Is that bill for 1 month? Do you have the thing on 24/7? Cause 750hrs/30days=25 hours/day. The increase may not be due to this unit alone though, your other usage may have gone up a bit accounting for why its 25 and not 24 hours.
24 x 30 = 720, though Dec is 31 days so 31x24 = 744 hours in a month of 31 days.