What use is a battery in winter when there is barely any solar?

In winter a battery's bigger job is usually price arbitrage on a smart or night tariff rather than storing daytime solar. You charge it from the grid during the cheap overnight window — where night-rate units can be a fraction of the day rate — and run the house off it through the expensive daytime and evening hours. That works even on dark winter days when the panels make very little, which is exactly when a no-battery home is buying most of its power at the full day rate. Whether it pays comes down to your tariff spread and how much you use in those costly hours, so it suits higher-usage homes, EV owners and electric heating far more than a very low-usage household, where payback can stretch beyond ten years. It is a genuine judgement call, not a must-have.