The input/output power/voltage for voltage regulators was named
inconsistently as U_\mathrm{in}, U_\mathrm{out}, U_\mathrm{zu},
U_\mathrm{ab}, P_\mathrm{zu}, P_\mathrm{ab}. Use `in` and `out`
as unified subscript.
We do not need the equation environment. It introduces the equation
numbering, which we then disable by `\nonumber`. We just need the
split-environment.