Waste Heat - sswelm/KSPInterstellar GitHub Wiki
See waste heat management for more information.
Nothing is 100% efficient; much of the heat produced by a reactor is simply wasted. In space, this heat must be dissipated or it will lead it to overheating, causing ship malfunction and death. Once the Waste Heat bar reaches 95%, reactors, solar panels and microwave receivers will shut down to prevent further heat buildup. At 100%, components may explode.
This applies to all electric generating components except for the OX-STAT fixed solar panel since it cannot be retracted.