Efficiency is the ratio between the consumed input and given output power. The emerged difference is called energy loss.
Causes of energy loss include:
Ohmic losses by transformer and leads
Switching losses of SMPS
Inserted filters for EMC activities
Losses by multi stage (eg PFC & current control of LED drivers)
Losses by processing the IC
Ottima's LED drivers have efficiency levels of up to 87% (full load).