Comment by hgomersall

There's always going to be excess supply at night, because demand is lower. At the moment the difference is large because battery capacity is low, but even as more batteries come online, it will stay lower because battery capacity will respond to the differential (since that's where the profit is). What's interesting is domestic battery installation is so cheap that you can capture almost all the benefits as a private buyer.

https://www.fogstar.co.uk/collections/solar-battery-storage

FWIW Good Energy are outside the price cap on their standard tariff as well. You don't have to do that many miles for the EV tariff to pay off without any additional storage.