How to Calculate KWH from instananeous KW

[rPraveenkum]

HI any one tell me how to How to Calculate KWH from instananeous KW
thanks in advance

[Steve Bailey]

Accumulate the instantaneous kW readings over time.

If you sample the kW value once per second and add the sampled value to a running total, you're accumulating kW-seconds. There are 3600 kW-seconds per kW-hour.

[rPraveenkum]

I have to Multiply with 3600 with accumulated value to find KWH?

[SimonGoldsworthy]

No, you have to divide the KWSeconds by 3600 to get KWHours.

[rPraveenkum]

Thank you everybody!!!!!!!!!!!!!!

[bob1371]

Here is a little usefull online power consumption calculator.

http://www.cactus2000.de/uk/unit/massstr.shtml

[TJS]

You can't get an immediate calculation result. You have to integrate over time.

Make a 1 minute clock in logic (or use plc clock SEC=0 to get minute pulses).

On every Minute pulse, add KW value to a memory.
Keep track of how many pulses youve added (index a counter)

After 60 minutes (60 additions), Take the memory KW summation and divide by the counter (60). This will give you the average KWH over the last hour. The user has to wait a whole hour to get an updated KWH value.

You can instead add for 5 minutes (5 counts) then divide the sum by the counter, then multiply by 12 (12- 5 minute periods in an hour). This will give you an 5 minute KWH average that they only have to wait 5 minutes to see a change.


Better yet buy a KWH instrument that does all this for you and delivers a 4-20mA signal or Modbus data. Get it all PF KW KVA KWH continuoulsy.

[rPraveenkum]

HI TJS

this to display in RSView

[DJM]

I sum my kw value every .5 seconds to my accumulated value and divide the accumulated by 7200, also at .5 second intervals to provide a dynamic KWH display. This is done for batching and is cleared at the end of each batch. The batch interval is several minutes. This can also be accomplished by doing it every second and dividing by 3600. You will probably have to use double precision registers to accomplish this.

[russrmartin]

I just did this exact same thing. Check out my thread on this topic. What I finally ended up doing is in the final post.

Russ

[Alaric]

Originally Posted by TJS

The user has to wait a whole hour to get an updated KWH value.

Why?

A 1KW motor running for 30 seconds will use 8.3wH of electricity. You should be able to integrate this and display real time results.

kWH is not a time domain unit, despite the presence of the word "hour" in the term. It is a measurement of a quantity of energy and time is irrelevant to the unit itself. OTOH Watts is a time domain unit because it is not a unit of energy, it is a rate of energy usage. The water analogy would be kWH is like a bucket of a specific quantity of water, while kW is like GPM or LPM.

1 kWH = 3,600,000 joules.
1 KW = 1,000 joules/second.

For a 1kW motor running 30 seconds:
1000 joule/s * 30s = 30,000 joule.

Notice how the time units algebraically cancel. They are no longer a part of the unit.

Since seconds and hours are both time units then multiplying the instantaneous kW by a totalizing period in hours gives us a unit of kW * H, or kwH, but time is not a part of the unit. Thus for a 1kW motor we would add .00833kWH to the energy consumption total every 30 seconds.

There is also apparently some confusion over whether to multiply or divide. Technically, you are multiplying by hours, HOWEVER, there are 3600 seconds in a hour, so the 1 second fraction of an hour is 1/3600, or in other words, dividing by 3600. When working with integers in a PLC, division is prefered and 1/3600 is an exact value while .003333333 is not.

I used a 30 second example, but I recommend totalizing at a faster rate than than.