This was asking about energy efficiency, though. The vast majority of the energy ever consumed by an animal is used in processes other than growth. The ability to grow meat without it needing to be part of an organism that has to be able to also grow a skeleton, reproductive system, be able to move and think, and other energetically expensive processes in addition to growing muscle, seemingly has to be possible for less energy cost than supporting an entire animal lifecycle.

Whether it can be done entirely using solar is another question, but free-grazing animals that only consume energy from local grass are not anywhere near prolific enough to feed the entire planet.