Iíve been using UPS for my entire career, so know a decent amount about them. I have four of varying sizes (450, 550, 1000, 1400 VA).
Yes, bigger is generally better, but there are other considerations. UPSs with high VA ratings usually also have more battery capacity but one does not necessarily equate to the other.
As a rule of thumb I usually total up the peak wattage of everything to be connected to the UPS, and use that as a starting point. Peak wattage for a computer can vary a lot depending on what you are using it for, with the greatest variable generally being the graphics card (if youíre a gamer), or the CPU if youíre not. A single high end graphics card can draw upwards of 400 watts and a high end CPU almost as much. But if you donít know where to look for that info, then a suitable substitute is look at the wattage of the power supply on your computer (knowing that it will almost always be higher than the total peak wattage expected to be running in the computer.
Everything else should be able to pick off of the equipment labels. Flat screens draw a lot of power also, especially the big ones. Mine take about 200 watts each.
Anyway, once you have your total load I would advise you to look for something up to twice that amount in VA. Thereís a simple reason for that; the more capacity generally means longer run time.
For instance a 550 VA UPS might supporting a load of 550 watts might only run for less than five minutes with the power out, while it might run for 15 or 20 minutes at half load. The manufacture should be able to supply you with run times at half and full loads, so that you can compare. If you are there to shut down reasonably quickly, run time isnít much of an issue. But if you walk away from your computer leaving it on, it may very well be.
Or, if you are like me and run a cable or DSL modem and a router, a substantial run time can keep both of those going for quite a while, with the main computers shut down and you accessing the internet from a laptop.
If you do rely on your UPS for more than a few minutes, you should know that VA does not equal watts for alternating current (AC) except for short bursts. So you should make sure that your UPSs actual W rating (which is more or less average wattage over time) is higher than the load you are putting on it. As a guess, I would say that your 550 VA UPS was only rated at 350 W. (mine is 550VA / 330 W)
But donít just look at VA or W. Compare run times also.
That being said, you can probably run a single computer, 20Ē flat screen, modem, router for like 550-600 or so VA, if you donít have a high end graphics card that you use for games, and expect to shut down quickly after a power failure, 1000-1500 if you have high end graphics, play high-res shooters, and use bigger flat screen(s) at high resolution, or want longer run time.
At the same time the price differential between 600 and 1000 VA may not be all that great and, in that range, bigger is better. The last time I looked, 1000 VA was the sweet spot in the market in terms of value for dollar spent, but that may not be the case now.
Caveat: my UPS experience is entirely with APC