We consider the impact of transceiver power consumption on the energy efficiency (EE) of the Zero Forcing (ZF) detector in the uplink of massive MIMO systems, where a base station (BS) with $M$ antennas communicates coherently with $K$ single antenna user terminals (UTs). We consider the problem of maximizing the EE with respect to (M,K) for a fixed sum spectral efficiency. Through analysis we study the impact of system parameters on the optimal EE. System parameters consists of the average channel gain to the users and the power consumption parameters (PCPs) (e.g., power consumed by each RF antenna/receiver at BS). When the average user channel gain is high or else the BS/UT design is power inefficient, our analysis reveals that it is optimal to have a few BS antennas and a single user, i.e., non-massive MIMO regime. Similarly, when the channel gain is small or else the BS/UT design is power efficient, it is optimal of have a larger (M,K), i.e., massive MIMO regime. Tight analytical bounds on the optimal EE are proposed for both these regimes. The impact of the system parameters on the optimal EE is studied and several interesting insights are drawn.