An energy saving problem in heterogeneous cellular networks (HetNet) is considered. In HetNet macro and small base stations (BSs) are deployed to operate at peak busy hours and are under-utilized during the rest of the day. The BSs have high power consumption even at a time with low load and traffic profiles. HetNet should adapt to load variations and some of the BSs should be switched off or put into sleep mode to save energy. The active BSs must provide a minimum quality of service (QoS) to all users in HetNet. We define QoS as the minimum data rate requirement of users. We consider BS power consumption model with two terms: static and dynamic. The former is fixed power consumption term and the latter is proportional to BS load. The macro and small BSs have different performance. The optimization problem is stated as joint user association and BS switching-on/off. The objective is to minimize total network power consumption while satisfying QoS constraints. The obtained integer optimization problem is solved using a Lagrangian heuristic. The discussion is illustrated by numerical examples.