sorry but that is completely false, watts is power while volt amps is apparent power so you need the power factor as part of the equation. For a UPS, such as a 1500VA, you will see on listings it can do up to 900W. The VA rating is only the same as watts in a DC circuit and in that circuit it does not change. Also you cant say 'its almost always this give or take some' for a set mathematical calculation, it doesn't just change as you please.