The reason none of those are a problem is pretty straight forward. Each of those systems were designed to be powered by the aircraft's gen-sets, rather than the

The reason none of those are a problem is pretty straight forward. Each of those systems were designed to be powered by the aircraft's gen-sets, rather than the other way around. When designing an aircraft and its sub-systems, the engineer provides power availability specifications. The people who make the headlights, avionics, and coffee machines are all aware of the common power configurations and design their equipment to work with that specification directly. I used to work on light aircraft (private), which all used 24 VDC. Everything in the plane was designed to run on 24 VDC. Commercial airliners likely have more than one voltage specification available, but I'd imagine it's all DC. Any time you have to convert voltage, you're going to lose some efficiency. Inverters (DC to AC) have some particularly nasty characteristics that make them undesirable on an aircraft. A DC voltage converter would work, but you still introduce some degree of inefficiency. A good switching DC voltage converter can exceed 90% efficiency, but so can an AC inverter. Neither operates at that efficiency all the time though. I know that AC inverters perform best when run at close to nominal load. At lower current levels, efficiency drops. I don't know enough about DC converters to say. Add to this the fact that inverters and converters are heavy, and you can begin to understand why a power outlet at each seat is problematic.