Measurements of electricity use can be used in many ways. They allow a utility customer to control electricity consumption (and demand), determine typical consumption, evaluate opportunities for improving energy efficiency, and confirm savings from building and/or building systems improvements.
Measurements require a measuring device, the most common of which is the utility revenue, or billing, meter. However, the revenue meter may not provide the information needed for some cases. Special purpose data loggers are available for measuring electricity use, and someone who has experience with these devices can give recommendations about equipment.
Utility Watt-Hour Meters
Utility revenue Watt-hour meters are, by far, the most popular instrument for measuring electricity demand and consumption. For buildings, these meters are most often located at the electrical service entrance or at the transformer. Advantages and disadvantages of utility revenue meters are given in Table 1.
|Standard mechanical||Low cost
Widely in use
|Manual read, most read
No data storage
Designed for permanent installation
|Moderate cost, accurate
Available, but few in use
Can use to automate data collection
|Designed for permanent
Automatic data collection requires data acquisition equipment
|Standard electronic||Low to moderate cost,
Optional pulse initiator
Some with short-term data storage
Designed for permanent installation
Pulse initiator drives up cost
|Programmable electronic||Programmable measurement intervals
for data storage
Electronic data down loading
|Designed for permanent
Need software to down load data
Mechanical meters that indicate electricity use in kWh (kiloWatt-hours) and electricity demand in kVA (kiloVolt-amps) are the most common. Mechanical meters do not have data storage capability and therefore provide only an instantaneous readings.
Electronic meters have capabilities ranging from single kWh and kVAR measurements up to programmable logic with internal, longer-term data storage. Their costs vary widely depending on these capabilities. Except for the simplest of electronic meters for demand measurement, electronic meters are not widely installed.
Utility revenue meters are designed for permanent-type installations and often require power interruption to install.
Measuring Power Using Electric Meters
For meters with a visible, rotating disk, inside the meter you will see a disk which spins at a speed relative to the instantaneous power being measured. It spins faster when more electricity is being used and slower when less is used. The disk usually has a black marking on it that can be used as a reference for counting revolutions.
Use this process to calculate the average power and the kiloWatt-hours of electric use from the revolutions of the disk:
1. Start a stopwatch when the black reference mark is seen and measure the time required to count 10 revolutions. Lets say this requires 42 seconds. The disk is then turning at 14.3 revolutions per minute (60/42 x 10).
2. Find the value of kh, the Watt-hour constant, on the faceplate of the meter. This constant is the number of Watt-hours represented by one disk revolution. Let's assume that it is 7.6.
3. Multiply kh by the number of revolutions per minute to calculate the number of Watt-hours in one minute. For our example, the meter counted 109 Watt-hours in one minute.
4. Multiply this value by 60 to get the number of Watt-hours per hour, or Watts, the average power required by the building over our measurement period. This is 6500 Watt-hours per hour, or 6500 Watts.
5. Divide by 1000 to convert Watts to kW, or 6.5 kW (6.5 kWh per hour).
6. Many commercial building electric meters have a meter multiplier, indicating that the power measured by the meter (and rotating disk) is a multiple of the power required by the building. This multiplier is sometimes on the meter faceplate. In this case, multiply your result from Step 5 by this multiplier to get the actual building power. If the multiplier is not shown, you may have to obtain it from the utility company (or you can do some experimenting to see what it is approximately, by turning known loads on and off and measuring the changes that occur).
Levels of Monitoring
Energy use data are collected for most buildings on a monthly basis as utility billing data. Monthly data collection provides 12 measurements, which cover the heating, cooling, and two in-between periods that occur during a year.
During a cooling or heating season, only four or five points may be collected, as shown in Figure 1. This number of points is often too small to provide much confidence in heating or cooling energy use estimates, or other end-use load estimate, when seasonal temperature dependence is evident.
For building systems such as lights or process loads, where loads are near constant and temperature dependence does not occur, it may not be necessary to make measurements over a long period to determine energy savings.
