Using your Electric Meter as a Watt Meter

Tom Stockebrand

©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

Watts =

Kh x 3600 x Rev Sec 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 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: 2400 watts makes 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 it's 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 buy 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 volt 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 • 505-292-4261 • FAX 505-293-5724 /±v

ANANDA POWER TECHNOLOGIES camera ready

ENERGY SYSTEMS & DESIGN camera ready

BACKWOODS SOLAR ELECTRIC SYSTEMS camera ready

Mick Sagrillo

©1993 Mick Sagrillo

Wind power has taken a lot of bashing in recent years, for a variety of reasons. Some of the criticism has been justified, but most of it is completely unfounded. A fresh look at wind power and its place in a renewable energy (RE) system is in order.

Dispelling Myths

Let's take the most frequently used excuses for not considering wind power in an RE system and analyze the whys and why nots. By the time we're finished, you will have clearer understanding of wind's place in the whole RE picture.

"Wind doesn't work"

One reason given for not wanting to use wind power is that the equipment apparently doesn't work. "I drove past this wind generator and the blades weren't spinning." Well, maybe the wind wasn't blowing, or maybe it was blowing too hard. Maybe the wind system was shut down for routine maintenance. Maybe the owners' batteries were charged up and the power was not needed. Maybe...I could go on and on, but I'm sure that you get the picture. We humans are very visual yet blind creatures in that first impressions are often the most lasting impressions. When one glances quickly at a PV array, a solar hot water heater, or even a hydro plant, you have no idea if it's operating or not. You can't see what's going on inside. But it is always obvious when a wind generator is or is not operating.

This isn't to say that wind generators always work. In the late '70s and early '80s, there were scores of companies who set up shop building wind generators. In many cases, their motivation was greed. They were attempting to cash in on the 40% federal tax credit that was granted to buyers of RE equipment. Most states also offered tax incentives for RE purchases. Homeowners could get up to 70% of a $10,000 investment back from the government. The problem was that there were no standards for most of the RE equipment that folks bought. Consumers were in the mood to spend lots of cash, and manufacturers were all too willing to take their money. No standards, lots of money, and the tax credits all combined for what would ultimately be a major setback for renewables. There is no question that a lot of junk was being sold. Many of these travesties still hang from their towers as a dismal reminder that their owners had been had. Not only wind power, but all forms of renewables got a black eye that is now just healing. Unfortunately, many people developed a sour taste for RE. Ronald Reagan even got a laugh at one of his State of the Union speeches by poking fun at wind farms and Jimmy Carter's solar water heaters.

Six years after the sunset of the tax credits, virtually all of the 80 or so wind generator companies doing business in the U.S. are gone. Only a handful of manufacturers have survived the shake-down. And these survivors are still cranking out the same reliable equipment that the fly-by-nighters tried to emulate a decade ago. There is no question that the equipment available today is reliable, and their manufacturers stand behind their designs. These people know that a wind generator manufacturer is in the business to help generate renewable electricity, not just build and sell RE equipment. Today's wind generators work!

"No Power!"

Many folks have the misconception that there is no real power available in the wind. Therefore, why bother with a wind generator. Let's look at this more closely. The equation for determining the power available to a wind generator is as follows:

where P is the power in the wind, d is the density of the air, A is the swept area of the rotor, and V is the wind speed or velocity.

What we are interested in here is V3 or V x V x V. When we double the wind speed for a given location, say from 5 to 10 mph, we increase the value of V3 by a factor of eight! Multiply 5 x 5 x 5 and you get 125. Double it to 10. Multiply 10 x 10 x 10 and the result is 1000, an 800% increase. There are two lessons here. The first is that even small gains in wind speed can result in enormous increases in the power in the wind. As the wind steadily increases, the power in the wind increases exponentially.

The second lesson is that, because the power gets so large so fast, it must be awfully difficult to build a wind system that will survive high winds. Any do-it-yourselfer who has tried to design a wind generator has learned this lesson, probably the hard way. So have all those bankrupt manufacturers that were mentioned earlier. But today's wind generators know how to behave without self-destructing in high winds.

"No Wind?"

A lot of people have the idea that wind power is so site specific that they probably have little or no wind at their site. Richard and Karen Perez even fell into this category, until a manufacturer gave them a wind generator to test at their home. Lo and behold, they now have more power than they oftentimes know what to do with!

The chief culprit here is that wind power has been saddled with a very poor yardstick: average wind speed. Dealers will ask a prospective buyer what their average wind is. If it's down around eight or nine miles per hour, the dealer will usually disregard the wind system in favor of photovoltaics (PVs). But average wind speed is the wrong question to ask. An example is in order.

Let's assume two locations, each with an average wind speed of 10 mph. We are only interested in the average for one month, and we are going to take readings on a weekly basis. The wind speeds for our two imaginary sites are as follows:

Wind Speed for two Sites

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