Utility kWH vs kW

[JoeStillman]

But if a load of 1,220 persisted for 15 mins on a 2000A breaker I would be very impressed that the wire is still in solid. Its worth noting that there was an Siemens Energy meter that measured the maximum demand load as 350 kW. The building doesn't have any outages or breaker trips so that's why I'm dubious about trusting the 1,220 number.

@JoeStillman & @ron In post #8 by "ratcheted" value do you mean that they pad the number out based on demand loads for that month? I pretty sure I understand what you are saying but the next month the Billed and Measured demand are the same number at 239.8 kW.

So are you saying that between 239.8 kW and 280 kW there is a service change were they round up just charge you for 1220 kW because its the next size up?

You're right, that is too much for a 208V service and surely would have tripped the main. How many months did the 1220 show up on the bills?

Yes, I meant exactly what you said about "ratcheted". They pad out the bill because of the infrastructure that the utility has to have to supply that many watts. Here in PA, you have "contract minimum and maximum demand" with the utility - the amount that the POCO agrees to build capacity for. If your minimum demand is 1220 kW and your metered 15 minute peak is 280 kW, you would pay for 1220 - and request a new contract!

I would call the utility and ask them to explain the anomoly.

 

[Revous]

You're right, that is too much for a 208V service and surely would have tripped the main. How many months did the 1220 show up on the bills?

Yes, I meant exactly what you said about "ratcheted". They pad out the bill because of the infrastructure that the utility has to have to supply that many watts. Here in PA, you have "contract minimum and maximum demand" with the utility - the amount that the POCO agrees to build capacity for. If your minimum demand is 1220 kW and your metered 15 minute peak is 280 kW, you would pay for 1220 - and request a new contract!

I would call the utility and ask them to explain the anomoly.

It only happened only once in a 12 month period. For every other month the billed vs measured demand loads were the same and less than 280 kW.

Thanks for taking the time to explain, I really appreciate it. So now..... I'm going to wait on hold for a few years to get answers out of PSE&G!

 

[Besoeker3]

Here in the US, where electric bills have a component for demand and a component for usage, you pay a certain amount for your highest 15 minute demand (kW) and another amount for your usage, which is the total kWH for the month. It's how the meters work.

A 15 period minute is energy. kWh. A kW is instantaneous, not energy.

 

[Revous]

A 15 period minute is energy. kWh. A kW is instantaneous, not energy.

I think the utility averages the kW readings for a 15 min period

 

[wwhitney]

I disagree. The exception makes no such implication. It only addresses power, not the total energy used.

The exception parenthetically defines "maximum demand" as "the highest average kilowatts reached and maintained for a 15-minute interval". Average power = Energy used / measurement period, and the measurement period is specified as 15 minutes. So the exception does, in fact, refer to energy or average power, and not instantaneous power.

Cheers, Wayne

 

[Besoeker3]

I think the utility averages the kW readings for a 15 min period

Possibly so but that isn't instantaneous - a fifteen minute period is energy.

 

[retirede]

Possibly so but that isn't instantaneous - a fifteen minute period is energy.

Think of it this way….take an infinite number of instantaneous power (KW) readings over a 15 (or any) minute period and average them.

Since “infinite” isn’t a practical number of measurements to take, it’s just some big number of readings.

 

[Besoeker3]

Think of it this way….take an infinite number of instantaneous power (KW) readings over a 15 (or any) minute period and average them.

Since “infinite” isn’t a practical number of measurements to take, it’s just some big number of readings.

The kW is just a unit. That's it.

 

[jaggedben]

The exception parenthetically defines "maximum demand" as "the highest average kilowatts reached and maintained for a 15-minute interval". Average power = Energy used / measurement period, and the measurement period is specified as 15 minutes. So the exception does, in fact, refer to energy or average power, and not instantaneous power.

Cheers, Wayne

What he said. Thanks Wayne.

Theoretically any metering device has a measurement interval, so the code should define one if it wants something realistically enforceable. The exception seems to imply 15 minutes is okay. Well, at least for the exception. But if that's what the utility is doing anyways...

We've discussed this in at least one other thread.

 

[Charged]

Thanks all for the good information in this thread. It’s something I’m dealing with now myself. I read something earlier in another thread but I figured I’d throw it out here because it’s associated with the topic. If the data is presented in the bill as KW do you apply the power factor before the 1.25 required by the calculation since it’s not presented as kva?

I saw an explanation in another post that the 1.25 was a way to account for pf. Made sense.

 

[Jraef]

There is no way to extrapolate peak kW from a kWh value.

1000kWh in a monthly billing cycle can be 2kW steady for 500 hours, in which case the peak is 2kW, or 1kW for 729 hours and 271kW for an hour, in which case the peak is 271kW. There is no way to tell unless you were measuring the kW and recording it.

