You do the very same things that electric grids has been successfully doing for the last 40 years:
- Exchange electricity with other network on national and international level to balance out the load.
- Modulate big industrial loads. It's standard practice in many countries to have fine-grained tariffs for large consumers that are willing to adapt to fluctuating energy availability and prices.
- Modulate residential consumer loads around house and water heating. Many countries have been doing day/night tariffs for decades and people store energy as heat using boilers and storage heaters.
- Store energy using pumped hydro & similar
And now we can also start modulating consumer loads more precisely using smart appliances and charging EVs at the best time of the day.
Also, despite the FUD around wind/solar the reality is that:
- currently most countries have excess energy during the night rather than not enough. In general, using more solar would help rather that create a problem!
- solar+wind combined are not going to drop down to 30% for a whole day or week in a whole continent, only on a local level. You can find detailed statistics online.
- solar is very cheap and can be overscaled to compensate for low production times
- many industrial loads can be made more flexible if that means cheaper electricity on average