There are a couple of things to consider.
- Are you on an FTTN or FTTP connection? If you are on a FTTN connection, it is not really worth investing in a UPS as the signal disappears during a blackout anyway and your modem would be sitting there powered up with nothing to do.
- Which model modem do you have? They are each slightly different in power consumption.
The easiest way to check is to look at the rating on the power pack. Unfortunately I'm at work and son't have access to one at the moment.