Machine powering redstone when a adjacent BB/MFE/MFS reaches 70% capacity.
Similar to how electrolyzer works. It would allow, for example, to send only excess power to a mass fabricator by powering a transformer with redstone.
An alternative:
Instead of redstone power, the Machine could simply send EU. That way the storage unit would essentially always send power through one cable and if over a certain capacity, also through the cable connected to the Machine.