Why is my consumption higher in summer, when I leave my thermostat at the same setting as in winter?

In the summer the AC has to work harder, and therefore consumes more chilled water, to maintain the temperature in your unit as the outside temperature is much higher. It’s a bit like driving a car at 100KPH along a level road – when you come to a hill you need to put your foot down harder on the accelerator to maintain the speed, which uses more gas.