mm writes:

I'd like to better understand that and what Dave said. I once asked
about the difference between volt-amps and watts and didn't get a real
answer iirc. I haven't googled.

To calculate watts, you take instantaneous measurements of voltage and
current and multiply them together, then integrate (average) the result
over some time period. You can do this in analog circuitry using an
analog multiplier device, or you can do it digitally by making many
measurements of voltage and current per cycle and multiplying them
digitally before filtering. Either way, you need a special meter that
measures and multiplies two quantities instant by instant.

To calculate volt-amps, you measure and integrate voltage and current
separately, then multiply the two numbers together. This can be done
with two independent ordinary multimeters.

If the load is resistive (e.g. an oven, baseboard heater, etc) there is
no difference between watts and VA. The current is always in phase with
the voltage, the product of the two is always positive, and the two
different calculation methods give the same answer.

But that's not true of capacitive and inductive loads. In capacitive
loads, the current waveform is up to 90 degrees in phase ahead of the
voltage waveform (i.e. peak current happens as voltage passes through
zero, where its rate of change is greatest). If the phase shift is
exactly 90 degrees (pure capacitance), then for half of the cycle the
sign of the current and voltage are actually different, and the product
of the two is negative for that portion of the cycle. The same is true
for inductive loads (e.g. transformer with no load on it, unloaded
motor) except the current waveform lags the voltage one by up to 90
degrees.

Whenever there is a phase shift between current and voltage, the VA
remains the same, but the watts measured are reduced. Effectively, for
one portion of the AC cycle the circuit accepts power from the utility,
and for another portion of the cycle it feeds some of that power back to
the utility. If the phase shift is a full 90 degrees (either ahead or
behind), the net power is zero!

Both measures are important. Watts is the amount of actual power
consumed by your device and converted into heat, light, or mechanical
motion. Voltage determines the amount of insulation needed on wires and
the number of turns of wire in a transformer, while amps determine the
size of wire needed to carry the current and the resistive losses in
that wire resulting from current flow.

So VA is generally the right measure to use when sizing transformers and
wiring, not watts.

Dave