The statement was no X in Summer, no Y in winter. It was not talking about a daily or even weekly phenomena but seasonal one.

There is no summer which have had no winds, no winters that have had no insolation or vice versa. Instead of making up numbers, you can use a site like this: https://www.solar-electric.com/learning-center/solar-insolat....

You'll find that by comparing seasonal best case to seasonal worst case it is something like, on average, 50% difference. It gets worse the closer you to get to the pole obviously.

Furthermore, surplus production does not go into dev/null. Excess resources create markets, and there will be consumers that take advantage of cheaper surplus power (smelting, heavy industrial, desalination, hydrogen production, and whatever else anybody cooks up).

> There is no summer which have had no winds, no winters that have had no insolation or vice versa.

The part of "no production" were not meant to be taken literally. If you want a more precise statement then let me put it this way:

For PV in central europe even an overcapacity factor of 5 would be insufficient to cover the difference between summer and winter. Germany's PV plants produced 7.3TWh during June 2019 and 0.58TWh during december 2018.

Wind does look a bit better as far as seasonal differences go (about 4-5x) but it suffers from more short-term variability, so you might need less seasonal storage for it but it would require more medium-term (multi-day or -week) storage compared to solar which is a bit less variable during its peak months compared to wind's peak months.

Building out the grid will help out a lot. Put those solar panels in Portugal and Spain and the difference between winter and summer is much less. I ran some numbers recently for Portugal's grid and was surprised to find out that with the current seasonality of solar, wind and hydro we have on the grid, we could scale up solar and wind to have 100% renewables with very little overcapacity and we could keep going and produce twice our current needs while exporting about the same amount of energy every month of the year. Solar and wind's seasonality match up quite well and hydro gives great flexibility.

24 hours worth of storage and 2x the wind turbines would smooth out the lack of supply in the graph you cited. A 5x seasonal variation would be more serious. I guess that exists at the poles with solar, but that’s getting into a small total percentage of global energy demand (and they could probably still use wind, unless snow prevented it).

24 hours worth of storage worldwide is 60 TWh currently. Once Asia and Africa become more developed this will likely become closer to 100 TWh. By comparison annual global battery production is just under 0.3 TWh - and only a small fraction of this goes to grid storage, the bulk goes to electronics and electric vehicles.

This leaves experimental solutions like the Sabatier process, Hydrogen storage, or more exotic things like lifting concrete cylinders with pulleys.