Electricity bill

Thread Starter

Erica

Joined May 18, 2011
15
On the utility electricity bill the unit is kWh. Is the power measured a apparent power (kVA) or a true power (kW)?
 

bountyhunter

Joined Sep 7, 2009
2,512
It is supposed to be true power (Watts). If the power factor of your house gets too low (and you are pulling to many VARs), they can install a VAR meter and charge for those too. Most houses have PFs around .85 so it's not a problem.
 

Adjuster

Joined Dec 26, 2010
2,148
Mercifully, the electricity companies do not generally charge private customers for apparent power, nor penalise them for poor power factors.

At least, that's the current state of play in my country (UK). Perhaps though in these modern days of smart meters and a quest for ever greater efficiency, one day this may come to us. How is it done in the USA?
 

Adjuster

Joined Dec 26, 2010
2,148
It is supposed to be true power (Watts). If the power factor of your house gets too low (and you are pulling to many VARs), they can install a VAR meter and charge for those too. Most houses have PFs around .85 so it's not a problem.
That's intriguing: I had never heard of a domestic customer managing to annoy the supply people in that way. Would this be likely to happen with a truly domestic load, say with a lot of air conditioning, or more likely a small business using machines being run out of somebody's home?
 

Evil Lurker

Joined Aug 25, 2011
116
Personally I believe after my smartmeter was installed and the bill went up that the power company is now charging for apparent power.

Those smartmeters are some tricky little bastards. One wrong setting in its firmware and you get billed out the wazoo.
 

Thread Starter

Erica

Joined May 18, 2011
15
Thanks for the reply. I also believe it is true power as the unit is kWh in the electricity bill. I am wondering how the traditional meter works and how the power is measured.

The meter may be able to measure the r.m.s values of the voltage and current. But how can it get the power factor?
 

Wendy

Joined Mar 24, 2008
22,164
I don't have all the answers, but one short one is they don't care. It the power company can squeeze more money out of people they will. It is like being presumed guilty before a court of law, it is up to you to prove their meters are not all they claim to be.
 

bountyhunter

Joined Sep 7, 2009
2,512
That's intriguing: I had never heard of a domestic customer managing to annoy the supply people in that way. Would this be likely to happen with a truly domestic load, say with a lot of air conditioning, or more likely a small business using machines being run out of somebody's home?
Small business, especially if it has a lot of motors running. It would be unusual, but the reason the power company cares is because it causes too much phase shift along the line. They have to put giant correction caps next to a house that looks too inductive. They care about the phase because it affects the generator's efficiency.
 

bountyhunter

Joined Sep 7, 2009
2,512
Thanks for the reply. I also believe it is true power as the unit is kWh in the electricity bill. I am wondering how the traditional meter works and how the power is measured.

The meter may be able to measure the r.m.s values of the voltage and current. But how can it get the power factor?
I recall from 40 years back, there is a way to get the true power by putting the inductors of the meter motors at right angles (?) The old mechanical meters were supposedly true watt power. The new electronic ones are totally different.

I guess my old brain was right:

Electromechanical meters

The most common type of electricity meter is the electromechanical induction watt-hour meter.[15][16]
The electromechanical induction meter operates by counting the revolutions of an aluminium disc which is made to rotate at a speed proportional to the power. The number of revolutions is thus proportional to the energy usage. It consumes a small amount of power, typically around 2 watts.


The metallic disc is acted upon by two coils. One coil is connected in such a way that it produces a magnetic flux in proportion to the voltage and the other produces a magnetic flux in proportion to the current. The field of the voltage coil is delayed by 90 degrees using a lag coil.[17] This produces eddy currents in the disc and the effect is such that a force is exerted on the disc in proportion to the product of the instantaneous current and voltage. A permanent magnet exerts an opposing force proportional to the speed of rotation of the disc. The equilibrium between these two opposing forces results in the disc rotating at a speed proportional to the power being used. The disc drives a register mechanism which integrates the speed of the disc over time by counting revolutions, much like the odometer in a car, in order to render a measurement of the total energy used over a period of time.
http://en.wikipedia.org/wiki/Electricity_meter
 
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#12

Joined Nov 30, 2010
18,210
There is no rule against asking your local power company how your meter works, and they probably know more than we do.
 
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