Since heating is by far the biggest consumer of energy in winter, just taking care of that with stored thermal energy will deal with a big fraction of the energy storage needs.
We need more districting heating systems.
The largest battery based energy storage in the world is 3000 MWh so we just need to build thousand of those (or a 1000x larger one). Good luck with that.
The largest heat storage under construction is 90000 MWh so that is a step in the right direction but still falls short by around 35x and even more if we have to turn that heat into electricity (While a lot of this consumption is district heating it is not all of it. And a large chunk of new district heating is just massive heat pumps)
And now that you have built these tools to make wind/solar work you have to factor the price of these into the price of wind/solar and suddenly they do not look that cheap when compared to nuclear. This is why the government and Vattenfall in Sweden want to build more nuclear.
Realistically the only cost effective energy storage on this scale we have available is hydro and pretty much all the places where it is cheap in Sweden have already been built.
In general I think Sweden is on the right track. They have a lot of wind and are building a lot more of it. They can also build more nuclear at the same time.
Though Sweden (just like here in Finland) are slowly reaching the point that when it is windy the wind farms don't really make much money as the price on the spot markets is very close to 0 so we also need a lot of new intermittent energy consumption (industrial processes that can be slowed down/ramped up cost effectively, make hydrogen, fill the heat storages with heat pumps, etc)
[1] https://www.malarenergi.se/om-malarenergi/framtidens-samhall... [2] https://www.svt.se/nyheter/lokalt/ost/de-lagrar-varme-i-berg...
For example in Stockholm you get around 6h of daylight. Malmö (can’t really get further south than that) a bit over 7h.
Basically the northern most point of mainland US is a lot further south than the most southern place in Sweden. Thinking about how good solar panels are in the winter in Anchorage gives you some idea (which has slightly slightly shorter days then Stockholm at around 5h30min at their shortest)
I don’t know the numbers for southern Sweden but for Stockholm level you get maybe 5% to 10% of the production you get in the summer.
Here in Finland we have it even worse but I know where to find the numbers.
https://www.fingrid.fi/en/electricity-market-information/sol...
Basically if we are lucky we hit 5% of a typical summer summer day for an hour maybe two. As the days are less then half as long you end up with ~1% the production vs summer.
But in reverse the panels are really good during the summer as the days are very long. This is why people bother installing solar here.