Using your Electric Meter as a Watt Meter Tom Stockebrand c. 1993 Tom Stockebrand Sometimes you would like to know the wattage of a load on your grid-connected system. You can do it by timing the rotation of the disc on your watt meter. If you can determine the on-time of the load each day you can figure out the energy use of that load in watt-hours per day. Like me, you probably do not own a watt meter and are not about to buy one since they are expensive. You can determine what you need to know by applying the "Meter person's Equation": INSERT EQUATION where Kh, which is the watt-hours per revolution of the disc, is a number printed on the meter. My meter has a Kh of 7.2 watt- hours per revolution. Thus if the disc spins once per second it will go 3600 revolutions in an hour yielding 3600 x 7.2 or 25,920 W-h per hour. That's equal to a load of 25.92 kW. Power To find the power being used, measure how long it takes the wheel to make some convenient number of revolutions and plug it into the equation. If for example your meter had a Kh of 7.2 like mine and it took 35.4 seconds to make 1 revolution then the power consumption is 3600 x 7.2 x 1 / 35.4 = 732 watts. If 5 revolutions took 54.8 seconds then you'd be consuming energy at a rate of 3600 x 7.2 x 5 / 54.8 = 2.39 kilowatts. Meters come in various sizes. I have one whose Kh is 1.8 and another which says Kh = 2/3. This last one is a smallish meter: a load of 2400 watts will make the disc spin once per second. In order to measure a particular load while others are on you need to measure the wattage consumption first with the load "off" and then with the load "on" and determine the difference, which is therefore due to the load that you added. You should make the measurements several times to be sure that the background load has not changed in the meantime. Energy Once you know the wattage drawn by a load, then to determine the watt-hours used each day by that load you need only to measure how long it is on during the day. One way to do this is to find an old electric clock and splice it into the load. Set the clock to read 12:00 at a particular starting time, wait one day more or less, depending on the accuracy you want, then determine the hours the appliance runs each day by reading the time on the clock! If you read it about the same time each day for awhile and record the readings you will get a very good feel for the hours it runs per day. Multiplying this by the wattage measured above will give you the watt-hours used per day. One difficulty with this method of determining energy use is that its sometimes hard to find a place to wire in the clock since it has to be AFTER the switch on the appliance. Estimates of the on-time may suffice, however. Those of you with electronic backgrounds can rig an op-amp across a sense resistor and put it in series with the load externally and then use its output to run an hour meter. This scheme will not work for measuring variable loads, such as electric ranges. If you really want to know about one of them, you can by a used meter for $15-20 from a place like C&H Sales in Pasadena and wire it into the range. This idea only works with 220 volt appliances, however. 110 v electric meters are hard to come by. What's needed is a Watt/kWh/Time meter that you can just plug in the line between your appliance and the wall. One fancy device was described in Home Power a few months ago but this one could be of limited function and could trade accuracy for cost, stay tuned. Access Author: Tom Stockebrand, LGK Corp., 1013 Tramway Lane NE, Albuquerque, NM 87122-1316 ù 505-292-4261 ù FAX 505-293- 5724