I'm trying to find replacement batteries for my UPS, I checked my local store and they dont' have this particular model I need so I'm looking for other brand that does exactly the same. What I usually see on the battery label is something like 12V 7Ah or 12V 9Ah. However, this one is 12V 21W. Does anyone know how can I convert the 21W to the equivalent Ah? Also, what is the reason of specifying the spec in Watt rather than Ampere-hour?
Below is the spec sheet of the battery in my UPS: