So inevitably, the first batteries will always prioritize daily arbitrage, and only once that market is capped out will some battery projects target weekly/monthly/yearly arbitrage.
In countries with cold winters, the obvious solution is heat-energy storage systems, which don't output electricity but instead store and output heat directly; they're basically just a big pile of sand/stones/bricks wrapped in a ton of insulation. Thanks to the cube-square law, they scale up unbelievably well and can easily store months worth of heat.
Due to that scale they don't make much sense without district heating, but energy storage is a numbers-game and lots of cold places already have district heating that could be quite easily retrofitted.
Getting to France's level of nuclear decarbonization with batteries is cheap and easy with current prices. Using existing thermal plants for a few weeks a year and renewables for the rest is quite similar overall to France's mix.
What's challenging is the final 10%, 5%, and 1%. But it will take 15-20 years of deployment of our current cheap renewables+storage technology before we need to solve those final percents. In that time, technology will have advanced tremendously and we don't know what the cheapest solution will be, just that it will be cheaper than current tech. Plus it would take much longer than 15 years to even build nuclear in any significant quantity! France said a few years ago that they would be building handful of new reactors but I still have not seen progress!
The major factor was reducing the use of gas which lowers gas prices. As a result the main beneficiaries weren't electricity users but gas users paying lower prices and saving 133 Billion.
We also have periods in the winter (so solar of little to no use), where we can have a week or two of no wind.
As the gas generators are not run constantly, they're more expensive than if they were. There are various (at least 3) UK "gridwatch" sites available, offering real time and historical generation mix. Maybe have a look.
From memory, so probably flawed, we still tend to depend upon nuclear and gas for around 40 - 50 % of our generation (nuke being low - say between 5 and 10).
Coal has gone from 32% to 0.
Gas has gone from 40% to 30%
How do you square these numbers with wind being responsible for the amount of gas burned?
A new wind turbine is built and plugged into the grid. Does this cause more gas to be burned or less?
[edit] and we might be talking at cross purpose here. I think most of the new capacity built now is to expand the production, rather than to reduce other forms of productions (in which case you might just keep around existing gas capacity if it was there, to your point).
It illustrates both the volatility of wind (which regularly goes to zero for at least a week), and how it is currently pretty much 100% offset with gas.
There are also other ways to store energy. For polar regions sand batteries are capable of storing heat for months. High grade heat to the point they can siphon off that heat for power generation.
As such they are essentially massive switching-mode PSUs, and there is no possibility of having a synchronised connection, as the AC has to be synthesised, following the local spinning iron.