Renewable generation is variable because of the resources it relies on. In California, a photovoltaic system will produce the most electricity in spring through fall when sunlight hours are the longest and the sun is positioned higher in the sky. It will produce less in the winter months or when the sun is obstructed by poor weather.
It will stop generating all together at night. Because of this, you need backup generation to provide power when your system is under-generating, or to absorb power when your system is over-generating. If sized correctly, your system can generate all the energy your property needs over the course of a year.