Utility kWH vs kW

[Revous]

Hello I just need a quick sanity check on this.

When looking at an existing building sometimes we are lucky enough to get utility bill which include both the kWH and kW for the service. As far as I know, the peak kW*1.25 is what in calculations for the existing loads per 220.87.

Is there anything that kWH is used to determine other than consumption? I've seen some calculations where people assume some peak hours and intervals days then use that to extrapolate to a kW value larger than what the utility peak demand is calling for. I don't see any example calculations in the NEC for it so I'm just curious if I'm missing something.

 

[PD1972]

You are correct; per 220.87 you should be using utility peak kW * 1.25 to determine existing capacity.

The 30-day kWh on utility bills may only be used for energy consumption. There is no way to extract peak demand from the kWh shown on a utility bill. In my opinion, any attempt at deriving a kW value from a 30-day kWh is a wasted exercise as there are too many assumptions taken. There isn't anything in the NEC with this type of calculation because it is not a valid calculation.

 

[Besoeker3]

Hello I just need a quick sanity check on this.

When looking at an existing building sometimes we are lucky enough to get utility bill which include both the kWH and kW for the service. As far as I know, the peak kW*1.25 is what in calculations for the existing loads per 220.87.

Is there anything that kWH is used to determine other than consumption? I've seen some calculations where people assume some peak hours and intervals days then use that to extrapolate to a kW value larger than what the utility peak demand is calling for. I don't see any example calculations in the NEC for it so I'm just curious if I'm missing something.

I use kWhr to help people to size their energy use. For UK a kWhr is the typical energy use for a 24-hour day is 10 kWhr. I use this to explain the difference when people decide to invest on an EV. The EV can easily double the home energy bill.

 

[Revous]

You are correct; per 220.87 you should be using utility peak kW * 1.25 to determine existing capacity.

The 30-day kWh on utility bills may only be used for energy consumption. There is no way to extract peak demand from the kWh shown on a utility bill. In my opinion, any attempt at deriving a kW value from a 30-day kWh is a wasted exercise as there are too many assumptions taken. There isn't anything in the NEC with this type of calculation because it is not a valid calculation.

Perfect, this is what I assumed but you never know what you don't know.

 

[jaggedben]

The language in the exception implies that if you have 15 minute kWh data you can use that to derive kW. (Multiplying by four is how you would do that.) I've multiplied 1-hour kW data by four for due diligence on resi projects. I've never had to justify that method to an AHJ, but it's extremely conservative.

30-day kWh data is indeed useless for estimating peak demand.

 

[ron]

If you try to use a months worth of kWh to extract kW, would be similar to taking a long road trip with an average speed of 55 MPH, and claiming to the police officer that you never were going 90MPH at some point.

 

[Revous]

As a quick follow-up, the utility bills I have break out the demand loads between "billed kW" and "measured kW". There is one month where the billed demand is 1220.4 kW but the measured demand is 280 kW. We only have a 2000A 208Y/120V 3 Phase service so a demand of 1220.4 kw is around 3,400A.

Is the billed demand just based on so utility provider magic number to charge people more? The utility in question here is PSE&G.

 

[ron]

The difference between billed demand and measured demand could be 2 things in my experience.

1. The billed demand is a ratcheted value which is a value based on a perilously measured demand and only resets quarterly or yearly, based on the billing plan.

and/or

2. The difference is a way to bill for poor power factor, that they penalize the measured demand with a multiplier (based on how bad it is) to result in the billed demand.

 

[don_resqcapt19]

As a quick follow-up, the utility bills I have break out the demand loads between "billed kW" and "measured kW". There is one month where the billed demand is 1220.4 kW but the measured demand is 280 kW. We only have a 2000A 208Y/120V 3 Phase service so a demand of 1220.4 kw is around 3,400A.

Is the billed demand just based on so utility provider magic number to charge people more? The utility in question here is PSE&G.

