the other devices, kettles and so on. So you have to make some kind of estimate. With lamps, you'll have to work out how many lamps there are, and so forth. So estimate the number of hours you use it for. Then, look at the power, but the power has to be in kilowatts, not in watts. Kilowatts, well, typical lamp is a hundred watts. That's 0.1 kilowatts. An iron, fifteen hundred watts, or 1.5 kilowatts. A kettle, 2.5 kilowatts. Quite powerful are kettles, and they take a lot of current, too. Something like twelve ounce, thirteen ounce. What we do is we calculate by calculating the daily hour consumption, six hours for example, by the power, .1. Six times .1 is .6, and we do the same for each of the devices. .2 times l.5 gives you .3, and .5 times 2.5 gives you l.25. This is kilowatts times hours, and this is the basic unit, the kilowatt hour. It's sometimes just called a 'unit'. So if it says so many units, it means kilowatt hours. So, the next job is to go to your electricity provider, because every one is different. And they will tell you how much it costs for the first so much electricity, and for the next so much electricity. It'll be a cost per kilowatt hour. I'm using a value of ten cents here, for your benefit. So, if we've got .6 kilowatt hours, and each once costs ten cents, we're only paying six cents for the light. We're paying even less for the iron, 'cause we don't use it a lot, three cents. The kettle, bit more expensive, 12.5 cents for the kettle. So, this would be a daily average. You could do, if you want, over a month and estimate the hours over a month, and work out your monthly bill. It's an interesting thing to do, because then you can get the bill from the electricity company, and work it out for yourself, and see how they charge you for the electricity you use. So that is how you calculate the energy used.


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