How to Calculate UPS Batteries DC to AC Efficiency?
A 66 Watt-hour battery generally can't supply 66 Watts for 1 hour. Probably it can do 660mW for 100 hours, and 6.6W for almost 10 hours, but after drawing 66 Watts for 39 minutes, it can't supply that much power anymore.
This is because batteries are physical things that rely on chemical reactions to re-generate the charge that they supply. The chemical reactions happen at a finite rate, and as you discharge the battery, the rate slows down.
This isn't really an efficiency thing, the other 25% or so of the battery charge is still in there, but it can't be extracted at a fast enough rate to power a 66W load.
At higher rates the same problem becomes even worse, which is why most UPSes only last a couple minutes at rated load, unless they are provided with very generous batteries relative to the power handling capacity of their electronics.