A lot depends on whether you pay the bill direct or via the landlord/building owner. I rented a studio apartment in Bangkok and the building charged for the power and it was frequently well over 1500THB per month. Now I rent a two bedroom condo and pay the bill direct and it is rarely ever over 1000THB. That's having ac on all night in the bedroom and probably 4-6 hours during the day in the living area.