I don't know the calculation either, and have been trying to get hard numbers on various potential large scale storage such as https://en.wikipedia.org/wiki/Thermal_energy_storage and Lithium ion, as well as some sort of learning curve for costs. So far I've found a pretty compelling and well backed case for solar, but the storage component is always hand waved.

Perhaps this is hard to calculate because it's unbounded?

Typically:

Known costs for (anything) < ( some storage cost * forever )

Maybe there's something wrong with my math?

Well, sure, storage costs depend in part on how long you have to store the energy, and how often. But surely someone has computed some kind of curve about how much storage is needed for PV to displace increasingly large percentages of peaker and baseload generation, assuming no demand response or other stabilization measures like rolling blackouts?

I hope you aren't making the assumption that storage = batteries, then arguing because weeks and months of batteries would be needed, it's impossibly expensive.

It would be very interesting to see what kind of storage capacity these agreements specify