The power consumed by electronic devices is measured in watts (W) or in volt-ampere (VA). Power consumed in watts is the real power consumed by the device, as volt-ampere is the “apparent power”; that is to say, the result of the tension applied to the device and the electricity consumed by it. Usually, W=0.9VA (0.9 is the power factor). Some high-quality devices have an efficency near to 1 (0.9), what makes watts very similar to volt-ampere; but this almost never occurs.
The Watts in a device, as a source of electric power, show the maximum consumption the device can support. If there are many devices plugged to a CPU, a more powerfull source of electric power is required.
A regular PC (personal computer) consumes around 300 watts, inspite the fact that it may have a source of 500 or 550 watts (meaning the maximun power it can support, not the power it actually consumes). Normally, a PC consumes around 60% to 70% watts worth in VA (power factor 0.7). In order to determine the consumption of a device, it is necessary to multiply its amperage (A) by its nominal voltage (V). For example, a regular laptop transformer requires 18.5 volts and 3.5 amperes, which results in VA= 18.5 x 3.5= 64.75 VA; and as transformers are electronic, its power factor is near 1 (0.9) so we can take W=VA. So, the maximum consuption of the transformer is 64.75Watts (65W).
65 Watts is the maximum consuption a transformer can support, but it is not the real consuption of the device. It must be taken into account that the device will not consume more than 80% of this quantity, hence the laptop or device will consume about 50watts. This number of consumption is the one that must be taken into account when assessing a UPS or voltage stabilazer for this device. A 500 va UPS (480 watts according to the power factor) will be able to support around 9 laptops plugged to it.
Usually, UPS measure their nominal value in watts and volt-amperes; none of this values can be exceeded when plugging in devices.