Low sun angles result in reduced solar energy because:

Solar energy is a form of renewable energy that is harnessed using the technology of the photovoltaic cells. These cells are composed of semiconductors. When the light from the sun hits the semiconductor, the energy from the light particles, or photons, cause the electrons in the semiconductor to be 'excited'. When the electron moves across the semiconductor platforms, the electron flows, thus creating electricity.

Therefore, as much as possible, you want to capture all of the sun's light. This can be achieved when the Sun is high up the sky. But if there is low sun angle, then the concentration of the sun's energy that strike the PV cells would be lesser.