 

[GoldDigger]

Just a note: In the US peak-based billing (demand charge) is pretty much exclusively for commercial customers and is not used for residential accounts.
POCO generally does not worry about the effect of a short high demand on a residential circuit where a single transformer serves multiple customers. They will make the transformer larger if and when it burns up.

 

[JoeStillman]

The kW is just a unit. That's it.

In the context of an electric bill, it's an average that gets calculated every 15 minutes, not the integral over 15 minutes time.

 

[Jraef]

In the context of an electric bill, it's an average that gets calculated every 15 minutes, not the integral over 15 minutes time.

Well, technically that is kW Demand.

 

[ggunn]

Well, technically that is kW Demand.

Well, yes, but it is using a short term kWh measurement to approximate an instantaneous kW demand.

 

[jaggedben]

There's another thread that's been going on this.

Apparently in at least some cases when the utility gives you peak demand it is just the highest kwh in a 15min interval, mutliplied by 4 to convert to kW. So if that's all they're doing, no reason we shouldn't be able to do it too, if you have 15min interval data.

 

[ggunn]

There's another thread that's been going on this.

Apparently in at least some cases when the utility gives you peak demand it is just the highest kwh in a 15min interval, mutliplied by 4 to convert to kW. So if that's all they're doing, no reason we shouldn't be able to do it too, if you have 15min interval data.

Dimensionally, kWh/15min X (4)(15min)/h = kWh/h and the h's cancel.

 

[rlqdot]

i know that i am coming in late on this conversation, but these comments may be worth considering.

KW is always an instantaneous measurement. when a "demand tariff" rate is the chosen utility rate structure, the utility will install a demand meter that measures the KW. those meters reset to zero at a regular interval (often every 15 minutes) and store the largest measured demand KW that occurred during any interval in the entire billing period. SO, it is possible that a demand representing an amperage that exceeds the service overcurrent device rating could occur - that demand may only be present on the service for milliseconds, so it does not cause the fuse or circuit breaker to trip, but the utility supplied that demand. i personally have never found this to be the case, probably because most meters are not measuring the demand in that short of a time period. that measured peak value is stored and, until it is exceeded, it becomes the peak demand value for that particular billing period. at the end of the billing period, the entire demand history resets to zero and the process starts all over again.

the other thing to remember is that the CTs used to measure the demand may or may not be sensitive enough to measure the full value - so there may be adjustment factors. i have seen meters with the "CT Multiplier" written right on the meter. in that case, the measured demand may be a given value, but that value has to be multiplied by the CT Multiplier to reveal the actual demand.

then, based on the utility tariff document, the peak demand for a given billing period may be the one that is applied for one billing period, for one season (heating or cooling season for example, with the actual months defined in the tariff documents) or an entire year - that all depends on how the tariff is written.

Like you, i am a design engineer, and when we do a building addition or add more equipment to an existing service, we frequently use existing demand measurements to establish the actual service demand - multiplying the measured value by 125% as directed by code. we then add the new calculated demand load to that value to arrive at the total new service demand. in general, i have found that you can use the highest demand KW that is reported on the utility bill for the preceding 12 months and that will be acceptable to most permit officials.

let me know if this helps or how it compares to what you may have learned about the measurements from PSE&G. i have also found that in most cases, the utility area engineer and / or the metering department will be glad to explain how their demand measurements are collected and what the numbers on the monthly bills really mean.

 

[wwhitney]

KW is always an instantaneous measurement.

Abstractly, that is true, but no measuring device can record it instantaneously. A measuring device will always have some averaging period, be it in microsecond, milliseconds, or seconds.

The principal point of uncertainty in this thread and the thread linked to in post #36 seems to be what averaging period is used for "demand" as specified in utility tariffs and for NEC 220.87 (which could be a priori be different). So far the tariffs that have been discussed are actually using a 15 minute average period--i.e. energy is billed on 15 minute intervals, and "demand" is just the highest such interval during a billing period.

If you have any pointers to a utility tariff that actually specifies a "demand" value determined over a shorter averaging period, that would be very interesting.

Cheers, Wayne

 

[jaggedben]

i know that i am coming in late on this conversation, but these comments may be worth considering.

KW is always an instantaneous measurement. when a "demand tariff" rate is the chosen utility rate structure, the utility will install a demand meter that measures the KW. those meters reset to zero at a regular interval (often every 15 minutes) and store the largest measured demand KW that occurred during any interval in the entire billing period. ...

That description just raises more questions for me.

Why would a meter reset every 15 mins if the max demand charge is for a whole billing period? Wouldn't the only reason be because it measures demand as kWh consumed over a 15min interval?

Why and what would a digital meter that reports interval kWh data need to reset? Are you therefore describing an analog meter?

I have loads of curiosity about how utilities actually handle demand metering, and how it had changed as we've moved from analog to digital meters. I'm all ears for direct evidence of how they do it. But so far, the only official utility sources I've seen seem to described measuring average energy consumption over 15min or sometimes 5 min intervals, not actually directly measuring max power, for which an interval of that timescale would not make sense.