KWH to KVA conversion

I am working on a transformer load balancing program, and need to find some info that I have not been able to find on the internet. Meters read in kWh, but the transformers are rated in KVA. For Single-Phase meters, I have been told that you multiply the kWh times .0035 (customer wasn't sure where this number came from - if anybody has any ideas, please shed some light on this subject). For example, 4,760 kWh is equal to 16.66 KVA (4760 * .0035 =

16.66). Is this correct? Assuming that part is correct, how do I calculate the KVA from the kWh for 3-phase power? The customer said that the .0035 conversion factor doesn't work for 3-phase meters. For example, if I have a 3 phase meter that used 46,951 kWh, does it overload a transformer with a capacity of 150 KVA? How do I calculate that? Thanks, Wes
Reply to
Wes Faul
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NO.

The *easiest* way to get kVA for a single phase load is to measure the current it draws and the voltage supplied. kVA then is just current times voltage, divided by 1000.

For three phase, use line to line voltage, and the highest phase's current reading and calculate.... kVA = Vline * Iline * sqrt(3) / 1000

To use the kWh, first, you must find the kW of the load, not the kWh. To do this you need to know over what time period it took to accumulate those

4,760 kWh. And you need to know if that was running continuously at a fixed loading, or if it varied. If fixed, just divide kWh by the number of hours and you have kW. If varying or cycling on/off, then you're much better off getting a kW/kVA meter. Many such meters exist and can display either kW or kVA (some even will measure kWh, but not all).

If you don't get a kW/kVA meter, but deduce the kW rating some other way, then to convert kW to kVA, simply divide by the power factor. Problem there is that to know the power factor, you need to have a meter that measures that. Again, a good kW/kVA meter often has the option for reading the power factor, but if you have kVA meter, you don't need to proceed any further.

Using 46,951 kWh in a steady, continuous load for 30 day billing cycle of the utility (720 hours), would mean your load draws 46951 kWh / 720 h =>

65.2 kW. If it runs at a 0.9 power factor, that would be 65.2 kW/ 0.9 => 72.5 kVA.

But if it uses those 46,951 kWh by running only 1/3 of the time, then 1/3 of

720 hours is only 240 hours. *That* would be a load of 46951 kWh / 240 h => 195.6 kW. If it runs at a power factor of only 0.7, that 195.6 kW load would be 195.6 / 0.7 => 279.5 kVA.

You really need a proper kW/kVA meter to do this. Guessing at the power factor or the number of hours the load runs to use 46,951 kWh is just that, guessing.

Frankly, that 0.0035 'conversion factor' sounds like BS. It could *only* be right if the customer knew how many hours and exactly what power factor it takes to use that 4760 kWh. Chances are, he/she doesn't. If they did, they wouldn't be asking you to figure the kVA rating.

daestrom

Reply to
daestrom

| I am working on a transformer load balancing program, and need to find some | info that I have not been able to find on the internet. Meters read in kWh, | but the transformers are rated in KVA. For Single-Phase meters, I have been | told that you multiply the kWh times .0035 (customer wasn't sure where this | number came from - if anybody has any ideas, please shed some light on this | subject). For example, 4,760 kWh is equal to 16.66 KVA (4760 * .0035 = | 16.66). Is this correct?

There are two unknowns you need to convert with. Time and power factor.

Reply to
phil-news-nospam

Time is usually 30 days (720 hours). Where would I get documentation on the power factor at? Should it be in the documentation for the meters?

Does this sound correct?

4760 KWH / 720 H = 6.61 KW. Power Factor = 6.61 KW / 16.66 KVA Assuming that the KVA they gave me was correct, that means the power factor is roughly .4? Everwhere I've seen defaults to .85 for the power factor. Is anybody out there familiar with electricity meters that power companies, pud's, etc... use? Thanks, Wes

Reply to
Wes Faul

| Time is usually 30 days (720 hours). Where would I get documentation on the | power factor at? Should it be in the documentation for the meters?

Power factor is the relationship between volt-amps (power drawn) and watts (power used). It affects the phase lag (or lead) of the current relative to the voltage. A power factor of 1.0 indicates all drawn power is used, and the current is in phase with the voltage. Resistive loads have this characteristic. Reactive loads will drawn more power then give some back at other parts of the voltage wave cycle.

| Does this sound correct? | 4760 KWH / 720 H = 6.61 KW.

6.61111111111111 :-)

| Power Factor = 6.61 KW / 16.66 KVA | Assuming that the KVA they gave me was correct, that means the power factor | is roughly .4? Everwhere I've seen defaults to .85 for the power factor. | Is anybody out there familiar with electricity meters that power companies, | pud's, etc... use?

That's quite a low power factor, but motors could do that. Perhaps default values listed are just typical sums resulting from a common mix of resistive and reactive loads.

Power drawn and returned is still costing the power company something, such as I2R line losses in transmission and distribution.

Reply to
phil-news-nospam

I have the customer checking with his engineer to see if this sounds about right. Is the calculation from KWH to KVA the same for 3 phase as it is for

1 phase? Or do I need to divide it by 1.732 (square root of 3)? Thanks, Wes
Reply to
Wes Faul

Reply to
Phil Munro

Hi, Dr. Munro,

He wouldn't be taking EE at YSU, would he?

Al Stiver, still recover> There is NO conversion from KWH to KVA! Those are two different

Reply to
Alan Stiver, PE

kWh is a measure of energy consumed. 4760 kWh over what period. One month? Lets assume that the formula is based on the monthly consumption. The

0.0035 factor appears to be a rule of thumb relationship between kWh and peak consumption and would only be valid for a certain class of loads, in this case probably residential loads. The 0.0035 factor is based (among other things) on a peak to average load ratio of 2.5.

Or for different load types either. Even single phase commercial loads will have different load characteristics and power factors. So don't use that number if you don't know what its for.

Is 46,951 kWh a monthly consumption? If so, divide by 720 (hours per month) to get the average kW demand. Then multiply by the proper peak to average load factor for the particular load type (good luck finding these numbers). Finally, divide by the load power factor (which also depends on the load type). Using your example, if the load factor is greater than 2.3, or the power factor worse than 1.0, your transformer is overloaded.

Reply to
Paul Hovnanian P.E.

The period is 1 month (720 hours - 30 days/month * 24 hrs/day). This works out to be roughly 6.61 kW. Assuming the 16.66 kVA is correct, this works out to roughly .4 for the power factor. Does this sound right?

Thanks for the info. I just scheduled a conference call with this customer's lead engineer to try to hammer some more of this stuff out. Wes

Reply to
Wes Faul

No. A power factor of 0.4 is quite unusual for most electrical loads.

You are forgetting that the 6.61 kW you calculate is an average over 720 hours. Most loads vary over time and can be (roughly) characterized by a peak to average load factor. Your 0.0035 kVA/kWh factor works out to a peak of 2.5 times average (assuming unity power factor).

Assuming a unity power factor for residential loads is pretty good for what you are doing. For commercial and industrial loads (both single and three phase) both your peak to average load factor and the load power factor will require more careful consideration. Simple rule of thumb factors may not be sufficient. [snip]

Ask about how they classify their customer types and what sorts of load factors they apply for each. The better you can break down customer types (like residential vs commercial, type of space heat, etc.) the better. Your application should allow these rule of thumb factors to be overridden for special case customers or if actual demand and/or reactive power metering data is available.

Reply to
Paul Hovnanian P.E.

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