Solar energy is the first type of energy structure you get in the game.
To upgrade solar, go to your research center and look for the next level of solar upgrades. Upgrading increases the efficiency of your solar structures 20%. In the slider below you can see the 3 upgrades you can give to your basic solar technology.
Solar Upgrade Level 1
Solar Upgrade Level 2
Solar Upgrade Level 3
Once you get to a certain level, upgrading does not matter anymore. Now you need to make the structures you have more efficient. Placing the solar pinnacle structure doubles the production of the solar structures around it.