Can I set my electrical system up to divert surplus electricity into the car when my house isn’t using it. This is what I do with my hot water iBoost – when household energy usage is low, it automatically switches on my immersion heater. Can I rig up something similar for my car?
I’ve been thinking about this – and I’m not quite sure that it’s worth doing. Hear me out!
Here comes the science bit…
Most UK domestic solar installs can generate a maximum of 4kW. The i3’s batteries have a capacity of around 19kWh.
If you install a fast 7kW charger you’ll be able to completely fill the car in 4 hours – but you’ll never entirely max that capacity with solar juice.
So we’re back to using a standard 3 pin socket. Generally, chargers can draw a max of 3kW and take around 8 hours to fill the car. How often do solar panels supply 3kW or over?
On the best solar day I’ve had this year, I got 27kWh out of the panels – sufficient to fully charge the car from flat.
— Edent's Solar Panels (@Edent_Solar) July 9, 2015
But only around 4 hours were at or above 3kW. That’s just about enough to top up the battery by half.
On a cool but bright day, it’s just about possible to get 6-7 hours of >=3kW out of the panels.
— Edent's Solar Panels (@Edent_Solar) July 4, 2015
Which, in fairness, would usually be sufficient to completely charge the car.
From looking over my generation records, days like that are a bit of a rarity.
It would be possible to hook up a sensor, like the iBoost, and divert energy from being fed back into the mains grid. But I think it isn’t worth it.
Electricity is fungible. A Watt is a Watt. It doesn’t really make much difference whether it comes from the panels or the grid. If I generate 18kWh during the day, and I charge my car with the same amount, it doesn’t matter if those two events occur simultaneously.
In the UK, exporting electricity attracts a fixed rate payment (of around 4p/kWh) based on 50% of generation. That is, there is no export meter. So whether the electricity flows back into the grid or not, you still get paid.
In terms of cost, I pay around 13p/kWh from Ovo Energy. The total cost to fill the car is around £2.50.
- If the day is sunny enough to generate 18kWh…
- and if I can match the rate of generation to the rate of charge…
- and if I don’t need the energy for anything else…
…then my electricity meter will be at a standstill and I will be charging for free and have saved myself £2.50.
More realistically, I’ll probably be charging half my battery – so the savings are around £1.25 per charge (which won’t be every day).
So, while direct charging would be theoretically cheaper, in practice it doesn’t make enough difference to offset the cost of buying the equipment to make it work, nor is it worth the risk to the electronics of constantly stopping and starting the charging. In addition, it can be a bit inconvenient not knowing when the car will be fully charged.
Therefore, my plan is…
- If it looks to be a nice sunny day and I’m not in a rush, use the standard lead to charge up throughout the day.
- For the rest of the time, use the fast charger. Preferably around midday to offset some of the energy use.
(NB – if you’re on an Economy 7 tariff with cheaper electricity at night – it may be worth just charging overnight.)
In summary – if you have solar panels and an electric car, don’t worry too much about matching your charging time to your generation time.