i need to roughly calculate kwh 2

[kfoyil]

I am trying to correct a bad power bill I know the amount of ampa i pulled and for the time period and i know the voltage of the incoming line and i know the pf how do i calculate the total kwh for a certain amount of time

 

[dpc]

If 3-phase: current x kilovolts(line-to-line) x pf x 1.732 gives you kW. Multiply that by the number of hours and you have kWh.

 

[sslobodan]

In one of my previous jobs I needed to calculate exact power consumption on television Repetitor Stations trough country so they could pay bills in front. You take Universal measuring device, first you measure voltage on a single phase (although some would say it is a balanced system in 90% of cases phases are not so balanced) than you measure Amperage consumption on that phase. Than you multiply them, do that on each phase and you will get the power consumption in KW for each phase add them and that is consumption you have you take that much kilo vats per hour KW/h also. Now this goes for constant consumption, but if you have devices that goes on and off that is entirely different story, but you can get their consumption on same way than measure how much they are on and off so if they have 4KW consumption they spend 1kW/h for 15 minutes and 4kw/h for hour

 

[itsmoked]

sslobodan; If your measure the voltage and the current and multiply them.. You get the kVA not the KW. You need the power factor to sort out the kW.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[sslobodan]

The way the power meter in Europe (and I doubt Us is different) is working is the principle above. I am not discussing what he will calculate but what he should pay to a power company and they are not crazy to charge him only active power but they charge reactive power. he made doubt that his power meter is calculating correctly this is how he can check it.

 

[slavag]

Slobodan, as I know, in all world customer pay to utilities per kWh and some additional ( is depend on country) per
bad PF , if your PF is OK ( probably more from 0.92) you haven't pay additionals.
Of course you pay also according to TOU.
Biling report possible only with revenue meter.
But for some value, DPC recommendation OK, we check in this way utilities meter.
Regards.
Slava

 

[sslobodan]

http://www.allaboutcircuits.com/vol_2/chpt_12/3.html

This is scheme of a standard power counter used from all electro power companies in the world (electronically differ now but they work on basically same principle)

 

[Skogsgurra]

Slobodan,

You are rather ill-informed for an electric engineer.

It is true that the device shown in your reference multiplies U and I. But it goes that microsecond for microsecond, not using an average or RMS value.

You could say that your method is scalar multiplication while the method used in kWh meters is vector multiplication. As we all know, you need to include the angle between the units to arrive at a correct answer when vectors are multiplied.

The angle, in this case, is the phase angle, phi, between U and I. And it enters the multiplication as cos(phi).

As you also should know, PF = cos(phi). We are limiting this discussion to sine waves and linear loads. Distorted waveforms define PF in another way.

So, dpc (the guy that first answered the OP) gave a complete and correct answer. No need to confuse things like you have done. Please do not ruin this great forum by adding misconceptions, downright mistakes and "feelings" about technical facts.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[peebee]

Either dpc's or sslobodan's answer should be good enough. The time and current measurement error will probably be much worse than the 100% PF assumption error.

In the US, most utilities monitor kWh. Some charge penalties for low PF, with different penalties kicking in at perhaps 95%, 90%, 85%, 80%, etc -- stepped.

I'm not aware of a single US utility that measures kVA (except to apply stepped PF fees) or that bills based on kVAh.

 

[jraef]

Either dpc's or sslobodan's answer should be good enough.

Let's not forget, we are all supposed to be Engineers here. "Good enough" is for technicians and electricians and sslobodan's answer is just plain inaccurate. If kVA and kW were calculated the same and "close enough" why would we have two separate terminologies to begin with?


Nope, sorry, I have to agree with Gunnar. Only dpc's answer fits the question and deserving of that star just for being the first responder. To be honest though, it's an answer right out of every text book on AC Electricity 101, page 2, right after the cute little diagram of a Sine Wave that seems to exist in every version. This wasn't a difficult question people; no need to make it one.

 

[kfoyil]

Thanks very much jraef you are so right. The main problem i had was i didn't have the text book with me. Thanks

 

[peebee]

jraef: I absolutely disagree. Unless kfoyil has some pretty sophisticated power metering equipment available, or some pretty sophisticated time vs. current plotting going on -- and it's pretty clear from his question that he has neither -- then the inaccuracy due to neglecting power factor will be absolutely meaningless compared to the inaccuracy in his assumptions regarding his actual voltage, load profile, demand charges, etc.

Engineers use "good enough" assumptions all the time. We'd never get anything done if we designed everything to 10 significant digits. And for you to state otherwise is just plain inaccurate.

 

[scottf]

To say he knows the voltage of the incoming line ought to be a red flag that we're not dealing with exactitudes.

Clearly there is a big difference between knowing the rated voltage of the line and the voltage needed to calculate kWh

He doesn't even seem to have enough info/measurements to calculate instantaneous kW, much less kWh, with any kind of precision needed, unless he thinks his power bill is off by a factor of 2 or 3 times.

 

[peebee]

Exactly. For kfoyil's purposes, he only needs enough detail to show his utility that his power bill has a meaningful inaccuracy, or could have a meaningful inaccuracy.

He doesn't need to calculate the bill down to the cent (and any inaccuracy smaller than half a cent is meaningless to everyone concerned). He just needs someone at the utility to take him seriously and say "hmmm, kfoyil, you just might be on to something, we'll take another look at this bill and get back to you". So long as his calc does that, it's good enough.

 

[stevenal]

A meter should be within a half percent, which is dificult to test without the right equipment. The utility in question should have the test record for the meter using references traceable to NIST, at least in the USA. If not tested recently, they should be willing to do it for no charge.

Something else to consider: If usage went high one billing period and low the next, or vice versa, it was do to a reading error. This error is self correcting, as one period compensates for the other. The meter continues to increase its register regardless of what the reader wrote down.