You can get close to the answer with some basic math. If you have a power station that is rated at 1,000 watt-hours, and you plug in a device, let's say a tv, rated at 100 watts, then you can divide that 1,000 by 100 and say that it will run for 10 hours.  However, this isn't usually the case. The industry 'standard' is to say that you should take 85% of the total capacity for that math.  In that case, 850 watt-hours divided by 100 watts for the tv would be 8.5 hours.  The reality is that you should expect somewhere between 8.5 and 10 hours in this example.