> This sounds like enough grid scale storage/dispatch to do 100% renewables.

I might cover short-term variability but not seasonal differences. In Germany PV generation varies by 5x between summer and winter and in the Nordics it's obviously even more extreme. Unlike some other locations peak consumption also doesn't coincide with peak production since more energy is needed for heating during winter than for ACs during summer.

Hydro and wind have the opposite seasonal variability to solar. And since some hydro allows shifting energy during the day even in summer it may be enough for a mix that solves both seasonal and daily/weekly variability. Build enough wind and solar to cover the total energy needs evenly during the year and use those 6+ days of hydro to shift energy during the day/week to solve the unreliability. Maybe 6 days of hydro is not enough for that but it's probably not 60 days either.

Additionally we will need to live with a supply constrained electricity net. Sometimes there just isn't more available and variable pricing will be needed to reduce demand. Of course the Texas power crisis showed that even if prices rise 50x some people will stubbornly keep using power regardless and basically go bankrupt. But perhaps this is just because nobody is used to actually matching demand to supply vs the current situation where supply is always matched to the demand at all costs with (rolling) blackouts as the only effective measure to reduce demand.

I imagine that in a near future where most cars are electric (and thus transportation is a significant part of electricity demand) it should be quite easy to temporarily reduce demand, even in winter: just stop commuting for a few days (at least all office workers). Also big industrial consumers could be incentivized to reduce demand when supply is too low. It might require some design adjustments, but shutting down most industry and working from home for say 0-5 days a year is surely more cost effective than massively overbuilding renewable energy supply, making giant batteries.

Priorities should probably be 1. More grid interconnections (even out supply variability), 2. cheap long term storage like hydro, 3. demand reduction. I think we need all three to transition to a 100% fossil free future.

> Hydro and wind have the opposite seasonal variability to solar.

There isn't much growth potential for hydro since it's more dependent on geography than the others and many sites are already occupied. Short of mega-projects trying to exploit ocean currents.

> Build enough wind and solar to cover the total energy needs evenly during the year

That means you need to overbuild PV and wind independently, which may be an option but a costly one. And you'd still need more storage, 6 days are too short, there are lulls that last longer than that.

If seasonal storage can be built then it would complement both sources and could decrease the required overbuild factor. So it's still good idea to pursue it to reduce cost and reliance on a single source type.

> There isn't much growth potential for hydro

But there's significant potential in wind, particularly offshore.

> That means you need to overbuild PV and wind independently, which may be an option but a costly one.

Quite the opposite. Exactly because wind and solar have opposite seasonality you need to overbuild less in total with the sum of both than you would independently. See the spreadsheet. The 100% renewable scenario only requires 15% overbuild in total and it's all in solar which is now the cheapest electricity source ever. It also maintains net exports to the rest of Europe over winter which is probably when those exports are most valuable.

> And you'd still need more storage, 6 days are too short, there are lulls that last longer than that.

Even when in a "lull" the sources aren't at 0. 6 days of full load fills gaps much larger than 6 days.

> If seasonal storage can be built then it would complement both sources and could decrease the required overbuild factor.

It would have to be very cheap to be competitive with only 15% of overbuild, particularly of solar. I don't think any of the existing or proposed technologies come even close.

> Hydro and wind have the opposite seasonal variability to solar.

What? North eastern India (e.g. Leh) has hydro only in summer as the snowpack melts. In winter it can be -30 C with limited solar resource.