Kill A Watt P4400
The “instrument” referred to here can either be a stand-alone device doing AC power metrology exclusively or a subsystem performing that function on a secondary basis as part of something else, e.g. as part of a solar PV system inverter.
There is tremendous breadth of AC power metrology instruments – you may recognize the image of the PCB in a Kill-A-Watt (Model P4400.01 from P3 International) which does a great job in helping folks get a handle on energy usage in their homes. On the other hand, you would have to be a serious aficionado of this technology to recognize the other image which is the inside of a Powerlogic Ion 7350 from Schneider Electric which would typically be found in electric utility substations. Both units sample the incoming signals and deliver values for voltage, current, real power (watts), reactive power (VARs) and apparent power (VA). The biggest difference between the two is that the Schneider instrument costs of the order of 100 times as much as the Kill-A-Watt.
The “knee jerk” response to the 100X ratio above (about $20 for a new Kill-A-Watt and about $2,000 for the Ion 7350 (on the “used” market) would be an assertion that the Schneider instrument is more accurate. That happens to be true for this particular example but in terms of ability to measure real power and therefore accumulate electrical energy accurately the ubiquitous $50 utility revenue meter on the side of most homes is tough to beat. In fact, most of what makes the Schneider instrument so expensive is strict compliance with Class A of the IEC 61000-4-30 international standard about which the typical homeowner could care less.
The point is that with modern sampling methods one can do a lot for not much money and therefore what really matters is building the instrument best suited for the application. If, for example, a homeowner tried to use the Schneider instrument for a survey of their energy use they would find it grossly cumbersome while the Kill-A-Watt does exactly what they need right out of the box.
DJA has considerable expertise in making the best engineering tradeoffs for the myriad of power metrology applications out there. One great tool that can be very helpful is the use of integrated metrology engines (sometimes called, “meter chips”) available from several semiconductor manufacturers. We are huge fans of the ADE78xx and 79xx parts from Analog Devices, Inc. These ICs handle the sampling and most of the basic power metrology computation leaving the majority of engineering cycles to figure out how to build the best instrument for the application at the lowest cost.