A quick note: I realised that the 18% I quoted above may look to be the energy efficiency of the PT6A-140A engine.
That's untrue; it's a ratio between apple and pears.
On one side, the useful mechanical energy per kg of the entire propulsive system and, on the other side, the chemical energy per kilogram of fuel.
The efficiency of the engine is actually hidden in the ESFC number.
0.566 lb/ESHP/hr is equivalent to 0.25673328 kg per 0,735499 kW per hour,.
That's 0,735499 kWh / 0.25673328 kg = 2.86 kWh/kg or 23.9% of the energy contained in the fuel that is converted in mechanical energy provided to the propeller shaft.
(Actually, ESFC is about mechanical energy to the shaft plus mechanical energy from the exhaust)
And 23.9% is damn good for a small engine.