This article provided information about the backup time for your UPS
Please access the following link to see a graphic of the backup time depending on the UPS you have or you select: Runtime Calculator
NOTE: This tool will use the watts provided by the UPS. Depending on the load you are going to consume from the UPS this will give an estimated runtime (backup time) from the UPS to your devices.
When you are on the website look for the option SELECT A MODEL here select from the drop list the model of the UPS you have or the one you want to investigate the backup time. In this case we will choose NT-511
Once you have selected the UPS from the drop list, you will see in the graphic the values change to the UPS selected.
In the specification of the UPS you will see the capacity supported by this UPS can support. The NT-511 only support up to 250watts.
This means if you go above the 250watts this UPS will not be able to provide backup time to your devices.
Lets take the example of the 77watts that you can see in the graphic. If the amount of watts is 77 from all the devices, then the NT-511 will be able to provided a backup time of 21 minutes. If the consuming is more than 77watts then the backup time is gonna be less and less time.