A 10A – 2.45kW Granny Charger takes about 3x longer than the usual 7kW wall box.
Lets say you have a larger battery, about 77kWh, usually you’d keep it charged between 20 and 80% (unless you were off on a long journey). That’s about 60% of its total capacity so 77 x .6 = 46kWh.
A 46kWh weekly charge at 2.45kWh would take around 18 hours. If you have Octopus Intelligent tariff (11:30pm to 6:30am) that’s 3 nights a week at only 7.5p per kWh so £3.45
Even if you charged it during the day at 26p per kWh it’s only £12. At a running cost of about 3.7 miles per kWh that’s 170 miles for £12 compared to almost 5 gallons of petrol at £35. Charging often overnight at the cheaper rate makes it about 1/10th of the cost of petrol.
Please excuse spelling/typos. Apart from being a clot it turns out I had one on my cerebellum that's now causing various problems!