APPLICATION NOTE 3793

Flexible Tariff Scheduling for Multirate Electricity Meters

By: Ben Smith

Abstract: Utility companies employ a variable pricing structure to encourage energy conservation during peak periods. These pricing periods are called tariff periods, and the process is called flexible tariff scheduling. An electricity-meter reference design uses the MAXQ3120 microcontroller to meet all requirements for a multirate electricity meter, and provides even more flexible capabilities for monitoring electrical usage.

The only tool that utilities have to encourage energy conservation during peak periods is variable pricing. Specifically, variable pricing means raising the price of energy during peak periods, and reducing the price during slack periods. These pricing periods are called tariff periods, and the process is called flexible tariff scheduling.

The multirate meter is important for flexible tariff scheduling. In fact, the multirate meter actually exists for one reason: it tracks electrical usage during several tariff periods and thus maintains multiple energy accumulators each with its own register. The multirate meter, consequently, is the best mechanism for utility companies to enforce variable pricing.

An electricity meter capable of supporting variable pricing differs from a conventional meter in several important respects:

  • A multirate meter must contain an accurate time-of-day clock so that it knows when to stop accumulating energy usage to one rate register and begin accumulating energy usage to another rate register.
  • A simple mechanical counter is no longer a suitable display for a multirate meter. Instead, some type of electronic display is needed so that each tariff rate register can be independently displayed.
  • Reading a single usage value from the meter is no longer sufficient. In single-rate meters, the previous cumulative reading is subtracted from the current reading, and the result is the total usage for the latest period. With several tariff periods monitored in a multirate meter, several usage values must be conveyed back to the billing authority. This latter requirement strongly argues for automatic meter reading.
The MAXQ3120 reference-design electricity meter meets all these requirements for a multirate electricity meter, and provides even more flexible capabilities for monitoring electrical usage.

Scheduling Mechanism

Every day of the year is governed by a day schedule. During the day, energy is accumulated to one of several tariff registers in the meter, depending on the instructions in the day schedule. In the illustration above, Tariff 2 is the peak tariff. Energy used from 5:30 AM to 6:00 PM is recorded to Tariff 2 (and presumably, billed at an elevated rate.) Energy used from 6:00 PM to 10:00 PM is accumulated to Tariff 3, the normal rate tariff. Finally, usage from 10:00 PM to 5:30 AM is accumulated to the off-peak tariff, and would typically be billed at a reduced rate. In this way, customers are encouraged to conserve energy during peak times, possibly motivating them to move usage-intensive operations to off-peak times.

The day schedule used to define tariff periods can change throughout the year and be preempted for weekends and holidays. In the example here, the highlighted day in May is governed by Day Schedule 5. Day Schedule 5 will continue to be used, except for weekend and holiday changes, until mid-July when Day Schedule 3 takes effect.

The calendar works for normal weekdays. What about weekends? The meter in the reference design contains a setting that designates certain days of the week as weekend days with a separate day schedule that supersedes the normal calendar schedule. Holidays are given independent day schedules, which supersede both the normal calendar and the weekend schedules.

Using these tools, utilities can establish a set of rules that automatically enforce a flexible pricing policy.

Automatic Meter Reading

There are two communication mechanisms in the reference design for the electricity meter: an IR link for use with handheld terminals, and an RS-485 network connection for networking several meters and a host PC. For more information about these communication methods, see IR and RS-485 Communication Channels for an Electricity Meter.

It is a simple task to configure either the handheld reader or the network host to read any, or all the tariff registers from the meter in an on-demand or scheduled basis. Once read, the information can be transferred to a billing management system without any key-entry step. This data transfer ensures accuracy and timeliness in billing. There will also be a reduction in the personnel involved, either because meter reading is quicker and more efficient (just point a handheld terminal at the meter and pull the trigger), or because meter reading is entirely eliminated (when an Internet-connected host controls several networked meters remotely).