Again no. The utility here routinely measure KVA as quantity and bill it also. In their one bill attached herewith, the contracted MD is 120 KVA. For the month of February 2013, the actual demand was 108 KVA and was billed accordingly ( underlined line-8 in the attached file.)
You have to be clear on what VA we are talking about. What they are recording is peak Vrms*Irms values which is not the same as instantaneous readings of volts * amps (not the same as recording V*A*time then dividing by the interval).
For example, a 1.2 + j1.0 ohm load on a 120V/60Hz source will yield the following:
S = 9,213 VA
P = 7,081 W
Q = 5,895 var
If looking at the instantaneous readings, you would see that:
The S, P, & Q values for VA, var, and W are all 1/2 of the peak-peak values of their instantaneous plots in the time domain.
Consider the plot of VA(t) with a pure resistive load. It is equal to the W(t) plot and the power value S = P = (VA(t)_max-VA(t)_min)/2 = (W(t)_max-W(t)_min)/2. Here W(t)_min equals zero so we get power equal to W(t)_max/2.
Now consider the plot of VA(t) with a pure reactive load. It is equal to the var(t) plot centered over the x-axis and the power value S = Q = (VA(t)_max-VA(t)_min)/2 = (var(t)_max-var(t)_min)/2. Note that since it is centered on the x-axis we get Q equal to var(t)_max.
Now consider the plot of VA(t) with a resistive and reactive load. It is not centered over the x-axis and the power value S = (VA(t)_max-VA(t)_min)/2. We also have P = (W(t)_max-W(t)_min)/2 and Q = (var(t)_max-var(t)_min)/2.
These are the same values we get by using RMS measurements of voltage and current. The VA (or S) here is supposed to represent the worst case scenario of what the system might be required to handle so is what we use to size equipment.
As for metering the 120 V / 60 Hz source feeding a 1.2 + j1.0 Ω load we would see that the metering shows:
Vrms = 120
Irms = 76.78
S = VA = Vrms*Irms = 9,213 VA
P = VA_average = 7,081 W
Q = sqrt(S
2-P
2) = 5,895 var
pf = 0.77
You may also note:
1) metering |VA|s over a period yields 7,704 VA
2) metering |+VA|s over a period yields 9,490 VA
3) metering |-VA|s over a period yields 1,410 VA
4) metering Ws over a period yields 7,081 W
5) metering |var|s over a period yields 3,753 var
so you can see there is quite a difference and directly measuring VA quantities does not always give us what we are looking for if we are not careful. You might be tempted to say we need capacity of 7,704 VA or 9490+1410=10,900 VA or 9490-1410=8,080 VA instead of 9,213 VA.
A note about energy exchange:
We know that the |+VA|s is energy delivered and |-VA|s is energy returned and the net of the two produces the Ws consumed for the period.
We casually speak of reactive energy flowing back and forth from the load to the source in half cycles but that is not exactly what happens except on loads with no resistance (which don't exist). Consider the resistor-inductor load. As the magnetic field charges, the source has to deliver the real energy (Wh) plus the energy to charge the inductor (varh).
As the inductor discharges, the real energy needed by the load is supplied by both the source and the energy from the collapsing magnetic field (when VA(t) is positive and greater than W(t) in the plot). As the field discharges, it supplies all of its released energy to the real load (when VA(t) is positive and less than W(t) in the plot). Over time, the real energy needed is less that what is being released from the magnetic field and then this excess field energy is returned to the source (when VA(t) is negative in the plot).
