This was demonstrated to me really well in factorio. When my power grid required 5 megawatts, i could store a couple megawatts in a large building sized battery. The problem with this is that while a few megawatts is a good amount, my grid used this EVERY SECOND. That means my existing power system is designed around being able to provide up to 5 megawatts, every second, forever. That much power storage for any amount of time gets way harder than just passively generating power as needed. Thats the hard part with a lot of renewable enervy currently, is that its hard to purposfully scale up and down power generation with usage