It is a fact that we get heat from the sun, and it is a fact that that heat makes it possible for there to be life on earth. However, it is impossible to not get heat from the sun. We can’t stop that process. So not getting heat from the sun is an irrealis situation — something that goes against reality. Therefore it is expressed with a second conditional.
Zero and first conditionals are used for things that do or can happen. Sometimes both zero and first conditionals are possible; it’s mainly a matter of the speaker expressing a fact or a prediction.
In the case of getting heat from the sun, we can’t stop the sun from heating the whole earth. But we can make the sun shine on a particular thing (a tank of water, a window, or a solar panel, for example), and we can likewise prevent the sun from shining on those things, so those actions can be expressed in various ways. We choose which conditional to use based on whether we want to make a statement of fact or a prediction. If we think the predicted action is less likely or unlikely to happen, we use a second conditional.
If you install solar panels, you save money on electricity.
If you install solar panels, you will save money on electricity.
If you installed solar panels, you would save money on electricity.