Energy Audits

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watts audited since 05/27/07

One of the professional activities I do is conduct power quality and energy audits. I've developed a fairly unique method of doing comprehensive energy audits using very inexpensive equipment. I am going to give subject a great deal more treatment but for now, some photos of some of my equipment and a very handy spreadsheet for tracking energy usage.

My basic measuring instrument is an ordinary utility revenue-type watt-hour meter. These meters are very inexpensive while being remarkably accurate and rugged. This photo shows a meter assembled for use on a 120 volt circuit.

When I start the test, I use a Sharpie permanent magic marker to write the time, date and register reading on the glass face of the meter. This is the easiest place to keep track of it. The Sharpie ink wipes right off with lighter fluid, acetone, etc. Be sure to use the "industrial super Sharpie, Item 13601" version because it uses a pigmented ink that won't fade in sunlight or under fluorescent lighting. The ordinary Sharpie uses a dye ink and will fade rapidly. I usually also enter the starting values in the Energy Audit spreadsheet available below.

This photo shows the connection for 120vac operation. The astute will note that I'm metering both the hot and neutral leads. This nifty little trick allows me to use a 2 pole 240 volt meter (the most ubiquitous type) on 120 vac. The potential coil sees only half voltage but the current coils deliver two signals, one for the current going out and the other coming in. The two neatly compensate each other. I've verified this equivalence on my meter calibration bench.

Here is the face of one of my meters. This is a low range meter (15 amp rating) that I use for measuring household appliances. The closer the instrument range is to the actual load, the faster the dial will spin and thus the less effect friction and windage has.

An important factor for determining load is the Kh factor printed on the dial. The Kh factor is the number of watt-hours (not KWH) that one revolution of the dial represents. One can count the number of rotations during a known time interval, multiply times Kh and divide by the time interval in hours to get average watts drawn over the interval.

I like to accumulate 25 revolutions and preferably 100 to minimize quantization error so the faster the dial spins for a given load the better.

All of these instruments are periodically checked against a General Electric laboratory watt-hour standard. This standard is, in turn, calibrated by an outside lab and is NIST traceable.

Energy Audit Spreadsheet in zipped Excel format

This is one of several spreadsheets that I use to collect and reduce the data. This particular one is set up to collect raw data from the meter. For each run one enters a short description of the load, the start time and date, the start register reading, the end time and date and the end register reading. The spreadsheet computes the total KWH used and computes the daily and monthly usage and the daily and monthly cost, using the cost/kwh input at the top of the "Utility Costs" sheet.

This spreadsheet can also be used with electronic instruments such as the Kill-A-Watt, though long term measurements are difficult with the KAW, owing to its resetting itself on each power glitch or outage.

The "Utility Costs" sheet is designed to use your main power meter to log and compare your daily energy usage. To use, simply read your meter at about the same time every day and enter the reading time and the reading into the sheet. If you use a separate sheet for each month and carry forward the numbers at the top, you can track your energy usage for a year or any arbitrary amount of time.