When people are shopping for PC power supplies, most of them look at watts and maybe brands. While the max power is important, its important to know how much power you're really using in steady state for sizing UPS. If you already have an APC Smart-UPS, you can measure power with it. It's difficult to measure AC power. Before you say, Volt x amps = watts, let me say this only applies to DC. All but heaters and regular light bulbs need more sophisticated to calculate wattage. To measure the REAL power, you need to figure out the area of the product of voltage times the current and average it over time(every sec or so). A power analyzer takes both measurements and calculate the answer and give it to you in real time. They do it digitally or with an analog integrator. Simply put, you can't do it with your multimeter. You can't even measure the current(amps) of a computer power supply unless your meter is a true RMS type. VA = volts * amps Watts = volts * amps * power factor, in the case of computers, this is roughly 2/3. Ok, so you've got a SmartUPS.. Let's say your model is SUA750, which is rated at 500W/750VA. Connect it to a computer with a serial cable. If you don't have one, buy one from APC or make one. You just ought to cut a straight through serial cable (9pin type) and rearrange three wires. UPS(male connector)<-->PC(female connector) 2-2 1-3 9-5 Open HyperTerminal Use 2400 bps Flow control Xon/Xoff leave everything the default. enter shift-Y to activate communication port. enter shift-P and you'll get a true power reading percent. So, if this reads "45.2", it means your wattage load is 45.2% of the SmartUPS' rated capacity. If you have a 750VA/500W UPS, you would multiply 0.452 and 500 to get 226W load. I suggest you size the UPS to be around twice the combined idle power of all the devices you want to have on battery backup. This ensures you get a good runtime and prevent the UPS from overloading should the power go out while your computer is heavily pushed with games and such.