# How do I calculate KWH usage?



## MeLikeUmFire (Jan 18, 2010)

Im looking at a blower that is .92 amps for my insert. My KWH that I pay for electricity is .06486. How can I calculate how much per hour my blower will cost me to run?


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## mellow (Jan 18, 2010)

Most calculators I know of require the watts.  If you can find that out here are some you can use:

http://www.paystolivegreen.com/2008/09/energy-savings-calculator/
http://www.citytrf.net/costs_calculator.htm


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## woodgeek (Jan 18, 2010)

0.92A x 120V = 110W = 0.110 kW

in each hour, you use 0.110 kWhr * $0.0648 /kWhr = $0.0007/hr or 17 cents per day.

Your elec is either very cheap, or your mis-reading your bill--is there a delivery charge on top of that?


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## MeLikeUmFire (Jan 18, 2010)

oops, yup. distribution charge of .03547 and a "energy optimization" (WTH??) of .001081 on top of rate above


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## BrowningBAR (Jan 18, 2010)

RegencyR14 said:
			
		

> oops, yup. distribution charge of .03547 and a "energy optimization" (WTH??) of .001081 on top of rate above



Still cheaper than PA. I am at 15 - 16 cents per KWH.


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## Sleepy (Jan 19, 2010)

You cannot go by the rating if you want to be accurate. I have checked amperage on many blowers and the numbers are never at rated amps. Too many variables. The best way to measure watts is to kill everything else (which is a pain to do) and clock the meter, or to buy a small watt meter like: 

http://www.newegg.com/Product/Produ...ctronic+Gadgets-_-P3+International-_-82715001


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## Gooserider (Jan 19, 2010)

As stated, data plate info isn't really useful for this - generally it will be on the high side, as the plate listing will be for the "peak" load, such as when a motor is starting, and many appliances will draw less once they are running...  Also any appliance that cycles on and off will also be somewhat problematic.

OTOH, the data plate numbers will give you a "worst case" value, so it isn't totally useless.

For 110VAC plug in appliances, I suggest a "Kill-A-Watt" meter - it's a nice little unit that plugs in between the wall and whatever you want to measure, and does a reasonably accurate job of measuring the actual draw.  If you have something like a freezer that cycles, if you leave it plugged in for a while it can give you a pretty good idea of the average consumption over time.  Last time I priced them you can get them for around $25 from a lot of different sites.

Nominally, if you multiply current drawn by voltage, you should get watts, but this can get tricky with anything that has motors or transformers, as these cause a "phase shift" that will mess up the calculation a bit - however it will usually be "close enough" if you don't see a watt rating.  

Then it is simply a case of watts / 1,000 to get the number of kilowatts / hour the device uses, multiplied by the amount of time it's on...

Gooserider


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## russ bailey (Jan 21, 2010)

volts * amps = watts so 120 volts*,92 amps *24 hours*,065 unit cost/1000 kWh conversion = 17,2 cents per day 

The label is typically the 'full load' rating (when the blower is operating at maximum design flow) - you will probably be less than that 

Start up current will momentairly be higher but no big deal - ignore it.

İ am not sure what this 'phase shift' stuff is but don't worry about it. İf Gooserider is talking about power factor you can ignore it on something this small. 

You would have trouble measuring the difference.


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## jebatty (Jan 21, 2010)

In MN residential customers are charged for electricity based on watts and not volt-amps. Watts and volt-amps are the same for pure resistance AC loads but not for inductive loads. Inductive load volt-amps > watts. The watt reduction factor is power factor (PF). Many commercial/industrial customers are charged for electricity based on PF, which is why they often have capacitor banks and purchase high PF equipment to keep their electricity charges down. For residential customers, the loss to the power company in charging for watts rather than volt-amps is made up by a higher rate charge, so a residential customer has no rate incentive to purchase high PF equipment. I believe one factor of Energy Star for electrical equipment is meeting a higher PF standard.

If you have a Kill-A-Watt or similar meter, you can see the difference, as it shows both watts and volt-amps, as well as amps and PF.


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## Gooserider (Jan 21, 2010)

The phase shift is the action that is going on to cause your Power Factor / PF number...  Essentially, in a pure resistance load if you were to look at the current and voltage on an oscilloscope, you would see that both curves lined up exactly, or were in phase...  If you looked at a load with inductive or capacitive properties, you would find that the two curves would shift in one direction or the other in an amount and direction depending on if it were inductive or capacitive, and the ratio of capacitive / inductive properties to the resistance - in theory the hypothetical "pure" capacitive or inductive load would cause a 90° shift, and therefore consume ZERO "watts" of power, but still have a high Volt x Amps number

In almost all cases there is far less of a shift, I would guesstimate that you would typically see more on the order of less than 10° or so, and usually in the inductance direction since most of our loads tend to be motors and transformer based, which are inductive.

The Power Factor is simply the factor by which the VA and the Wattage numbers for a given load differ - and unless one is getting into really fussy numbers, it can usually be ignored so that you can treat Volt-amps and Watts as interchangeable...

As Jim mentioned, if you are curious, you can see both numbers on a Kill-a-watt meter.

Gooserider


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## pyper (Jan 21, 2010)

I have one of those Kill-a-watt devices and it's really neat.

Since it will keep a running tally you can use it for things like refrigerators. Mine seems to run all the time, but the cumulative power usage isn't too bad. I also found out my tv, stereo, ect., takes 12 watts when it's all off. 

The most interesting finding was regarding computers. All the ones here have 450 watt power supplies, but they actually only use about 100 watts -- the 450 watt rating is another worst-case example.


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## jebatty (Jan 21, 2010)

The volt-amp number is important for people who use a UPS as backup power. UPS ratings normally are in VA, not watts. So knowing the volt-amps of the equipment (boiler, circulators, etc.) to be provided backup power may be important in this case so as not to overdraw the UPS circuitry.


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## pdboilermaker (Jan 22, 2010)

Here is a little excel spreadsheet that one of my buddies and I made to determine KW use/ price to calculate payoff time for a windmill.  You may need to go into the formula and adjust your electric price.  Oops, a .xls wont upload PM me and I will email it to you


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