Solar Charging Electric Cars


Regular readers will know that I have solar panels and an electric car. Is there any way to hook the panels up so they directly charge the car?

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.

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.

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.

So...

  • 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...

  1. 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.
  2. 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.

8 thoughts on “Solar Charging Electric Cars

  1. Great breakdown, easy to understand and very informative. Being an electric car owner and also having solar panels, I have come to much the same conclusion as you. I charge my Leaf during the day in the summer when it's convenient, I have a 2.6 kWp system so I never reach the level needed for direct feed to the car but after 4 years I can tell it makes a difference.
    ! do have an off peak tariff, 5.3p kWh (with Ecotricity) so I generally charge at night.
    More recently I leased a Tesla Model S and my panels are woefully inadequate to deal with that, although in July this year I charged it from a 13amp socket (it would take about 40 hours to fill it) just for fun.
    A friend in Australia has 50 kWh battery system and a 15 kWp solar array on his rather large house in Adelaide, and 2 Model S' and a Roadster and an i3 in his spacious and spotless garage.
    He hasn't paid for electricity in 3 years. It's not fair.
    One day.

    1. Unfortunately for me the question was not answered. The excess from the solar panels is free and so a solar pv diverter can divert the excess at little extra cost as many pv owners already have the diverter so not much extra equipment required.

      I use my EV for say 4000mls/yr and therefore need less than 1000kwh/yr and this is very doable. I already divert 1000 kwh/yr to Hot water but charging a EV is a much better use as this is the equivalent of £400 diesel and loads of pollution.

      The ethos of the answer is also disappointing because on that basis its difficult to justify anything but believe me every little does help and adds up to a big. And when we get to V2G it really will be big.

  2. Useful, thanks, I've been puzzling over this. However maybe now we need to factor in that there's such a c*** up of energy policy that we cannot forever rely on the power grid. So is it going to be feasible to keep an EV on the road, say just for local errands, on a domestic PV array alone?

  3. I'm just in the process of having solar panels fitted mainly for my i3 but at least now I understand the expectations better so won't be disappointed. Cheers

  4. Great analysis. I have a 3kw system which provides 80% of my electricity. I don't have an electric car -yet, but I am planning on one. Given your analys, I will most likely have to roughly double my solar array size if I were to get an electric car and want to charge it with my solar panels.

    I appreciate your work, and you sharing it.

  5. Hi, just wondering will a solar diverted work with charging an electric car? We have a 4kwp solar system and a solar diverted that goes to the hot water cyclinder, it has a second output could i in theory run this through an evse into the car? Has anyone tried this? I'm thinking it may not be as straight forward as a simple resistive load such as an element because of the electric components in the evse and the cars on board charger before the power reaches the battery. Would really like an answer , thanks

    1. As my post says - it probably isn't worth it. Just treat energy as a fungible commodity and don't worry about it.

  6. Here in the U.S. I have a 3.2 kW PV system and am considering a BMW i3 to purchase that has a direct DC charge port. My goal isn't necessarily just to save money, as my PV already produces 110% of what I use. I would like to use the EV battery as an emergency back-up battery. The DC component could redirect to the vehicle when there is a power outage, which currently automatically turns off the DC-AC inverter. Then, a separate, small 1.8kW inverter could run off the DC-DC converter in the vehicle, powering critical loads in the house. This is a simplified version of V2H, which is used by Japan's version of Nissan Leaf after the Fukushima disaster. The vehicle manufacturers (except for Nissan) are slow to use the vehicle battery for anything other than powering the vehicle.

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