It’s free above your normal usage for that time of the day, they compare your recent activity (hence the need for a smart meter).
In my case the electricty use for the whole of Thursday last week was only about £1.50 including the 69p standing charge. I’ll not get a lot back but as they say, every little helps 🙂
Please excuse spelling/typos. Apart from being a clot it turns out I had one on my cerebellum that's now causing various problems!