I have since shifted but at my previous house, a 5 room executive appartment, with me, my hubby, my 2plus year old daughter and my mother-in-law, close to $300, actually we are also hardly at home, we leave for work at 9am, by the time we go home its usually 9pm plus. No water feature or anything that is consantly powered and we dont't even turn on the aircon. For us i think the bulk of it is from my mother-in-law, cos wen she is at hm, the standard TV/radio, Lights, fan must be on until 11pm when she sleeps.she turns on the lights even in the day.
I thougt it was quite high then for us since we were never at hm. But yurs is indeed on the high side.
You turn on the aircon? For me, i switch off the main switch for my kitchen appliances and only turn it on when i'm using. Except for the Tv cos that is a bit troublesome.