This gets really complicated if you want to understand solar at scale going forward.

First European countries don’t operate reliable and independent electric grids, it’s always been more cost effective to average our demand across huge areas. At the same time, at an organizational and technical level it’s useful to split the grid into multiple different chunks that all connect with each other to various degrees. Even the US shares it’s electric grid with Canada while connecting to the Mexican grid. https://en.wikipedia.org/wiki/Eastern_Interconnection A country may hit say 99% reliably on it’s own but 99.9X% when part of a larger grid.

Next reaching 99.9+% reliability requires significant redundancy no matter the scale. Normally you subsidize some percentage of generation so it’s there when you need it rather than trying to depend on second by second spot prices to handle extreme outlier events.

In that context solar is really vastly more reliable with a little extra capacity and a great deal of geographic separation between generation. Many US examples have 30+% capacity factors which is ~90% of their theoretical maximums ~(1/pi), but importing power east/west time shifts generation. Similarly aiming panels east or west trades off total generation to get more power either earlier or later.

All of the above doesn’t help at 2AM, but it still means your personal experience with solar doesn’t map very well to grid scale generation. California for example already sees 5% of all solar power being wasted, but because power is more valuable in the morning and evening than in the middle of the day that excess isn’t a big deal.

Sure, if you can use 100% of PV output then solar costs ~2c/kWh in a good location, but that number is largely irrelevant as you notice. The question is how much excess capacity do you add. If you can only use 50% of the output your paying 4c/kWh which is roughly the cost of just the natural gas needed to operate a natural gas power plant averaged over multiple years. Put another way if you own a natural gas power plant you currently save money installing PV simply to offset the cost of natural gas though that’s likely to change as PV gets more ubiquitous.

The question then becomes if you can meet say 60% of total monthly demand with solar but doing so means you have a lot of excess capacity should you then install batteries to cover some of the rest with batteries? Looking at how prices spike at some times of the day the answer seems to be yes, it’s only a niche but it’s still viable today. Which then brings the question of do you eventually build solar simply to charge batteries? We aren’t there yet, but with the way battery prices have been dropping that’s going to happen. (And when it does most other forms of generation get serious competition.)

Of course solar + batteries doesn’t operate in a vacuum. Hydroelectric generation is ~7% of total electricity generation in most areas, but while it basically provides a fixed amount of Wh per month it has a great deal of flexibility when within the month to generate power. This combined with weather forecasting caps how much battery power you want to set aside for unusual days as it’s cheaper to store energy in existing dams and tap that when needed. Hydro in in many ways the inverse of PV.

Similarly wind is it’s own thing and complements solar + batteries quite well.

Now Nuclear seems like a good fit on the surface but costs go up as capacity factor drops. In this model it’s really competing with Hydro and at vastly higher prices because most of the time it’s output is worth very little. Japan might subsidize nuclear because it’s grid is operating individually and so has more expensive and less reliable solar power. Similarly, it might be heavily subsidized for political reasons in some counties, but in general it’s unlikely to make up a large fraction of total generation long term.