A battery shifts midday surplus into the evening, which is when many EVs are actually plugged in. For commuters, that shift is the difference between exporting most generation and driving on it.
The trade-off is capital cost against the value of the units you time-shift, minus round-trip losses of roughly 10 percent. Households on a wide off-peak tariff spread sometimes find that cheap grid charging already covers the evening need at lower cost than storage.
Sizing matters too. An oversized battery rarely fills on UK winter days; an undersized one saturates by early afternoon in June. Matching capacity to your typical daily surplus is more useful than buying the biggest unit available.