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# Power consumption

## Power consumption

(OP)
Hi,

I have the following doubt: My client wants to record the power consumption in KW/Hr. I can take the power measurement from the relay in Kw, how I can use this value to get the Kw/Hr consumption?

Thanks

### RE: Power consumption

Since power in kW is the time rate change of energy,  kW/hr would be the second derivative of energy with respect to time. Sort of an energy acceleration. You sure you're not looking for energy in kWh?

### RE: Power consumption

If the relay has a load profile function, you could approximate kWh consumption by downloading the load profile and summing the kW for each period of the load profile then multiplying by the period in hours.

### RE: Power consumption

If you know the average kW for a specific time period, you can get the ENERGY consumpption (kWh) for that time period by:

kW * (time period, in seconds) * (1 hr / 3600 sec) = kWh

If you know the average kW for many specific time periods, just add do the above calculation and sum all of the periods.

(Note that kWh and kW/Hr are VERY different quantities).

### RE: Power consumption

It sounds as though you are dealing with someone who has a vague idea of what he wants but doesn't know exactly how to ask for it.  He may not even know it was what he had asked for if he were to get it.

A graph of kW versus time gives instantaneous consumption, which is what he is asking for.  kWh would be the integral of the kW graph.  Such a graph would provide a visual indication of demand fluctuations (kW/hr), but I have never seen a need to put a numerical value on such a quantity.

Depending on the particular rate schedule, utilities can charge for

Demand (kW)
Energy (kWh)
Fuel   (kWh)

and perhaps other things such as capital cost of supply, although this is usually reflected in the demand charge (a per kW charge based on peak kW usage during a moving period).  Decreasing the demand (peak shaving) is one of the most effective ways to cut the electric bill.

Regards,
William

### RE: Power consumption

One way to obtain the energy consumption for a particular period of time is plotting a graph KW vs. time. Simplification could be possible approximating the curve to regular shapes and adding the partial areas.

I am not sure if you should need the variation of power in time KW/Hr as you requested. If still this needs to be estimated determine the tangent of the curve in several points along the desired interval.

Suggestions:
1-    Check if the data for KW are in RMS or instantaneous value.
2-    Are you only interested in active power P (kW) or total power S (kVA)?

### RE: Power consumption

Does your client really want KW/hr, or kWh?  If kWh, there are many power meters that can do this for you.  You probably have one on the side of your house that your utility company uses to monitor YOUR power consumption.

Vince Socci
www.ontargettechnology.com

### RE: Power consumption

You can get revenue meters of different types for submetering such as in large office buildings. Some of these are hardwired units such as from Emon. Some of the more astute electrical supply houses can get you a conventional kilowatthour meter that you can hook up to your system.

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