That's assuming you're using the steady source to cover for the intermittent source when it isn't generating. If you're using the steady source to replace the intermittent source entirely (as France had been doing) then it is producing most of the time and you don't have that problem.
This is probably the largest impediment to a 100% renewable grid. You might be able to get over the solar day-night thing with batteries if you had to because you can justify a lot of batteries if they're getting used every night, but the thing where sometimes it's cloudy for a straight month is a serious issue.
Right, it's an either-or situation. And now, with renewables crashed in cost and continuing to fall, it look's like renewables are winning.
You bring up batteries, but they would not be used for long term load leveling. The optimal solution is some combination of overcapacity w. curtailment, batteries for short term leveling, and hydrogen for long term leveling. Simple cycle turbine power plants can have a construction cost maybe ~5% of the cost/kW of a nuclear power plant (CC, around 10%), so building them and letting them sit idle most of the time is not a big deal.
Not exactly, they're still complementary, just not in that specific way.
Suppose you have 100% solar generation with enough batteries to survive the night, and then it's cloudy for a month and they generate half as much power. You can suppress demand some by raising prices, but suppressing it by 50% is unreasonable. Suppose you can suppress demand by 20%. Well then you still need enough long-term storage to cover 30% of your total demand for a month, which is really expensive for something you only use once every two years.
Now suppose you have 50% nuclear and 50% solar. Immediately this gets you out of needing the day/night batteries because nuclear generates at night. Then it's cloudy for a month and you lose half the solar generating capacity, but now you're only down 25% instead of 50%. Suppress demand by 20% through pricing and you only need enough long-term storage to cover 5% of the load.
This also makes the leveling a lot less precarious. If two weeks into the cloudy month it turns out you've already burned up all your long-term storage fuel and it's still cloudy, you can lean a little harder on pricing and get to a 25% reduction even if people gripe about it some. If that happens with pure solar, pricing high enough to cut demand by the full 50% would have people rioting in the streets.
Long term storage of the kind I'm talking about has two parts: a capacity part, and a consumable part. The capacity part is the same whether you need it for an hour or a month, and would be MUCH cheaper than nuclear plants of the same capacity. The consumable part (in my example, hydrogen) would be expensive, if needed for a month. But that doesn't happen very often, so on average the cost is quite reasonable.
This is probably the largest impediment to a 100% renewable grid. You might be able to get over the solar day-night thing with batteries if you had to because you can justify a lot of batteries if they're getting used every night, but the thing where sometimes it's cloudy for a straight month is a serious issue.