Inverter not lasting

Clinton one of the managers at Crossways. Spar sold me an inverter in his private capacity he told me in a whattsup it would run for up to eight hours it went off after 90 minutes this morning. I whattsup him get a shirty reply that I.must buy a bigger one. I bought his one as he said it could handle tv and computer easily and I got rid of my two inverters and two new batteries cheap. Not good for a manager to act like this in his private capacity