Sorry, this might be a stupid question as I'm not an electrical person. I'm analyzing an electricity consumption dataset and the consumption is measured by apparent energy (VAh).
How do I convert this VAh to kWh as normally seen on a power bill? Or are they the same thing, just different ways to call?
Thanks.
You need to know the power factor, which is highly depedant on your application. Then you simply use this formula: $$ P = S * PF $$ where
P = actual energy
S = apparent energy
PF= Power Factor
This answer tells us, that power factor in general public households can be expected to be greater than 0.9.
You can’t convert them.
All you know is that the actual power(’s absolute value) is ≤ the apparent power. The actual power might be 0 (purely inductive or capacitive load) or negative (if you are analyzing a generator). It might also be = the apparent power (purely resistive load).
As for the units, VA and W are equivalent.
As for the energy, give than the power is the derivative of the energy, you have the same result that actual energy(’s absolute value) is ≤ the apparent energy, and that VAh and Wh are equivalent units.
Power: W=V*I
Effective power: VA=VIcos(Φ) where Φ is the phase angle between the current and the power.
For a pure capacitive load the current will lead the voltage by 90° or Π/2 Radians
cos(90)=0
Therefore for a purely capacitive load, although the effective power would be 0, the actual power drawn is V*I
The same goes for a purely inductive load, where the current lags the voltage by 90° and cos(-90) = cos(270) = 0
This would mean that under those conditions, although the system draws V*I watts out of the system, the work done is 0, which is what the meter on the incoming mains, will also read.
This is not a good state for either the consumer or the supplier, therefore power factor correction is applied. As most loads are inductive, the method used to correct the power factor, is by using a capacitor of the appropriate size, for the load.
Domestically, washing machines, drills, etc. have in built capacitors, and in anycase, their effect is minimal.
Industrially, where they have 3 phase power feeding large transformers, large motor driven machines, ones which require 10kVA or more, will also require the incoming mains to have power factor correction capacitor connected. These however tend to be dynamic, where the power factor is constantly monitored and capacitors are automatically switched in to keep the PF as close to 1 as possible.
References:
https://en.wikipedia.org/wiki/Power_factor
http://www.eaton.com/ecm/groups/public/@pub/@electrical/documents/content/sa02607001e.pdf
