I keep hearing this. How many twh do you think you need and why? And how much are you willing to pay for it? The sun comes out every day. On a bad day there are clouds and your panels are less effective but still produce power (except in the arctic regions). Wind blows every day somewhere but not necessarily everywhere. Well known facts. However, continent wide black clouds without any wind whatsoever is pretty rare and unlikely to last very long if it happens at all. So instead of storing energy locally, you can absorb a lot of this by moving energy around via wires.

So, you need batteries locally for fluctuations and you can cross connect sites with wires so you can import excess energy from elsewhere. Conveniently when it is cloudy, it's usually also pretty windy. Global warming is oddly helping here by creating even more wind.

So, a combination of a some over capacity, some wires for moving energy around, and some battery for short term fluctuations would be all you need. If it's not enough on the worst days, just add more windmills/solar/batteries. At the rate prices are dropping installing 2-3x the needed capacity would still be cheaper than keeping gas plants around. Those prices are projected to continue to drop of course making the whole thing only even more affordable.