The do not say it because Watts = Volts x Amps ONLY in DC or 100% resistive AC loads. If it is an inductive load, it isn't that simple, that is why they have `VA', `Volts x Amps', ratings. There can be a phase difference between the voltage and current, thus the `Wattage' is less than the `VA' rating. A `1500VA' UPS is going to list less than `1500 Watts'. Just check out any UPS manufacturerer's specs. and the `VA' and wattage ratings will be different. To be safe, I take the voltage rating of the equipment and multiply it by the current rating to come up with the `VA' rating. I know APC says to not go over 80% of the UPS rated load, so take your `VA' value and divide by 0.80 to get the `VA' rating you want your UPS to be. This will probably give you an UPS a little bigger than you need, but it is better to be a bit too large than too small.