Each daily cycle saves him 80 x 11.5 x 0.14 = £ 1.29 in electricity but costs 175/400 = £0.43 in battery wear and tear. So the maximum possible saving on this scheme is around 86p per day. I would be surprised if, in real-world changing conditions, you would achieve half that.
It's even more pointless than that - as there is no way he's going to burn through £1.29 of power per day just running the 12v lights in anycase.
The average UK home uses about that much per day across all circuits so...
It would genuinely be more sensible (or less stupid??) to leave the van running, and fit a cheap inverter to provide a 230ac supply from the van to the house.. That's an equally silly but far more economic way of turning company fuel into home power than pushing it into batteries and swapping them each day.
EDIT: Since seen @Shoei 's post and the decimal point mistake. Whatever, it's still more economical to take the power direct from the van than to try and store it
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