You don't need to worry overly much about the rating on the APC UPS.
Unless it might be an online UPS, which is more expensive.
The offline UPS will supply power from the mains, and will only cut in a very small fraction of a second after abnormal power conditions from the mains supply.
So, if you have 500 watts connected to an offline UPS, even one with a small battery, then no problem.
But, when the power from the mains fails, then you won't have much time available to turn off your equipment.
The advantage of the offline UPS is that it protects against short fluctuations in power, or very short power outages.
I would always go with an online UPS, but not in Thailand where they are so much more costly than most places like the USA or Taiwan, or HK, or.....so many other places.
I used to have an online APC UPS which I really loved. That model would connect to the computer to monitor and record power conditions.
A crazy price in Thailand, IMHO.
Therefore, I use only offline UPS in Thailand, where, maybe, one needs the online UPS....most.