A topic sentence needs to tell the reader what you are going to talk about in your paragraph. As the link below says, a topic sentence is
a general sentence that provides a very broad, sneak peek into the specifics of your paragraph.
This means that you have to decide why solar energy is good before you write your topic sentence. So, first you need to know what your argument will be.
For example, if you want to talk about environmental reasons for using solar power, you should say something like "As global climate change becomes a bigger and bigger problem, it becomes more important to support the use of solar energy."
Alternatively, you might want to talk about energy security. You might say "Because much of the fossil fuels we use has to be imported, it is important for us to develop domestic sources of energy such as solar power."
These sentences give the "broad sneak peek" mentioned in the link.