How to choose a UPS / calculate power for a new PC
As a very rough rule of thumb, the wattage rating of a UPS is approximately 0.6 * its VA rating so, as you have seen, a 700VA UPS is good for a power load of around (0.6 * 700) = 420W (your specs said 405W). Conversely, the minimum VA rating you need is approximately 1.6 * load wattage. Your 700W PSU gives you some idea of how much