# How to measure electricity usage

1. Mar 22, 2009

### hah2110

Ok, heres the deal. I am trying to measure how much energy (best in watts, i guess?) a device uses over a seven hour period. I bought a device from lowes that measures watts and amps but how do I figure out [for example] if it says a device is using 8 watts and .2 amps right now, how much that is over a period of time?

2. Mar 22, 2009

### Redbelly98

Staff Emeritus
Welcome to PF.

Electrical energy usage is typically measured in Watt-hours or kilowatt-hours. Multiply 8 W by 7 hours, and the energy used is 56 watt-hours. Assuming that the device maintains 8W over the entire time.

Power (the number of Watts) is a measure of the rate of energy usage, not of energy itself.

3. Mar 22, 2009

### dlgoff

Welcome to PF. Watts is the measure of power. Multiply that times the time in hours to get the energy usage. i.e. Watt-Hours

Check out this http://en.wikipedia.org/wiki/Electricity_meter" [Broken] wiki page.

Edit: RB beat me to it.

Last edited by a moderator: May 4, 2017
4. Mar 22, 2009

### hah2110

Hey guys. Thanks for your fast response. The issue is that I'm trying to see what uses more electricity:

A) Leaving a device turned on all night
B) Turning the device off and back on in the morning (when the device turns on, it does a calibration which I think is power intensive)

How would I measure that then because when I measure the watt usage as it turns on, it will only be for a few seconds.

5. Mar 22, 2009

### Bob S

There is (was?) a unit on the market called "Kill a Watt", costing about \$40, that plugs into the wall and measures volts, amps, watts, VA, frequency, power factor, and kilowatt-hours. It's good for monitoring refrigerators and for the small "dark" leakage currents in appliances like TV sets etc..

6. Mar 22, 2009

### Redbelly98

Staff Emeritus
7. Mar 22, 2009

### hah2110

What I have is similar to the Kill A Watt but not exactly the same. It is a relatively large power surge which is why I'm trying to figure it out. How would I do so?

8. Mar 22, 2009

### Redbelly98

Staff Emeritus
The Kill A Watt has a setting that will display the energy used, in kW-hours, since it was last plugged in. Does your unit have that setting?

9. Mar 23, 2009

### hah2110

Unfortunately, not. Isn't there a way to work this out?

10. Mar 23, 2009

### Redbelly98

Staff Emeritus
You would have to monitor and record the wattage over a period of time. But if, as you say, the wattage changes faster than either the unit or you can record the reading, I don't see a way to measure the energy accurately.

However, we could estimate a maximum for the energy. If you can get an idea of what the peak power is when you turn the unit on, and for how long it operates at peak power, then simply multiply
power x time (in hours)
to get the estimate. The extra energy consumed when you turn the device on will be no larger than this number.

Moreover ... you mentioned earlier considering leaving the device on overnight vs. turning it off at night, then back on in the morning. It's unlikely the power surge is large enough to warrant leaving it on:

Overnight, say 8 hours, the device will use
8 W x 8 hrs = 64 W-hrs​
We can ask, what would the power and current have to be to use this much energy in 3 seconds when the device is turned on? If the power is "P", then
P x (3/3600) hrs = 64 W-hrs
P = 64 x 3600 / 3 W
P = 77,000 W
And for current,
I ≥ P / V
I ≥ (77,000 / 120) Amps
I ≥ 640 Amps
This is at least 30 times what most household circuits (in USA) can handle, 15 to 20 A!

So leaving the device on overnight will use at least 30 times as much energy as is used in the surge when the device is turned on.

You did say "a few seconds", and I assumed 3 seconds in this calculation. I also assumed "overnight" is 8 hours, and a 120V household. You can adjust those numbers if they are different, but the basic conclusion won't change.

Hope that helps.

11. Mar 23, 2009

### Pumblechook

Power factor can complicate things.

12. Mar 23, 2009

### Staff: Mentor

If you know the wattage and time, you were already given the method for figuring out your answer. For example:

If the calibration period is 10 seconds and it uses 1000 watts during that time, it consumes 10*1000/3600 = 2.8 Watt-hours of energy.

If the device uses 10 watts constantly for 10 hours at night, it uses 10*10=100 w-h

13. Mar 23, 2009

### Staff: Mentor

For this calculation, not enough to matter.

14. Mar 24, 2009

### Pumblechook

Even if the max power available at home (about 14 kW) was drawn for 15 seconds it wouldn't be very much energy.

14 x 1/240 kWh = 60 wH ... about 1p worth of elec.

Medium power devices run for many hours account for the bulk of your elec use..fridges..freezers..high power lights..large TVs. Heaters on for a few hours.

Kettles, microwaves, cookers (unless used heavily), showers, vacuum cleaners wont use much because the running time is not long.