ASK THE EXPERTS: Power, Energy & Power Factor

Intermediate
Kilowatt-hour meter
A standard residential kilowatt-hour meter.

I am concerned about universal energy measurement standards that are used for certain consumer devices, residential service meters, and appliance rating guides.

Typical energy consumption measurements use watt-hours (Wh) or kilowatt-hours (kWh). Although the units are accurate for 100% linear (resistive) loads, accurate measurements need to calculate the total flow of electrons (amperage) through both the resistive and reactive portions of loads.

The reactive portion of inductive loads (most electronics, microwaves, fluorescent lights, refrigerators/freezers, pumps, etc.) increases the amperage to the load based on the load’s power factor (electrical efficiency). Adding a capacitive load (such as a capacitor) can offset the increased current by improving the power factor. There is no “free lunch” when using the wattage of a load and not accounting for any extra energy consumed by its inefficiency.

I’ve used a watt-hour meter to read the energy consumption of a static-load appliance. This device takes the measured wattage and multiplies it by the cumulative time recorded in the device. Although this same device can measure current, voltage, and power factor, it isn’t using these values for energy consumption calculations. I believe this results in very inaccurate readings and misleading information for the consumer.

Our typical service meters are billed using Wh or kWh units, but they are actually measuring volt-ampere-hours (VAh) or kilovolt-ampere-hours (kVAh), because they use the current in the meter. Using VA or kVA should be the true standard for electrical measurement, especially in today’s world, where nonlinear loads are far exceeding linear loads. A watt-hours measurement is antiquated and mostly inaccurate.

Rick Simpson • via email

The central issue is that power factor (PF) is not electrical efficiency. The two are distantly related, but they’re entirely different concepts. Because of power factor effects, any volt-amp measurements will often be wrong. Contrary to what you seem to be suggesting, volt-amps gives an incorrect overestimation of the actual energy consumption.

What’s PF? It’s the ratio of what is sometimes called “apparent” power to actual power. A PF of 1.0 means that the two are the same. A PF of 0.5 tells us that a 100 W load actually needs a 200 VA source to power it. But it does not mean that it uses 200 W, since some of the energy sent out by the source is returned to the source.

PF comes into play in non-resistive—inductive and capacitive—circuits. Ideal inductors consume zero energy, yet plugging inductors into AC outlets draws significant current. It’s the same with capacitors. If we multiply their voltage and current measurements, we’ll see major watts of energy flow. But this wattage is wrong, and the actual energy consumption is zero.

Inductors do draw energy from the utility grid—but then they give back every bit! During each cycle of AC, the electrical energy ends up “sloshing” back and forth between the distant generator and the inductor. Resistors don’t do this. Their current and voltage alternate at the same time, so the energy flows in just one direction.

While I appreciate your investigative zeal, the present energy meters we use are doing it right. When connected to an inductor, it ignores the volt-amperes and the reversing energy flow. We don’t want to measure the “sloshing energy” created by an inductor or capacitor. We only care about the total energy that moves, on average, from generator to appliance. In some specialized applications, having equipment that can measure VA and PF is helpful. But for almost everyone, focusing on watts and watt-hours is the best approach, since this is what we generate, move, and use.

Bill Beaty • amasci.com

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