This paper thoroughly examines the semiconductor power loss characteristics of modular multilevel converters (MMC). Power loss behavior is examined under different pulse width modulation (PWM) methods and operating conditions. The effects of stored energy level, circulating current control utilization, power factor and submodule voltage balancing method on power loss are studied. Furthermore, unbalanced power losses and specific semiconductor stresses within a submodule are visualized by investigating the loss distribution in a submodule. The paper aids in understanding and design of MMC converter modules, selecting PWM and control methods with power loss perspective.