For temperature-dependent performance evaluations, weekly or daily data may be used, so that enough data can be collected to establish statistical confidence in the relationship between weather and energy use. For evaluating constant loads (little variance over time), short-term weekly, daily, or hourly data might be used.
Using Data Loggers for More Detailed Data
Electronic data collection systems are often used to collect energy data at more frequent intervals (weekly, daily, hourly, and faster). Because the data collection interval of such systems is usually programmable, they can collect data across a wide range of intervals.
For evaluations of peak electric demand, monitoring is often done at 15-minute intervals to match the utility measurement interval used to establish demand charges.
Some research has been performed at very short intervals, exceeding 1 sample per second, to investigate the starting and stopping of end use loads for the purpose of load disaggregation from whole-building power.
Measuring electricity in a building or building system with minimal or no power interruption can be important. for this to occur, either the existing electric meter must provide the data, or devices are installed with minimal power interruption. Electricity is measured using devices called potential transformers (PTs) and current transformers (CTs). To minimize or eliminate power interruption, clamp-on CTs are used. This is in contrast to solid, ring-type transformers which require power conductors to be disconnected so that they can be slipped over the end of conductors. Often, PTs will not be installed without a power interruption, because of concern with high voltages being measured.
Electric energy profiles collected using utility meters or special data loggers can answer questions such as:
These results can be used to develop effective demand management strategies. An electricity measurement system could then be used to confirm the effectiveness of any implemented strategies.
Such measurements can also be used to confirm and evaluate building or building system operating strategies. Building operational patterns can be verified, nighttime operation of unneeded equipment can be detected, and the effectiveness or ineffectiveness of existing energy and demand control systems or strategies can be confirmed.
The overnight power profile in Figure 2:
The performance of electrical systems that have constant loads can be quantified by making short-term measurements using the system. These loads could include motors, lighting systems, exhaust/ventilation fans, and other equipment.
The performance evaluation of motor-driven systems such as heating, cooling, refrigeration, and air and fluid distribution systems can also be evaluated using electric power measurements. For motors with constant loads, one-time measurements of the power required by an existing motor could be compared to the power required by an efficient replacement. A follow up measurement could confirm savings.
Demand and energy savings from lighting improvements can be approximated from short-term power monitoring. The demand profile shown in Figure 3 shows that the near constant whole-building electrical loads measured two weeks before and after a major lighting upgrade in this all-electric building can be used to approximate the resulting lighting energy reduction. The approximate 18 kW reduction from an existing lighting load of around 30 kW is confirmed from the delamping and upgrading of existing lighting.
Lighting systems add heat to the occupied space. As a result, lighting efficiency improvements will increase building heating energy use and decrease cooling energy use. For moderate climates the annual increase in heating is about equal to the decrease in cooling. (However, the same may not be true for heating and cooling costs, if different fuels are used.)
Heating and cooling impacts, while smaller than the direct lighting electricity reduction that results, will contribute to measured, net whole-building power reductions like those shown in Figure 3. Their smaller magnitude, however, makes it difficult to extract them from short-term data wihout additional metering.
The data shown in figure 3 were taken in the early Fall, when little cooling or heating was occurring. For this instance, the reduction shown is essentially equal to the lighting reduction.
Thus, electricity use measurements can be important for evaluating building and building systems energy use, the resulting energy savings from building retrofits/improvements, or the resulting energy savings from changes in building or equipment operations.
By using a system that measures both real and reactive power, power factors for commercial/industrial buildings can be determined, and the reactive loads of building systems can be determined. Capacitors needed to correct for low power factor can be determined from these measurements.
If a utility meter is not practical, will not provide the data needed, or a less permanent-type of installation is desired, special equipment will have to be obtained. Typical equipment to measure one or multiple loads sets can be acquired for approximately $1000, although installation is additional, and more sophisticated (and often more expensive) equipment can also be used.
Source of Additional Information
Handbook For Electricity Metering, Edison Electric Institute, 701 Pennsylvania Avenue, NW, Washington, D.C., Publication No. 06-92-01