12VA should equal 12 watts. [VxA=WATTS]
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The measurement of electrical power that is computed by multiplying volts times amps. In a DC circuit, volt-amps (VA) and watts are the same because DC circuits do not add inductance and capacitance that take away power. In AC devices, volt-amps ratings are higher than watts. For example, in an uninterruptible power supply (UPS), the volt-amps rating is approximately 60% higher than the actual wattage.
Some power conditioners are rated in WATTS (PS Audio, Hydra, eg) and some are rated in VA (Equitech, eg).
To size a power conditioner rated in VA: add up all the equipment power ratings that are given in WATTS and double that value. Add the (2xwatts) to the total power rating of equipment given in VA. That's the size of the power conditioner in VA. Or in your case, 2x300+12=612VA. Pick the next size larger than 612 to be sure.
To size a power conditioner rated in WATTS: add up all the equipment power ratings that are given in WATTS. Add the watts to the total power rating of equipment given in VA. That's the size of the power conditioner in WATTS. Or, 300+12=312 Watts. A 300W unit will do it.
Also note that amps can have a power output greater than their nominal rating. Check the owners manual and see what's the maximum power draw/rating that the amp has. For example, if your amp is 300W into 8 ohms and 400 Watts into 4 ohms, then use 400 watts.
Best-bet-can-never-go-wrong: size your power conditioner for the wall power standard of 15 amps/120volt or 1800VA.
I understand the difference between true power and apparent power but I never knew about the difference in ratings from power conditioners rated in VA to those in watts.
GS, if you double the watts to get the approriate VA, why don't you halve the VA to get the appropriate watts. With 12VA or 12 watts it doesn't really matter that much but if the numbers were bigger it would. Using your math I would choose a 500 watt conditioner for a 500VA load when it looks like a 250 watt unit would suffice.