So you've got an issue, you generate a bunch of energy that's not being used, so it's thrown away. That means the effective cost of an average kwh of solar electricity goes up a lot because you're not just paying for when you're using it, you're also paying (not really, but in a way when you average it) for electricity that's generated and then wasted. i.e. if I build a machine that produces energy for $1 per unit on average, but half my units are generated when I'm not there (and wasted), my actual average cost per unit of energy is $2. The $1 may be competitive with non-renewables, but because half is wasted, the actual $2 price is not.
But of course you don't have to waste it, you can store it, but then you need large amounts of storage and that's not easy or cheap. Take the Tesla powerwall for example, roughly has the capacity to run 1 clothes washer + dryer cycle, and it costs $3k (and doesn't last forever, either). Paying for battery storage for everyone (for when renewables are a big part of total energy production), would put the cost in my hypothetical example somewhere between $1 and $2.
For the actual data (rather than my silly numbers above) you're better off using google, there are a ton of studies that look at the kwh premium when running storage in different contexts. A quick back of envelope calculation for example for home-consumer storage prices... the above powerwall, $3k, lifetime of around 3500 cycles, at 7 kwh capacity, you're talking about the ability to store and use about 24.5k kwh of electricity at a cost of $3k before the device dies, or 12c per kwh. Now 12c per kwh is roughly the average price of a kwh in the US, so a powerwall essentially doubles the cost of of any unit of electricity you want store (the excess power solar generates around noon which can become very significant if we increase renewables a lot). Doubling in price for those kwh is massive, on average it makes little economic sense. In Hawaii, it makes total sense because prices of electricity are much higher. But this is a consumer solution, utility scale is cheaper, and some things can be much, much cheaper (like pumped hydro, way cheaper than a powerwall in general, and way way cheaper if you've got the natural resources like Norway does to build cheap pumped hydro capacity).
So why not turn off the power plants (e.g. coal/natgas) when solar production is high at noon? Because these plants can take 24-48 hours to startup, they provide base power and you can't just shut them on and off.
So yeah, storage is essential and it's not quite there yet. Battery tech has always lagged behind, and now we need it at a massive scale.