How can I calculate if a UPS is sufficient for my requirements?

Solution 1:

If you have 8.5 Ah and draw 8.5 amps, you can do it for one hour. Conversely you can draw 5.8 Amps for 32% longer or approximately 81 minutes. You should only try to draw 80% of your max rating. Batteries get HOT under 100% load.

I would be nervous about operating so close to the stated maximum wattage of the UPS's. You should upgrade as soon as possible to the next available size (wattage).

The formula for wattage is very simple. It is Volts (electrical pressure) times Amps (electrical current). So a 120 Volt 5 Amp (maximum current draw) device would need a 600 Watt power supply.

What would all of this information provide you? Figure out the total in kilowatts, and then multiply that by the number of hours it is on per day (daily kilowatt hours). Then multiply that by the cost per kilowatt, and you now know what this rack of equipment costs you in electricity every day.

Solution 2:

First thing to do is to ignore the nameplate and actually measure the current draw. Get an electrician to do that if you don't have the gear for it. Make measurements at the heaviest load your servers normally experience. It's surprising how different (and generally lower) real-life figures are compared to the server specs.

Next ensure your calculated figure is no more that half of the UPS rating. While you could theoretically draw the full Ah rating of the battery for an hour, drawing anything more than half that figure will result in a drastic reduction of the battery life. Ideally even stay below one quarter of the rating.

Of course your UPS should also state the maximum current it can supply, so be sure to factor that in as well.