I think it costs around £20-£35 to fully charge a mid-large electric (ie. Tesla) from empty to full. I don't think this can be done over night on typical domestic supply (ie. not a specific ev charge point), but can maybe fully charge over a 24 hour period.
Despite our listing saying we weren't suitable for electric. Someone did arrive in a Tesla and asked to charge their car. They said they wanted just enough to give them peace of mind for their return journey. At least they asked, but they left it in for 24 hours and made no offer to cover any costs.
Some very rough maths, but here goes.... I think it's about 20-26p per KW (based on other EV point pricing). And it's roughly 1KW for 3.5 miles. This all massively depends on the vehicle but it's a guide! Therefore, to charge for 300 miles is c.£20-25.
I don't think that £5 is enough - unless it's one of the smallest electric vehicles with the 30KW battery versus the Tesla with a 100KW battery. I'm going to suggest £20 per day.