insulate the house, put ventilation fan and AC... if temp is damn hot your venti fan won't do much. but you can stop the ventilation fan and run the AC for maybe less than an hour(if you're not aiming for cool/cold) just to cool down the hot air.. maybe it'll increase your bill around less than RM30 i think if run not more than an hour everyday.. keluarla duit skit kalau mau selesa...