What is the earth's primary source of light energy?

The sun. The reason being is that there is no other star close enough to us to be able to transmit enough energy to power our earth. The sun is not a very large star, or very bright, but it is close, which allows it to transfer more energy than other stars.
example: If I put my food in a microwave, compared to right outside the door. Some energy is bound to escape, but how much would, to cook my food outside of the microwave? Very little.

The sun