Battery backup

I have the model X88 internet gateway.  It's the only device plugged into a 950VA battery backup.  During a recent power outage, the backup battery was drained after, maybe, two hours with no internet being used.  We also use Xfinity for landline service, so no gateway equals no phone service.  Before I invest testing and charging/replacing the current UPS battery, I'd like to calculate how many VA is required to power the gateway each hour of battery drain.  Does anyone have a formula for that calculation?

Thanks