I have just joined OVO anytime as I recently bought a hybrid car. The battery capacity is 25.9 kw but I noticed the savings on the app were being reflected to around 18kw and on days I charged the car the daily energy usage costs were significantly higher than days I did not charge.
I called OVO and they had the wrong capacity stored on the system at 17.9kw. I supplied confirmation that my model had a 25.9kw gross battery capacity and 21.9kw net usable battery - they said the latter was the figure they should be using for calculating the discounted tariff.
This is incorrect as the previous battery on older models of my car was 17.9kw gross and 14.1kw net usable.
Maybe worth checking that OVO have the correct capacity stored for your vehicle so you are also not being over charged.
Not a great first impression of this company and still awaiting them to sort out the issue