Energy is required to make your mines and synthesizers work. Energy is produced by Solar Plants, Fusion Reactors and Solar Satellites. It is displayed in the top frame of the Interface. The far right resource shown is Energy displayed as numbers x/y. Y would be the combined output of your Solar Plant, Fusion Reactor and Solar Satellites. x is the remaining unused (excess) power.
Energy is also required to research Graviton Technology and to upgrade the Terraformer and Space Dock. Energy costs only require the output of your solar plant, fusion reactor, and solar satellites to be at least that much to start the operation, and do not interfere with the energy that your mines and synthesizers are using.
If the available energy is negative (red), then your mines will not produce at full capacity. By going to the Resources Screen you can see Production Factor above the table of building outputs. If this is 1, all mines are operating at maximum efficiency. If the Production Factor is less, then the mines are not producing as much as they could. For example, if production factor is .5, then your mines are getting half the power they need and are producing 50% their maximum output.
The best way to fix a low production factor is to build a higher level solar plant, a higher level fusion plant, or more solar satellites. Also, you can lower the production percentage of one or two of your mines so that the remaining ones can continue to produce at maximum capacity.