Fortunately we've already got ~5GW+; but cold still winter days are tending to affect large areas at the same time for extended periods of time; those are the days when everyone is at peak usage and has ~no solar. The last time the UK power grid had to raise a warning (a few years back) was a cold still February when the whole continent was low on wind generation, and you could see that a couple of weeks back where we were running coal generation and exporting to France (well the EU grid) at the same time.