  While non-orthogonal multiple access (NOMA) improves spectral efficiency, it
results in a complexity at the receivers due to successive interference
cancellation (SIC). Prior studies on the energy efficiency of NOMA overlook the
SIC overhead and rely on simplistic power consumption models (PCM). To fill
this gap, we first introduce PCM-$\kappa$ that accounts for SIC-related power
expenditure, where $\kappa$ represents the average power consumption per SIC
layer. Then, to investigate the energy efficiency of NOMA and joint
transmission (JT)-CoMP NOMA, we formulate a power allocation problem for
maximizing the energy efficiency and consequently propose a global approach
running at a centralized entity and a local algorithm running at a base
station. We evaluate the energy efficiency using PCM-$\kappa$ and two PCMs
commonly used in the literature. Numerical analysis suggests that using
simplistic PCMs leads to a few orders of magnitude overestimation of energy
efficiency, especially when the receivers have low rate requirements. Despite
the superiority of JT-CoMP NOMA over conventional NOMA in finding a feasible
power allocation, the difference in their energy efficiency is only marginal.
Moreover, when conventional NOMA is feasible, the optimal solution for JT-CoMP
NOMA converges to conventional NOMA and NOMA schemes favour the users with the
best channel quality.
