Stygmata wrotehow can i find the wattage needed and the wattage of a ups ?i am thinking about using this ups for a server cabinet
Watts= VA*0.6
If your ups is rated 600 VA then it can power a max output of 360 watts for a couple of minutes. The inverter in the UPS (especially the generic ones) isn't made to handle max output for prolonged periods of time. Therefore in order to protect both your house and ups from a potential fire, it's recommended to use half of the output of the UPS.
For example if you want to power your PC for long periods of time and you know it needs 500 watts to operate, then you need at least a 1650 VA UPS (notice that the max power output will be almost 1000 watts, but to play it safe you're only going to use half of that output).
And please make sure to check the videos and tutorials the other guy posted, they include some important info about wire gauge and how to store the batteries.
One more thing, if I were you I would seriously consider getting a sine wave inverter instead of salvaging an old UPS. These sine wave inverters are built to last and to handle lots of power, they produce a sine wave identical to that of the AC outlet, unlike the cheap UPS's that give out square waves which could potentially damage sensitive electronics.
Additionally, I would stay away from car batteries; car batteries can't hold a voltage that long to be used for a power back up system, instead go with one of those deep cycle batteries (also known as marine batteries).
If you want to save yourself a headache,
check out a store in Beirut - Msaitbe called "Sonics". It's run by an electrical engineer who specializes in building custom UPS's for households and companies. Tell him what you have in mind and he'll give you a quote including installation. It shouldn't cost you a fortune, of course it all depends on the wattage you want to run.