Things that Work! Home Power tests the Thomson & Howe Ampere-hour Meter Ever wonder exactly how much electricity your PVs or windmachine produced on a partly cloudy or gusty day? Ever wonder how much electricity your refrigerator or inverter consumed? Without an Ampere-hour meter it is very difficult to measure the current production or consumption of intermittent sources or loads. This Ampere-hour meter accurately measures and totalizes the flow of DC electrical current. It works and it can answer many questions about system performance. Of Amps and Hours The Ampere is the unit of the rate of electrical current flow. An Ampere is some fantastically large number (Å1018) of electrons moving past a point in space per second of time. An Ampere-hour is the measurement of the actual number of electrons produced, stored or consumed in a one hour interval. If a load consumes one Ampere for one hour, then it has consumed one Ampere-hour. If the same load operates for two hours, then it consumes two Ampere-hours. Consider a power source that produces 4 Amperes. If it produces for 12 hours, then it has produced 48 Ampere-hours (4 Amperes times 12 hours = 48 Ampere-hours). Ampere-hours are most meaningful to those of us making our own electricity. Consider a battery for example. Its electrical capacity is rated in Ampere-hours. This is a measure of the number of electrons available from a fully charged battery. The electrical consumption of appliances can be measured in Ampere-hours. Consider appliances which consume electricity at changing rates like refrigerators and freezers. Consider power sources that produce energy at varying rates, like PVs and windmachines. The flow of electrical current, in all these cases, can be measured and summed by an Ampere-hour meter. An Ampere-hour meter is a cumulative electron counter. It can accurately measure the number of Ampere-hours produced, converted or consumed by a device. The Ampere-hour meter is useful in monitoring the electrical flow through many system components. We chose our PV array for measurement during this 90 day test. How the Ampere-hour Meter Works The Thomson & Howe meter uses a shunt to measure current flow. The voltage loss across this shunt is run through a voltage to frequency converter. The output of this converter is totalized on an electromechanical counter. A variety of shunts (50 mV. precision types) are available for this meter, from 1 to 300 Amperes, all with 0.25% accuracy. We used the 15 Ampere shunt to measure the output of our PV array. All shunts are capable of handling twice their current rating. The shunt is externally mounted and can be easily changed to suit the measurement task at hand. The shunt can also be mounted remote from the meter. The meter is powered by the system under measurement and doesn't require its own battery. It will operate on any DC voltage from 12 to 48 Volts. Since the display is electromechanical, the meter has very low power consumption. The electromechanical display is nonvolatile and will not lose its reading should the meter be disconnect from its power source. The Test System We installed the Thomson & Howe Ampere-hour Meter in series with our PV array. This array is an assortment of PV modules we have purchased over the years. It is made up of one Sensor Tech 8 W., one Kyocera 59W., and three Kyocera 48W modules. On a good sunny day our array will produce over 15 peak Amperes. Installation of the meter was fairly simple with only three wires to connect as shown in the schematic below. INSERTSchematic We noted the numerical reading on the counter and recorded this data with the date. The next day, after sundown, we again noted the reading on the counter. By subtracting the previous day's reading from the current reading and multiplying this figure by the current rating of the shunt, we had an accurate measurement of the number of Ampere- hours the array produced that day. We continued to take the data offered by the meter for a 90 day period. As an example of the information we got from this meter, look at the graph below. This graph details our PV array production during the month of April 1989. INSERT Graph Thomson & Howe Ampere-hour Meter Performance We tested the meter and found it to be within the 1% accuracy claimed by the manufacturer. Operation was automatic, reliable and fairly silent, the only sound being the soft click of the counter as it operated once a minute. The documentation supplied with the meter is adequate. It offers all info necessary for wiring it into your system, meter adjustment and for remote mounting the shunt if desired. The physical construction of the meter is rugged and it should last a long while. The Cost & Access At $208. ($250. Canadian) this meter is a bargain costing much less than comparable products. The meter has a one year warranty. The exclusive distributor for these Thomson & Howe Ampere-hour meters is Bob Mathews at Appropriate Energy Systems, R.R. 1, Site 9, Comp. 16, Chase, British Columbia V0E 1M0, Canada or telephone (604) 679-8350. Bob was very helpful with telephone support and installation instructions regarding this meter. Conclusion Ampere-hour metering is essential for the accurate monitoring of intermittent power producers and consumers. The Ampere-hour meter can perform a wide variety to measurement tasksÐ from monitoring power producers to actual measurement of varying electrical consumption. Technically minded system users will appreciate this information and can use it to fine tune their systems. Dealers and system specifiers will find this instrument invaluable in evaluating system components. The Thomson & Howe meter is accurate, efficient and low in cost. We're using ours daily and find that we've been under estimating the long term power production of our PV array. I can't wait to hook it up to the inverter and find out our actual consumption. RP