GENERATOR INFORMATION: Measuring Building Electrical Load Using a Stopwatch

If you intend on powering most of the items fed by your utility electric meter you can measure your total building load at any time using simply a stop watch while observing your meter. Follow the steps below to make this measurement.

First locate the electric meter which feeds your building. To use this method, it must be a traditional style kilowatthour meter with a rotating disk. The meters shown here are typical for small to medium residential services.

Next, read the constant on the face of the nameplate shown as Kh. This value is the number of watt-hours equivalent to one rotation of the disk.

Now, start the desired appliances, heating or air conditioning for the condition to be measured.

Using a stopwatch while watching for the black mark on the meter's disk, measure the time it takes for one or more disk rotations. If the disk is rotating rapidly, better accuracy will be attained if you time more than one rotation.

Finally, take the three values and use the equation below to calculate the watts seen by the electric meter.

Here is an example calculation for the first meter above. From the meter's face, Kh = 7.2. The time measured for 5 rotations of the disk was 24 seconds. Thus, Rev = 5 and T = 24 seconds. Solving for the electrical demand we have:

This is within the capability of common portable generators.

Note that this method is not practical for measuring the peak in-rush watts needed to start most loads because the transient happens so fast. Typical acceleration times for most motor loads are less than 1 second. Unless the response time of your eye and your thumb is extremely fast, you won't be able to measure these starting wattages without special recording instrumentation. For a sample of these measurements, see Test Reports below. .

Test Reports

This section contains test results and measurements for various electrical loads. Note that these measurements are representative only for the specific models shown and under the particular test conditions. Different conditions can alter both the current values and starting times.

Slightly different line voltages will affect results as will the wire size and length feeding the load. Ambient temperatures and thermostat settings will affect refrigeration compressor current demands. For well pumps, the pressure settings and the depth that the pump is positioned will affect measured results.

Refrigerator Starting Current

Refrigerator Starting Current
This test measured the starting current for a refrigerator with the following nameplate data.

Voltage 115 V AC
Frequency 60 Hz
Amps 5.0
Mfr. Date 3 / 86

Results of one measurement are shown in the plot below. It shows a maximum inrush current of about 13 amps which lasts only about one-half second. Also, the running current is significantly less than the nameplate value. It should be noted that most refrigerators (including this one) are "frost free." This means that on a regular basis a timer shuts off the compressor and turns on resistance heaters to clear frost buildup in the freezer section. This defrost current was not measured and is probably greater than the compressor running current. This may explain the large difference between the nameplate current value and the measured value.

Freezer Starting Current

This test measured the starting current for a freezer with the following nameplate data.

Voltage 115 V AC
Frequency 60 Hz
Amps 5.0
Mfr. Date early to mid 1970s

Results of one measurement are shown in the plot below. It shows a maximum inrush current of less than 5 amps which lasts only about 0.3 seconds. Also, the running current is significantly less than the nameplate value. It should be noted that most freezers are "frost free." However, this one is not.