this one is easy. just buy a 750 va unit. or a 1000va. thats more then enough for your iMac.
if you spend more $$ like 1500 va your just adding more room for more devices down the road and you will get
more run time. the bigger va capacity of your ups the longer the run time you get on batteries.
the ups selector says a g5 is 299 watts. and it says 750 va is the one you want. apple says the iMac is about 350 watts full power so that about right.
thats 350 watts on a load, like doing handbrake or doing cpu intensive stuff . if your iMac is just idling, it doesn't use all 350 watts
apc has a deal where when your batteries die, and they will, if you buy the first replacement set of batteries from them, they will add a free extended warranty to your ups. after that you can just keep
throwing more batteries at it until it dies. my smart ups is still going strong 10 years later, i just give it new batteries.
back ups = basic ups , with i think like a regular power button, lately these are all small power bar ones
back ups pro = some have better lights and a lcd screen, these are like towers
smart ups = the top of the line. it has pure sine wave output and a slot for accessories.
make sure you get one with free shipping or buy one locally. these things are very heavy\\
another thing to look for in a ups is AVR, they will regulate the voltage. some will boost power input.
like if you have a brown out, and your getting like 90 volts or 95 volts, some units can boost that up to 115 ac without having to use batteries
google says not to use backups / backups pro with motors , the stepped wave / sine wave approximation isn't as good as a pure sine wave., people also say the smart ups units will last longer.
the ups is a very big investment. what you are buying today will probably be the only ups you ever need for 12 years, so you don't want to skimp on it.
plan on spending $188 for a back ups pro 1000 to $300 for a smart ups 1500
1500 va = 900 watts = 2.46 imacs