I'm not sure how accurate these figures are. That £13.75 for off peak, off street charging looks very cheap to me!
Glad to hear you've got your Mini Bob, hope you enjoy it as much as I do mine!
Apart from cost structure, the amount also depends of course on how much charge you're putting in every night.
I expect that we will see a decline in cheap overnight offers as more and more people want to charge and the demand rises. The only one offering a really cheap rate (Octopus) only offer it for 4 hours overnight so not enough time to get a large amount of charge into a battery even at 7.5KW.
I think it's best to do all the cost calculations based on charging at peak rates. If it makes sense then, it'll make sense any time. I'm paying a relatively eye-watering 40p/KWh with Ecotricity which is a flat rate regardless of time. Principles cost!
As for charge amount, I drive relatively economically, so I get around 4.2miles/KWh and I allow for about 15% losses in the charging process, so getting 1KWh into the battery means I take 1.15KWh from the grid.
So this £13.75 a month for me would mean 13.75/.40 = 34.37 Kwh / 1.15 = 29KWh into the car x 4.2 = 125 miles of range added per month - nowhere near enough me, at a cost of around 11p/mile. The bonus for me of course is that I can charge from my solar panels a lot of the time with spare power that would otherwise go to the grid and earn me nothing, so it's free motoring for much of the year..
In the 5 months I've owned my car I've done around 1700 miles, so about 340 miles a month and I don't use it for commuting to work, so around 3 times as much as this model would indicate. With average mileages of around 8k/year or c.650 miles a month people are going to be putting more like 180KWh/month into their batteries, more if they drive with a heavy foot.
But even as electricity prices rise, we can expect the costs of running ICE to increase faster as they introduce more and more disincentives to their use. An efficient ICE car doing an average of 45mpg with diesel at £1.80 or £8.28 a gallon costs 18p/mile, and we can expect that difference to widen further over the next few years.
