Run time for UPS

I go to events where there is no electricity. I need a power supply to keep my laptop and monitor showing video clips for about 8 hours. Any suggestions of what I could use?
 
8 HRS is extraordinary - - the object of a UPS is:
  1. to force a controlled shutdown while power is still available -- OR
  2. to give the user time to end tasks, save all docs and then shutdown.
This is NOT the same as a mainframe powerstation used to avoid outages.
 
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