So, how do we put together a high efficiency power system? Well, there are 4 main parts of a CubeSat power system. Solar Cells, Batteries, Chargers, and Regulators. All of these components need to be chosen and designed to maintain the highest possible efficiency.
How do we do that? For starters, we need to be smart in out component selection. Lithium batteries are an obvious choice for this due to their high power density. Unfortunately, they have a nominal output voltage of 3.7V. While that isn't really a problem for 3.3V electronics, it's unlikely we will be able to design a full satellite that only utilizes a 3.3V bus.
Unlikely, but possible. This is something that should be investigated. If a 5V bus is not needed, it could simplify the design of the power system tremendously.
Overall, the fewer regulators used, the better. And oh boy, these will be some regulators. You won't find linear regulators here, no sir, not at all. This will need the highest efficiency switching regulators that can be found at a reasonable price point. Will this create other design challenges (noise, increased component count, debugging, etc), probably. Despite the increased effort needed, this project isn't really freasible without switching regulators.