By the time much storage is being built, renewable power will be the cheapest choice, by far. So, you are worried that somebody up the line will choose to use expensive fossil-generated power instead of cheap renewable power.
Just by preferring the cheaper product, you bias your choice toward being made with renewables. It is not a guarantee, but in aggregate it is good enough.
Every single kWh that goes into making any piece of kit, whether battery, wind turbine, solar panel, e-car, or what-have-you goes onto its price tag.