Overbilled, probably due to very fast meter. It cannot be 1300 units.
17 hours AC usage daily. 17x30=510 hours of ac usage per month.
Now if they are non inverter and not in healthy state (not serviced or improper gas charge), it will run continuously at peak power (7-10 amp) and will consume around 1.75 units per units in worst case scenario (full power throughout).
510 hours x 1.75 = 892 units out of 1300 units.
Still not possible, because non inverter runs at full power for first hour and then keep cycling on and off in continuous use (if in healthy state) and achievs about 3.5-5A average pretty easily (0.75-1 unit per hour) and even lower if properly maintained.
Switching to inverter will bring some benefits, but your meter is very fast, faster than it should.
We checked multiple healthy non inverter and inverter with advanced power analysers/monitors that recorded the average consumption, even a healthy non inverter consumes average 0.7-0.8 units on continuous use (day+night) and a non inverter consumes (0.5-0.6 units).