Net metering with a home energy monitor simply means to take the before and after power readings when the device to be measured is turned on and off and net out the difference.
The benefit is that you do not have to wire current transformers (CT's) or purchase additional metering equipment to measure individual loads. The disadvantage is that you are limited to capturing only power readings, not the energy consumed.
Remember that power measurement refers to instantaneous load in watts or kilowatts while energy refers to power used over time and is expressed in kilowatt-hours. Think of power being measured with a speedometer and energy with an odometer. Converting power to energy requires us to introduce run time.
Daily run times can be very difficult to measure with net metering alone because there are other loads are turning on and off at the same time. Most run times will need to be estimated by the energy auditor or clocked with other means.
Try to isolate the load you are measuring. Do this by taking net meter readings, at a time when your home is quiet and the indoor temperature has stabilized. The objective is to have only the device you are monitoring turn on and off during the measurement period.
Construct a table, similar to the one shown here, to take multiple readings and average the results:
|Measurement||Iteration #1||Iteration #2||Iteration #3||Iteration #4||Iteration #5|
(ON - OFF)
A good practice is to perform three to five iterations of recording the initial kilowatt load, the load when the device is turned on and the remaining load after the device is turned off. Discard measurements if the initial and final off-state power levels do not match. Average the good readings to determine the watts of power the load draws. If you are using our Power Panel Profiler enter the power measurement in the appropriate circuit.
The smallest increment of accurate measurement, or resolution, that the home energy monitor is capable of detecting is an important consideration when measuring electric usage. Monitors with a resolution of 100 watts will only be able to detect your home's larger loads. Resolution of 10 watts is OK but one watt resolution works best for the purpose of net metering.
Just because a home energy monitor system has a resolution of one watt does not mean it can accurately measure every load of one or more watts. Consider moving up two orders of magnitude for good net metering. In other words, if the resolution is one watt do not net meter loads below 100 watts. If the resolution is 10 watts do not net meter loads below 1000 watts. Use a plug-in meter to measure these small loads individually as it will be much more accurate.
Measuring a chandelier or bank of recessed lights will be straight forward using net metering provided your home energy monitor has good resolution. Measuring electric usage for an oven/range or a variable speed heat pump can be a little trickier because the electrical load is not always constant.
Lets look at a typical electric range with four burners. How many burners are normally used for cooking a meal at your home? How often is the oven used?
Start by net metering each individual burner and the oven separately to quantify the power consumption for each element. Estimate the amount of time each burner and the oven are on when preparing a meal. Breakfast, lunch and dinner will most likely have different profiles. When viewed in this manner, it should be easier to make an accurate estimate the amount of energy the oven/range uses each day.
Variable-speed or multi-stage heat pumps get a bit more complicated because they operate at different power levels as the various components in the system cycle on and off. For example, the motor for the air handler may or may not come on exactly when the compressor does. Staged compressors use different amounts of power depending upon how many stages are running. Supplemental heat strips can add a significant electrical load during cold weather. How can all these different loads be sorted out?
The trick is to break the total electrical usage load down into its component parts. While watching your home energy monitor, bump up your thermostat one or two degrees. Watch for a small power increase (air handler motor) followed by a larger increase (compressor motor). Record the power level at each step. Lower the thermostat and allow the system to turn off while recording power levels each step of the way.
There may be a lag of several minutes before the compressor and air handler shut off due to time delays programmed into the circuit. It is important to have a "quiet house" during this lag period so other devices do not skew your measurements as they cycle on and off.
After recording three sets of consistent measurements, turn your thermostat up five to six degrees to activate the auxiliary heat strips. Outside temperatures will normally need to be under 40 degrees F. for these heat strips to turn on unless you can find a way to over ride that constraint in the circuitry.
Watch your home energy monitor and closely noting the large increase in power over and above the compressor and air handler load. This big jump is what your auxiliary heat strip consumes. Record it as a separate heat strip load. Repeat the process two more times to verify consistency and then average the results.
If you are using our Power Panel Profiler record the average netted readings for each component load under the appropriate circuit on the spread sheet. Run times for each component will need to be estimated as net metering alone cannot produce this information. We recommend sub-metering or circuit level monitoring as a method to capture this information accurately.
With a little patience, net metering can provide power consumption readings for most electrical loads that cannot be measured with a plug-in meter. This includes both 240 volt loads and most 120 volt loads that do not have an accessible outlet. Run times for these power readings will need to be estimated or measured with another means in order to complete the Power Panel Profiler.