Want to only use a power source when your accumulator drops below a certain percentage? Well now you can!
Simply set the L (lower level) constant combinator and U (upper level) constant combinator to your thresholds and paste this puppy in there. Your reserve power connects to the right-most pole and your main power to the left substation.
Intended for use for large solar/accumulator arrays. When accumulator power drops below %L it will enable your reserve power (steam/nuclear) until it reaches %U