A register is the device that records the units that are measured by the meter, and there are a number of different register types. The most familiar type is an energy register, which records kilowatt hours or kWh. For large commercial and industrial customers with a demand rate, a demand register records the amount of power consumed, in kW, at a specific time. Many solid state or electronic displays can show energy, demand, apparent power in kVa, reactive power in kVARs, as well as other units.
Energy register
The energy register displays energy usage in kWh. There are three different types of registers in use. The pointer type of register uses a moving pointer on a dial to display usage. The meter reader reads the register each billing cycle and usage is determined by subtracting the current value indicated from the value displayed the last time the meter was read.
The typical pointer meter is read from right to left. To read the meter above, you would first start by looking at register A. Notice the pointer is moving in a clockwise direction and it’s past the five but hasn’t gotten to the six yet. So the unit associated with this register is five. Register B, which is the tens column, moves in the counterclockwise direction. Its pointer is past four but hasn’t gotten to five, so the unit associated with this register is four. Register C, which is the hundreds column, is again moving clockwise. As you can see the pointer is past two, but hasn’t gotten to three, and the unit associated with this register is two. And finally, register D, the thousands column, again moves in the counterclockwise direction. Its pointer is past three but hasn’t gotten to four so the unit associated with this register is three.
An issue with the pointer meter is that it creates the potential for error when read manually since the meter reader must interpret the position of the dials. An alternative is the cyclometer register, with numbers that roll up similar to an odometer on a car. In this case, the meter reader simply records the numbers shown on the cyclometer. Depending on the number of dials, the device can read a maximum of 9,999 or 99,999 kilowatt hours.
Many newer meters use solid state registers. Solid state registers use digital electronics and usually provide actual usage numbers on an LED readout. Since the data in these meters is already stored digitally, it is also easy to transmit this data directly to a billing management system thus eliminating the potential for error by a meter reader.
Time-of-use (TOU) register
Some power suppliers offer time-of-use electric rates that charge customers different prices for electricity during different periods of the day. Prior to smart meters, any customer on a time-of-use rate required a time-of-use register (which is actually two distinct registers) to measure the amount of energy used during each pricing period. The time-of-use register contains a clock that allows measurement of energy during different time periods.
For example, a customer’s rate might be $0.15 per kilowatt hour during the peak period from 2 p.m. to 10 p.m. and $0.05 per kilowatt hour during the off-peak period from 10 p.m. until 2 p.m. To accurately bill this customer’s usage the meter must be capable of measuring and recording usage during the two distinct rate periods. As you can see in the illustration below, this meter contains two separate energy registers: one for premium or peak usage and one for total usage. Off-peak usage can then be calculated by subtracting peak usage from total usage. Because smart meters can record usage and put a time stamp on all data recorded, they have emerged as the preferred way to meter energy consumption in different time periods where time-of-use rates are used.
Demand registers
Demand registers measure peak demand for customers who have demand rates. These registers record and display the highest demand or power usage measured by the meter over a certain period of time, for example the highest amount of power used over a 15-minute or 60-minute period during a billing month.
Older demand registers based on induction meter technology use either thermal or mechanical demand registers. Mechanical demand registers contain devices relying on gears, motors, and pointers to register the maximum peak demand. Thermal demand registers rely on increased amounts of heat produced from higher current flows to sense and register peak demand. Newer solid state meters contain digital equipment capable of displaying demand levels based on electronic pulses. Thermal and mechanical demand registers can be used only on induction type meters while an electronic demand register can be used on both induction and solid state meters with the appropriate equipment.