Single Point Metering

Single point metering refers to the energy measurement of a single appliance or device.   This is accomplished with a simple plug-in meter which is a very handy tool for the energy auditor. The meter plugs into a 120 volt wall outlet and the appliance or device plugs into the meter.

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Power is displayed in watts or kilowatts depending upon the size of the load.  Over time energy is recorded as kilowatt-hours.  Most models also display energy cost after the utility's charge per kilowatt-hour is entered into the meter.

This allows you to get a feel for what type of impact each appliance has on your monthly electric bill.  Small electric loads such as a refrigerator, wine fridge or even a night light can add up over time because they are always on.

Analyze Your Electric Rate
If you do not know your kilowatt-hour rate you can obtain it in one of a couple ways.  First, check your utility's web site and look for the Residential Rate or Rate Schedule page.  The rate schedule will explain how the electric bill is calculated using the customer charge, energy charge, fuel cost adjustment and any other applicable charges excluding state and local taxes.  You can also call your local utility and have a customer service representative give this information to you.

Another option is look at your electric bill.  Find the total kilowatt-hour charge and divide it into the total number of kilowatt-hours used for the period. Look to see if there is a fuel cost adjustment (FCA).  If so, divide it by the total kilowatt-hours used.   Add this amount to your base kilowatt-hour charge, or subtract if negative, and apply any applicable taxes for a total cost per kilowatt-hour.

Kill-A-Watt Model 4400
Model 4400

The kilowatt-hour charge may be computed as a flat rate, a tiered rate or a time-of-use rate.   The flat rate is simply one flat charge applied to all of the kilowatt hours.  A tiered rate charges one rate for for the first range of kilowatt-hours and a second rate for all kilowatt-hours used over that amount. For example, the first 1000 kilowatt hours may be billed at $0.095 and all remaining kilowatt-hours at $0.085.  Tiered rates are not affected by the time period in which the energy is used.

The time-of-use electric rate, on the other hand, is time sensitive.  Its charges are expressed as peak or off-peak.  Peak rates are higher and generally occur during the period of greatest demand.  Off-peak rates apply to all other times and holidays.  For example, a time-of use rate may have a peak charge $0.105 per kilowatt-hour from 8:00 AM to 6:00 PM and $.075 per kilowatt-hour for all other times.

Most single-point-metering devices are not set up to handle tiered or time of use rates.   It is still possible to to obtain a cost estimate but you will have to average the different rates.  This averaging will make electricity cost estimates less accurate because the actual kilowatt-hours used are not linked to a specific time slot and rate.   If you have a tiered or time-of-use electric rate with we would recommend moving up to a smart meter with software capable of calculating these distinctive rates.

The Process
The primary purpose of single point metering in the home energy audit is to get an accurate measurement of the amount of electricity used by a specific device or appliance.  Whole house energy monitor systems have difficulty capturing the actual amount of power being used by a single device, especially if the load is small.   This is because there are many different devices turning on and off in a typical house making it difficult to isolate the measurement you are trying to take.

Tracking the energy used by an appliance such as a refrigerator, freezer or wash machine that frequently turns on and off by itself is a good application for single point metering.   Monitor the load over several days or weeks to get a realistic measurement of the typical amount of power used.  Take this kilowatt-hour reading and divide it by the elapsed time in hours to determine an average watt-hours.

If using our Power Panel Profiler enter this watt-hour value, divided by 1,000, for average hourly load and use 24 for the number of hours of operation per day. Another approach with single point metering is to take the normal operating power in watts, or kilowatts, and divide it into the total kilowatt-hours logged in a 24 hour period.   This will give you an average daily run time for the device which can be entered as normal operating power, instead of the average hourly load just mentioned.

Single point metering is generally limited to 120 volt plug loads.  Larger 240 volt loads can be sub-metered with extra channels on home energy monitoring systems but require a dedicated set of current transformers or CT's for each sub-panel or circuit.   Net metering with a home energy monitor system is another option to consider for sizing larger loads.  However, net metering will only give you power readings, not energy. Run time will need to be estimated separately to determine energy used.

Load profiling software is another option that can be used track larger loads with a detectable on/off signature provided the load is relatively constant.  If none of these techniques seem to work, which is rare, simply use the data from the electrical placard to estimate energy consumption for the device and enter it into the Power Panel Profiler.

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