[Matthyze]

I'll give a somewhat simpler answer than Filligree has. The problem with renewable energy sources is that they are typically both highly variable and not dispatchable (i.e., controllable). The former leads to supply peaks that can exceed transmission capacity or supply lows that require compensating generation elsewhere. The latter means that energy generation can easily be increased or decreased as required, which is of course very helpful for grid management. Dispatchable generation can be increased if supply requires it, or multiple dispatchable generators can be 'redispatched' to relieve congestion in a part of the power grid (by rebalancing generation). Power from renewable sources can be decreased through curtailment, but that wastes the generated energy.

[ExoticPearTree]

The "problem" with renewables is that they are not dispatch-able, meaning you will never be 100% sure that tomorrow or a week for now there will be sun or wind in a specific region. On the other hand, a nuclear or a coal powered plant, know months in advance if they will be available for peak load or not.

Yes, if it is sunny or windy, they can be scaled in minutes, but only if conditions are met. The inverse is true for nuclear/coal - they cannot be scaled up & down in minutes.

[rich_sasha]

Yeah, hence I mention batteries specifically in my question. I guess a renewable grid will need a lot of batteries. And presumably these are highly dispatchable?

[pjc50]

Batteries are the ultimate in dispatchable. Provided they're charged.