how to determine eletrical usage?

Question

I understand that all electrical devices will draw a certain amount of current. For instance a given device that draws .15A at 120v consumes 18W. What I don't get is how to equate this to usage over time. Is that 18W for one second? One hour? One cycle? Certain that this is documented somewhere but I sure can't find it.

Once I know this I can equate this to all the devices in my house and get a better handle on my electricity usage.

I still dont' get it. For a watt-hour to mean anything there has to be a time constant. I got that leaving the device plugged in for an hour would be X watt-hours. If I leave it plugged in for .2 seconds? I dont' see the time connection. It's like saying you walk 20 miles without putting "per hour." There's no time constant.

Answer 1

A watt shows how fast it is using power -- The useage over time is a watt-hour (usually billed by the electric company by the thousand, or KiloWatt Hour)
Instead of your example, let's make the math easier to figure in our heads. . .

Say you have a ten 100-wat light bulbs, all turned on all at once. You would be be using one kilowatt of power.

If you had them all turned on for one hour, you would have used one kilowatt hour of electricity, costing around 15 cents or so, depending on what part of the country you live in.

Your 1500 watt blow dryer would use as much electricity as 15 100-watt light bulbs all turned on at the same time. If you used it for a solid hour, that would be 1.50 kilowatt hours, 1/3 of an hour (20 minutes) would be 0.500 kilowatt hours, half of that would be 1/6 of an hour, or 10 minutes, you would use 0.250 kilowatt hours. If electricity is 16 cents per kilowatt hour, it just cost you 4 cents to dry your hair for 10 minutes. . .

Answer 2

The label on electrical devices normally give the maximum power that the device could possibly use. Because most things rarely use that much power, those numbers aren't of much use in determining energy usage.

For about $20 you can buy a "Kill-A-Watt" meter that measures energy usage (see first link). Besides telling you haw much energy your appliance is using at the moment, it will add up all the energy that it uses over time. For example, a refrigerator cycles on and off. But if you plug it into a "KIll-A-Watt for a week, it will tell you how many kilowatt hours it used in that week.

In your example, if an appliance is using 18 watts, then when has been running for one hour it has used 18 watt-hours.

Electricity is usually billed in kilowatt hours ( 1000 watt-hours= 1 kilowatt-hour). So if you are paying $ 0.10 per kilowatt-hour, running you appliance for 1 hour uses:

18 watt-hours = 0.018 kilowatt-hours = $ 0.0018 on your electric bill

See links for more info.

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>I still dont' get it. For a watt-hour to mean anything there has to be a time constant.

A watt-hour is one watt for one hour, you mutiply it by whatever amount of time you are interested in.

An example:
If you had a space heater that used 1000 watts all the time (no thermostat), it would use 1 kilowatt per hour.

If you ran the same heater for 2 minutes, it would use:

1 kW/hr X 2 minutes X (1 hr/60 minutes)= 0.033 kilowatts

If you pay $0.10 per kilowatt:
Running it for one hour would cost you $0.10. for 1 kilowatt-hour
If you ran it for 2 minutes, it would cost you $0.0033 for 0.0333 kW-hr
If you ran it for 24 hours, it would use 24 kW-hr at a cost of $2.40

Answer 3

Hi Daniel C gets complicated eh!
Watts are units of electrical output not speed of output okay. Although the terminology "X" per unit etc is supposed to be understandable to the end user.
Speaking to a friend of mine (who actually reads our meters", he tells me that electrical units are measured in "units per hour" So my conclusion would be to forget volts amps etc and concentrate on the Watts and Kilowatts.
The first thing to know, is what is the cost per kilowatt then multiply by use and divide by cost example amount of kilowatts x cost divided by hours.
However, for an individual component use then the gadgeteer in one of your responses would be the ideal solution.....at least you would have accuracy without the need to understand the amps milliamps watts and what not.
Most electrical goods give some advice on power use on their labels and they too are pretty close.