On 11 November 2013 20:30, steve-ALUG@hst.me.uk wrote:
The lower the load, the longer the battery life. The higher the VA rating, the longer the battery life.
At least as I understand it, VA doesn't give any indication of battery life, and it's quite common to find cheap UPS's differentiating themselves on VA rating (1500VA is better than 1200VA, right?).
750 VA means it can deliver 250V at 3A. If your hardware needs 4A the UPS will fail to provide it and presumably go overcurrent and shut down. Whether it can provide 250V at 3A for 5 seconds or 5 hours is immaterial. So, for the same load, a 750VA UPS and a 1500VA UPS will last just as long (assuming all else is equal), but if you have a 4A load the former will fail to work at all where the latter will.
"Decent" UPS manufacturers like APC tend to put bigger batteries in to a UPS with a higher VA (logic: bigger battery means more stored energy, and whether you want it for a low load over a long period of time or a high load over a short period of time is up to you, so make it capable of supporting the higher load).
Frustratingly even the likes of APC seem pretty vague about battery capacity (or they did the last time I looked at it) making it quite hard to compare. Generally they'll tell you how long the UPS will last a full or half load (on which basis a 1500VA UPS would appear half as good as a 750VA UPS on the same battery capacity).
I'm sure someone will correct me if I have all this wrong.
Mark