Comment by ZeroGravitas

That 12 weeks almost certainly doesn't refer to what you think it refers to.

It appears to be a somewhat arbitrary notion of how long would it take the full storage to be completely depleted, if it was being partly offset by continuing renewable generation over that time.

This accounts for the most initially bizarre claim of the paper, that introducing bioenergy into the system (i.e. storage of natural gas from non-fossil sources) would increase this 12 week period to a full year:

> Interestingly, the decrease in renewable overcapacity in parallel to the increase in overall storage volume means that the period when storage is fully used, that is, the period that defines storage requirements, is prolonged to more than 1 year (10 October 1995 to 3 February 1997).

But obviously a longer period is actually better by this weird metric.

They give some more reasonable numbers of 12 days of energy storage elsewhere, which corresponds with figures given in models like this one, which suggest 13 days of power-to-X fuel would be a low cost optimum for Germany:

https://www.wartsila.com/energy/towards-100-renewable-energy...

i.e. the stored gas would if burned and used exclusively for electricity production would last 13 days as it equals 4% of the total electricity production. Of course, it wouldn't be used in that manner, but in concert with other energy sources, leading to the inflated number you quote from the paper.

And of course, an electricity system that burned 4% fossil gas would hardly be the end of the world. I personally would rather see nations do that and pay a carbon fee to let poorer nations achieve their low hanging goals than obsess about the last 4% in an unhealthy and (often seemingly intentionally) conuterproductive manner.