John Patson is right. In other words, there is always some kind of coupling between the engine and the rotor/propeller. The coupling has its own efficiency. In some cases mechanical coupling makes great losses, and becomes inefficient. (E.g. on multirotor aircraft, because of mechanical complexity.) Then engineers choose the “electric clutch” that is the electric generator+motor configuration. This coupling has a known weight, and a stabile efficiency value. (The latter is usually around 90%. For very high power this figure may further improve.) It is a relatively easy calculation if you shall, or shall not pick the electric clutch.
E.g. the military love, and have been using electric clutches for many decades. For trucks and tanks. (And submarines.) RR technology is quite likely to produce a version of the system that is both light and powerful enough for aviation.