Utility kWH vs kW

[drcampbell]

... 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? ...

It would be resetting its internal interval timer, not its kW or kWh accumulator.

A likely algorithm:
Each time the second hand advances from :59 to :00:
- Look at the kWh accumulated over the past 15 minutes.
- If it's more than the previous maximum value, make it the new maximum value.
At the end of each month: (billing period)
- Fetch the maximum number of kWhr accumulated during any 15-minute period, multiply by 4, and report that as the maximum kW demand for the month.
- Reset the kWh accumulator to zero.

The peak billing kW concept is motivated by how many more-costly generators the POCO needs to maintain in reserve and bring on line for short periods of time, (a few hours at a time) not by instantaneous motor-starting inrush currents.

 

[retirede]

It would be resetting its internal interval timer, not its kW or kWh accumulator.

A likely algorithm:
Each time the second hand advances from :59 to :00:
- Look at the kWh accumulated over the past 15 minutes.
- If it's more than the previous maximum value, make it the new maximum value.
At the end of each month: (billing period)
- Fetch the maximum number of kWhr accumulated during any 15-minute period, multiply by 4, and report that as the maximum kW demand for the month.
- Reset the kWh accumulator to zero.

The peak billing kW concept is motivated by how many more-costly generators the POCO needs to maintain in reserve and bring on line for short periods of time, (a few hours at a time) not by instantaneous motor-starting inrush currents.

When I dealt with this 40 years ago, that’s exactly how our utility (CIPS) did it. One small change: instead of looking back each time the second hand rolled over, it was every fifth time (5 minutes between readings).

 

[jaggedben]

Thanks for the info.

Do you think I would be correct in guessing that such demand billing algorithms are largely the legacy of old analog methods and that such counting methods are therefore followed by tradition even though digital meters could measure max kW on much shorter timescales?

 

[drcampbell]

Doubtful. Peak-demand billing is based on the cost of maintaining higher-cost peaking powerplants, which are brought online part-time to supply medium-duration (several hours) power-demand peaks, but which sit idle much of the year.

Peak power consumption on much-shorter timescales -- motor-starting inrush, for example -- doesn't cost the POCO anything extra. It's supplied by overloading the generation/transmission/distribution system for a moment, not by bringing more equipment online.

That, and demand-billing tariffs weren't restructured after digital meters became available.

An itemized, additional billing for peak power usage provides incentive for people to reduce their peak power consumption, which reduces the POCO's cost of generation.

For example: Large buildings sometimes make & store ice at night, then use that ice to meet part of the building's daytime cooling load instead of operating bigger air conditioners during the heat of the day.

 

[rlqdot]

replying to both posts 43 & 44 - i am not an utility expert, but i suspect the time period (5 to 15 minutes) during which the peak is measured still exists in most tariffs is simply due to the fact that most tariffs are re-written with focus on the rate structure (dollars) and not necessarily the billing mechanism. and, as stated in post 44, the primary reason for a demand charge is to reimburse the utility for the cost of building and maintaining the generation plants - and placing that cost on the customers who required the plant to be sized larger, those whose demand is higher.

many, many years ago, my Dad was a pumping station operator for a water utility - the water utility negotiated a peak DAYTIME and a peak NIGHTTIME rate on a monthly basis with the electric utility they purchased power from. every pumping station had a "ticker tape" electric meter in the operator's office space which typed the electric demand every 15 minutes. it was the pumping station operator's duty to make sure the demand did not exceed the agreed-to limit. the electric utility allowed the water company to use double the demand during nighttime hours at the same dollar rate as the daytime charge. when the demand was approaching the peak limit, electric pump motors were taken off line and pressure was maintained using diesel pumps.

i'm sure similar systems were, and still are, in place in other industries to control peak electric demands ... in today's world, those systems may be automatic, but their function remains the same - control the peak KW to help control electric costs.

as a design engineer, we frequently wrote project specifications that required contractors to provide their own power sources for things like welders and other heavy electric equipment during construction ... when we designed remodels for sites like grocery stores and other demand-metered locations, the owner did not want a contractor's welder to spike the demand on their electric service and result in them paying a higher demand charge on their electric bill for 6 to 12 months.

demand costs are on top of consumption and can be tricky to control.

 

[electrofelon]

I suspect the time period (5 to 15 minutes) during which the peak is measured ........i'm sure similar systems were, and still are, in place in other industries to control peak electric demands ........ control the peak KW to help control electric costs.
.

Remember, demand is an average not a peak!

 

[rlqdot]

Remember, demand is an average not a peak!

true - i guess i was saying "peak" in reference to the time period that any given utility measures it

 

[drcampbell]

Remember, demand is an average not a peak!

Yes & no. Depends on the timescale perspective you're observing it from. It's the peak consumption for any 15-minute period during the billing cycle, but it's not the peak one-cycle (16 millisecond) or half-cycle inrush current. The former requires peaking powerplants to carry the summer-afternoon peak demand; the latter requires low-impedance transmission & distribution to start big motors without excessive voltage drop.

... when the demand was approaching the peak limit, electric pump motors were taken off line and pressure was maintained using Diesel pumps. ...

That is an extremely excellent use of resources. The Diesel-engine-powered pumps need to be there anyway, to accommodate power outages, and using them whenever the price of Diesel fuel is less than the price of electricity is just prudent stewardship.
(I was half tempted to say "a no-brainer", but implementing this procedure took a lot of thought and a lot of attention to intangible details)