How do you write a topic sentence that supports solar energy?

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.