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Abstract
The Power Consumption and Energy Usage reports both measure kilowatt-hours (kWh). In theory, they should provide identical information; however that is not always the case. Usually, these reports differ only slightly, but sometimes the difference between them can be significant.
The purpose of this white paper is to demonstrate the reasons why these reports differ and also to identify best-use scenarios for each. Understanding how data is collected and processed for each report is important to understanding why these reports might generate different results.
The Energy Usage Report
The values displayed in the Energy Usage Report are generated directly within the recorder itself and are an accumulation of the data measured during each cycle. Each of the 60 cycles is accumulated for one second. Every second, consumption is then accumulated in the real, reactive, and apparent power measured by the recorder. The accumulation is always performed using the highest resolution data supported by the recorder being used. The accumulated real power is energy, in kilowatt-hours. The accumulated reactive and apparent powers are kilovar-hours and kilovolt-amp-hours. These totals are for the entire recording session, and are only available on recorders which can compute power.
In a wye hookup, the accumulated power is totaled separately for each individual channel. For this reason, the table shown in the Energy Usage Report displays the total accumulated power for each channel in different columns.
This report does not include a total three-phase power column. Only the total power can be measured when a delta hook-up is used. Individual phase power cannot be measured in a delta hook-up. In this case, Column 1 is used to report the three phase total, for real, reactive, and apparent power values. Also included are negative power values in the accumulation. There can be cases where a load can generate and then absorb power at different times. In this case, the accumulated power will record both the positive and negative values so the correct power is used is preserved. Figure 1 shows a screen capture of a typical Energy Usage Report.
Figure 1. Screen capture of a Energy Usage Report
The Power Consumption Report
Unlike the Energy Usage Report, the Power Consumption Report does not rely on the accumulation of power measurements by the recorder. Instead the Power Consumption Report is generated directly from within either ProVision or WinScan from data points that are recorded in stripchart format. For the software to generate the report, it uses the voltage, current, and phase angle interval graph data to calculate real, reactive and apparent power. Because this report is not generated directly by the analyzer and is calculated by the software from data that has been collected and downloaded, the Power Consumption Report may take a little longer to open verses the Energy Usage report, depending on computer speed.
The main advantage of the Power Consumption Report over the Energy Usage Report is that because the report is calculated from the interval graph data, it is possible to wait until after the data has been retrieved to choose the interval that the report is displayed in. This feature can is very helpful when it is necessary to view the power consumed over a certain day or a certain amount of time from a larger recording span. Figure 2 shows a screen capture of a typical Power Consumption Report.
Figure 2. Screen capture of a Power Consumption Report
Reasons For The Differences
There are several reasons that the Power Consumption report sometimes differs from the Energy Usage Report. Many times, the difference between the two reports is the result of differences in the recording duration. Typically, the interval graph does not display the data from the entire recording session. Enabling or disabling Interval Graph Overwrite in the Basic Settings window in ProVision affects the starting point of interval graph being recorded. Because the Power Consumption report is based solely on this interval graph data, the report only displays the power consumed during the time period of the interval graph. However, the Energy Usage Report keeps a running total during the entire recording session, and will continue to record and accumulate energy usage data for the entire recording session, no matter how long, providing the recorder has adequate memory. For example, if a recorder’s memory only allows for a week’s worth of data to be recorded, and the recorder is left in the field recording for one month, the Power Consumption Report will only display one weeks’ worth of data while the Energy Usage Report will show accumulated power over the entire month. In this example, the reports would be totally different.
It is also often possible for the Power Consumption Report and the Energy Usage Report to display data for the exact same time duration, and return values that are slightly different. This is because of differences in how these values are calculated. The Power Consumption Report calculations are based solely on interval data; therefore, there is much more rounding and averaging involved in deriving the final calculation. Since the Energy Usage Report is generated directly from data measured by the recorder, very little rounding is required. Generally the smaller the power levels being recorded, the larger the rounding discrepancies will be, resulting in a greater difference between the two reports. When these reports display data for the same duration of time with only nominal differences, the Energy Usage Report will be more accurate because it uses better data resolution and relies less on rounding.
The Power Consumption Report is generated with lower resolution data than the Energy Usage Report. The Power Consumption Report is produced by ProVision, using the real, reactive, and apparent power stripchart data from the recorded file. Stripchart data doesn’t span the entire recording session, if the stripchart fills up the recorder memory, it will either wrap around, or stop when full. In either case, the available power data will not cover the same time period as the Energy Usage Report (which is not affected by stripchart memory).
Stripchart power data is recorded at a lower resolution than the maximum internal recorder resolution. All of the internal measurements in PMI recorders are performed at a higher resolution than listed. When the values are stored in stripchart memory, they are rounded to resolution used for recording (e.g. voltage is computed to a 0.01V resolution internally, and rounded to the nearest 0.1V when recorded, in many of our recorders). This is to ensure that internal math calculations are done with the highest precision possible. However, this means that ProVision isn’t using the same high-precision raw data as is used to generate the Energy Usage Report. The accuracy is the same, but the resolution is different. This difference is more pronounced at very low power levels (relative to the full-scale current). If the absolute power is very low, then the resolution of the recorded stripchart data can be a significant fraction of the data itself. E.g. if the actual power is under 1kW on the 1000A range, the available stripchart resolution (and rounding) may become a factor in ProVision’s accumulation. Since the resolution is finer with smaller current ranges, the same power with the 100A range will be recorded with better resolution. Because the Energy Usage Report is performed with higher resolution values than stripchart, as the power gets smaller, it’s affected less quickly than the power stripcharts.
Conclusion
Simply stated, the Power Consumption Report and the Energy Usage Report can differ due to how the data is collected and processed. Both reports have advantages and disadvantages making either valuable depending the specific application. Because of how the data is collected, the Energy Usage Report is the most accurate of the two. The main advantage of the Power Consumption Report is that it allows the operator to see and display power consumed over a small period of time, such as a day, out of a larger recording span. The differences in the information that is displayed between the two reports are always more pronounced at lower power levels due to discrepancies in resolution and rounding.
Cowles Andrus, III
Communications Specialist
candrus@powermonitors.com
https://www.powermonitors.com
(800) 296-4120
