The term "Ah" found on acid-based batteries refers to amp hours. Amp hours is a form of measurement used to describe how much energy a specific battery can...
The term "Ah" found on acid-based batteries refers to amp hours. Amp hours is a form of measurement used to describe how much energy a specific battery can hold.
The amp hours describe the available capacity on a lead acid battery. The equation is broken down with the number before the Ah being divided by the hours printed on the battery. This yields the approximate power capacity the battery is able to maintain until it drains its power.
If a battery is labeled at 100 Ah at 1 Hr it does not mean the battery will be able to maintain 100 amps consistently at 1 hour. Lead-based acid batteries tend to decrease in capacity as the discharge rate increases.
Amps is how much power is being drawn at a given moment. Amp hour is how much is drawn over a period of time. Same idea as watts and watt hour.
So say you are drawing 1 amp from a battery, the more amp hours it has, the more hours you can draw an amp before it is depleted. For a UPS get the highest amp hour battery you can fit in the unit, though it will be close or the same to what came with it.
The only thing you really want to be concerned about is the voltage of the battery and the chemistry. As long as you get lead acid 12v the amp hour does not matter, but the higher it is, the longer it will run for. Of course, for cheap UPSes with no fan or active cooling you can run into overheat problems if you put a bigger battery and it runs longer than it was meant to, but electrically it will be fine. Some people modify their UPS to put big RV/Marine batteries and it's fine. Though I prefer to use an inverter-charger for that as it will have better cooling and overall be better designed for it.
Also as a side note the amp hour rating of a battery is typically tested at a specific load. The rating actually changes based on the load. The more amps you draw from a battery the lower the AH rating is. When they rate them it's usually tested at a specific load, like say 20 amps. (usually it's based on a time though). Basically it's not linear. The less amps you draw the higher the amp rating is. A 100AH battery might be like 120AH if you're only drawing a few ma but it might also be like 50AH if you're drawing 100 amps. I don't know the actual curves so my numbers are wrong, and are just examples.