We can do a pretty good job of predicting solar and wind production; that's done routinely. What's harder is predicting how load will change when electric energy is nearly free in most daytimes and expensive on calm nights.

That does not matter. You can only postpone washing your clothes for so long and heating in northern winters is non optional. The problem are things like: https://en.wikipedia.org/wiki/Dunkelflaute

Extreme weather events are what is important and we do have the data.

Heating in particular is an especially easy problem to solve; various kinds of thermal energy storage (sensible-heat, phase-change, TCES) can store heat for later climate-control purposes several orders of magnitude more cheaply than batteries. Washing your clothes is a harder problem, although a stockpile of clean clothes is easily stored.

Predicting how willing Swedes will be 10 years from now to buy extra jeans and install phase-change energy storage in their houses, however, that's beyond anybody's ability.