There has been a huge increase of interest in energy audits of
commercial and industrial buildings. The first step is usually
understanding how energy is being used in the facility. This requires
measuring power (kW), the rate at which energy is expended by the
facility, and demand (kVA), the total requirement that a facility
places on the utility to deliver voltage and current, without regard
to whether or not it does actual work. A digital multimeter (DMM) can
measure voltage and current, the product of which equals demand for a
simple single phase circuit with a stable load. For a real life load,
we need to also account for power factor (PF) which is equal to kW/
kVA. Utilities often collect a penalty charge is PF falls below 0.95.
To measure real power we need to simultaneously measure voltage,
current and Imaginary Power or Volt-Amps Reactive (VAR), a current
flow that is present on the distribution system but produces no work.
It's usually caused by motor inductance and is greater when motors are
not loaded to their full capacity. Harmonic currents, reflected back
into the system, also reduce power factor. Measuring real power
requires a meter that can simultaneously measure voltage, current, VAR
and harmonics on a one-second period. A DMM cannot do that but a power
quality analyzer (PQA). PQAs will typically measure power usage for
one hour, which makes it possible to estimate energy usage for a
month or year. A power logger, another type of power quality tool, can
perform a 30-day load study to provide an even better understanding of
power consumption over time.