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- Thread starter hah2110
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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.

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dlgoff

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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.

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

Edit: RB beat me to it.

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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.

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https://www.amazon.com/dp/B001OTA8TY/?tag=pfamazon01-20

However, if the power surge lasts only a few seconds, then I don't see how leaving something on overnight could possibly save power.

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https://www.amazon.com/dp/B001OTA8TY/?tag=pfamazon01-20

However, if the power surge lasts only a few seconds, then I don't see how leaving something on overnight could possibly save power.

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?

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- #9

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

- #10

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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 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", thenP x (3/3600) hrs = 64 W-hrs

P = 64 x 3600 / 3 W

P = 77,000 W

And for current,P = 64 x 3600 / 3 W

P = 77,000 W

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!I ≥ (77,000 / 120) Amps

I ≥ 640 Amps

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.

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Power factor can complicate things.

- #12

russ_watters

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

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.

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

Plug your numbers into the formula and you'll have your answer!

- #13

russ_watters

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For this calculation, not enough to matter.Power factor can complicate things.

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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.

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