One of those little one piece inverters with no wires?
No 2 of those inverters are the same.
Many seem to draw about 350ma no load.
So it should handle 8 hours. 8h X .35A = 2.8 amp-hours.
My luck with the little 'no wire' inverters has been
not too good for use past an hour or 2 at a time,
even with very low power circuits.
G-