Often the billed kW demand is based on the highest 15 minute demand over the previous 12 months. This is added because the utility has to have a system to supply that demand, but if you are using a lot less, they charge a demand fee to cover there costs for the larger supply system that is not being used.

 

[drcampbell]

As a quick follow-up, the utility bills I have break out the demand loads between "billed kW" and "measured kW". There is one month where the billed demand is 1220.4 kW but the measured demand is 280 kW. ...

Is the billed demand just based on so utility provider magic number to charge people more? The utility in question here is PSE&G.

I can't speak specifically to PSE&G, but it's common for each month's billed demand to be the highest demand during any of the past twelve months. Read the fine print in your tariff.

 

[Revous]

Based on the responses above I would assume that you apply 220.87 to the measured demand load and not the billed demand load. But I'm wondering why the numbers are so far apart on this bill. Could it be an error in their system?

 

[ron]

Based on the responses above I would assume that you apply 220.87 to the measured demand load and not the billed demand load. But I'm wondering why the numbers are so far apart on this bill. Could it be an error in their system?

Review Post #8

 

[Revous]

Review Post #8

Okay so, I guess the wrench here is this. From the month of June I have an "Annual Billed Demand" of 1,220 kW but in next month the "Annual Billed Demand" drops to 239.8. The month before was 560 kW. The measured demand load has never exceeded 300 kW for any of the last 12 months.

And after reviewing the PSE&G tarrif you are correct Annual Peak demand is defined as:
Annual Peak Demand: The customer’s Annual Peak Demand in kilowatts shall be the highest Monthly Peak Demand occurring in any time period of the current month and the preceding 11 months.

So I would expect 1,220 kW to be carried through the rest of the year but its not.... which again is confusing or a mistake on the utilities part

 

[JoeStillman]

Okay so, I guess the wrench here is this. From the month of June I have an "Annual Billed Demand" of 1,220 kW but in next month the "Annual Billed Demand" drops to 239.8. The month before was 560 kW. The measured demand load has never exceeded 300 kW for any of the last 12 months.

And after reviewing the PSE&G tarrif you are correct Annual Peak demand is defined as:
Annual Peak Demand: The customer’s Annual Peak Demand in kilowatts shall be the highest Monthly Peak Demand occurring in any time period of the current month and the preceding 11 months.

So I would expect 1,220 kW to be carried through the rest of the year but its not.... which again is confusing or a mistake on the utilities part

It's not a mistake. Ron explains it in #8.

The difference between billed demand and measured demand could be 2 things in my experience.

1. The billed demand is a ratcheted value which is a value based on a perilously measured demand and only resets quarterly or yearly, based on the billing plan.

and/or

2. The difference is a way to bill for poor power factor, that they penalize the measured demand with a multiplier (based on how bad it is) to result in the billed demand.

The delta between 280 kW and 1220 kW is so great I'm inclined to think it's #1 (ratchet) and not #2 (power factor). If you are doing a load calculation, use the highest peak kW for the year.

 

[Besoeker3]

So I would expect 1,220 kW to be carried through the rest of the year but its not.... which again is confusing or a mistake on the utilities part

1,220 kW is an instantaneous value.

 

[charlie b]

The language in the exception implies that if you have 15 minute kWh data you can use that to derive kW. (Multiplying by four is how you would do that.)

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

 

[JoeStillman]

1,220 kW is an instantaneous value.

If it's on the bill, it persisted for at least 15 minutes.

 

[Besoeker3]

If it's on the bill, it persisted for at least 15 minutes.

That is for kWh - kW is instantaneous.

 

[JoeStillman]

That is for kWh - kW is instantaneous.

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.

It's confusing to include a time interval when discussing power - it makes us think we're talking about energy instead. But the 15 minute ratchet saves you a lot of money because your instantaneous peaks can be much much higher. It's all related to the upstream infrastructure. You don't size your wire for motor inrush.

 

[Revous]

If it's on the bill, it persisted for at least 15 minutes.

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?