Re:Why are there different ways of measuring energy?
by otter-rock on Thursday July 07, 2022 @09:07AM
(#32013594)
You cover a lot of interesting topics. Some exposition:
Reporting power is easy with the power triangle. It's a mathematically exact concept.
Measuring power is a different story. Distortions of the assumed perfect sine wave create measurement errors. Meter bandwidth matters, but most have low bandwidth.
Grid meters at least use voltage phase, if not magnitude. Residential customers are billed only for real power, so the phase information is necessary to exclude the reactive power from the measurement.
One rating is for maximum output. The other describes a short-circuit that you can create with an overly reactive load.
The degrees of freedom complicate the implementation, but the bottom-line discussion abstracts away those details.
Your grid power consumption is probably measured in terms of current alone. That means that the number on your power bill is different from the amount of energy your devices dissipate as heat. That measurement is converted into power using the RMS trick, but there is no guarantee that it will exactly match the actual power consumption of your house.
Your UPSs has different limitations for real power, current, and peak current. And your house wiring (or better, any wiring) has limitations for (windowed) average current (with many different limits for different window sizes). Wiring also has a limitation for voltage, but it's usually high enough to not matter on normal usage.
As a rule, with AC, there are so many degrees of freedom that any question becomes hard to answer.