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Demand and Demand Registers

Billing Demand
Before discussing demand registers, it is important to understand why electric utilities bill customers for demand.  Let us begin by utilizing a simple formula to compare the electrical energy use of two customers. 

The following formula may be used to calculate kilowatt-hours to be billed:

            kW  X  Hours of Use  =  kWH to be billed

Example 1:
Let us compare two customers during a 24-hour period.  Customer number one has a steady 25kW load during the entire 24-hour period.

Customer number one:

            25kW  X  24 Hours  =  600 kWH accumulated on the kWH register

Customer number two has a 600 kW load but only utilizes his equipment for one hour during the entire 24-hour period.

Customer number two:

             600kW  X  1 Hour  =  600 kWH accumulated on the kWH register

If the electric utility billed these two customers for energy alone, they would each receive a bill for the same amount.  However, customer number one could be served with a single phase service with self-contained metering.  Customer number two would require a large three phase service with instrument rated metering.  In addition, customer number two would require larger conductors all the way back to the power plant providing the electrical energy.  Demand charges from the Generation and Transmission company may also contribute to higher costs associated with serving customer number two.  Therefore, to fairly bill larger customers, electrical meters are often required to record electrical quantities for both energy (kWH) and demand (kW).  
 

Mechanical Demand Registers
Another useful formula that helps explain the functionality of mechanical demand registers is:

                kWH 

-------------  =  kW (demand)

              H (hours)

Example:

Assume a customer has a steady load and the utility’s meter records 500 kWH during a 10 hour period.   Therefore:

            500 kWH

            ------------  =  50 kW demand

            10 hours

This formula has determined that the customer’s electrical equipment exposed the utility equipment to an average demand of 50 kW during the 10 hour period.

Electric utility equipment such as transformers, wire, metering equipment, etc… is sized to accommodate the customer’s load.  Small transformers for example, may heat up a great deal after just 15 minutes of load near their maximum rating.  Therefore, it is often desirable to have a mechanical demand register record demand readings in 15 minute blocks of time.  For example, in the previous example in which a customer averaged 50 kW per hour, the utility would like to have a meter record the 50 kW in just 15 minutes.

This is the basic concept behind all mechanical block interval demand registers.


Mechanical Demand Registers
Mechanical block interval demand registers have existed for many years.  Utilities could purchase registers designed to record demand in 15, 30, or 60 minute blocks of time.  The 15 minute blocks of time being more desirable for smaller services while 60 minutes was more appropriate where larger utility equipment existed such as in substations.  These registers accomplished their task by utilizing two mechanical gear trains.  Both of which were driven by a single gear that meshed with the worm gear on the disk shaft of the meter.  One gear train drove the kWH (energy) dials while the other controlled the kW (demand) indicator.  The demand reading is usually read and reset monthly.  The reset mechanism zero’s the demand reading while leaving the kWH (energy) dials unchanged.  Before reset, the demand reading was indicating the highest average demand for energy that took place during the entire billing period.

Early mechanical demand registers used a pointer to point to a numeric scale to indicate the demand reading.  Dial type registers were later developed to improve visual reading ease for meter readers.  These registers usually consisted of three dials similar to the dials used to indicate kWH (energy) use.   A long black line between two of the dials indicated where a decimal point should be placed in the reading.  Although the method of indicating demand changed, the pointer type and the dial type registers functioned in essentially the same manor.  

“Pegging” is a condition in which the demand registers pointer or dials reached the top of the demand indicating scale.  The most common reasons for pegging included overload, due to large customer load, and even more common was a failure of the demand timing motor.  A timing motor was used to essentially time each block interval of time.  If the motor failed, the demand registers indicators slowly moved to the top of the scale, yielding false reading.  Demand timing motor failure could often be recognized by observing the motors drive gear.  This gear should be turning slowly (approx. 1 rpm) anytime the meter is energized, even when the customer has no load.  While pegged conditions for pointer type demand registers could be easily recognized, the top of scale value for the dial type register was often misunderstood.  The far left hand dial often had part of its scale blacked out.  This dial could never enter the blackened out portion of the scale.  When it reached the blackened out portion of the scale, the dial type meter was at its maximum top of scale value and should be considered pegged.

One final note:
Mechanical block interval demand registers did not directly lock in on and record instantaneous load peaks.  They recorded the average load that took place during a block of time.  For example, during a 15 minute period, a customer’s load may have varied considerably.  During periods of light load the meter disk turned slowly.  During periods of heavy load the meter disk turned more rapidly.  These variations resulted in a demand reading on the register that yielded an average of all of the loads that took place during the 15 minute block of time.  Generally speaking, utility equipment should be sized for average load, not momentary peaks, which do little to cause equipment heating. 

Electronic Demand Registers
You may have noticed the previous section on mechanical demand registers was discussed in past tense.  Mechanical demand registers are rapidly being replaced by solid state electronic demand meters with far superior features at a lower price.  However, electronic demand registers still record average demand values in blocks of time just like their mechanical predecessors.