First don't listen to this poster, he has clearly no idea, just look at the text in bold, manages to compare a 10 year old UPS and generic PC a time (2012) which is different.
I was wrong, it turns out the Minuteman Enterprise series (and it's replacement, the Enterprise Plus) does have a sine-wave output.
What it isn't in off-line - it is "line interactive". Off-line runs the equipment off of the battery at all times. Off-line units are relatively inefficient. You shouldn't need an off-line UPS, assuming a proper design on a line-interactive unit. Line-interactive units connect the output directly to the mains and switch over if input is lost or out of spec. They have sufficient inductive/capacitive reserve to carry over across the switch.
Sine wave alone will not insure, though, that the UPS can run a reactive load, such as a refrigerator.
You can get a 1500VA unit used on eBay for $100, and I'd be completely comfortable buying one. You would likely have to invest in batteries. (Motorcycle Gel cells, as used in most UPSs.)
I did need a firmware update at some point that required replacing a chip, and the company paid the shipping both ways on the beast.
One thing is to simply look at the weight of the unit (factoring in batteries, though... How heavy is it after subtracting the weight of the batteries?). Heavier = bigger transformer, important to smooth-out variations and insure a smooth cut-over. You need a big transformer and big capacitors to provide adequate capacity for cut-over. This is the biggest place where UPS manufacturers scrimp, particularly those aimed at consumers, because these features are invisible to the vast majority of consumers.
I mentioned "generic PCs" only to give an idea of the load I have on this UPS - two custom-built (by me) servers with decent U.S.-built, non-commodity power supplies (PC Power & Cooling), graphics cards with hefty current draw, a MacBook, 4 22" monitors, plus all the various little router boxes, etc. and some room lights.
It has gone through several outages, and never once has anything glitched.
