In this work, we consider the downlink of a multiuser multiple-input multiple-output (MIMO) system and aim to find the jointly optimal number of base station (BS) antennas and transmission powers that minimize the power consumption while satisfying each user's signal-to-interference-and-noise-ratio (SINR) constraint and the BS's power constraint. Different from prior work, we consider a power consumption model that takes both transmitted and hardware-consumed power into account. We formulate the joint optimization problem for a singlecell system and derive closed-form expressions for the optimal number of BS antennas and transmission powers. The solution can be utilized in practice to turn on and off antennas depending on the traffic load variations. Substantial power savings are demonstrated by simulation.