There are very many ways energy can be sustainable and reliable at the same time.
Firstly, you can observe the fact that there is no renewable energy. Most so-called "renewables" (a misnomer) actually use fusion energy from the sun, although some less common "renewables" like geothermal power or tidal power use other finite sources of ...
If you want to include a means of energy storage within the system then yes.
Batteries can be used to store electrical energy for when the sun doesn't shine and when the wind doesn't blow, when the tide is not moving in tidal systems or when waves are flat in wave energy system.
Solar or wind powered pumps can be used to return water to hydroelectric dams.