Charging an Electric Car with Solar Power

Can I Charge my EV with SOLAR power from my PV Array?

Installing a solar PV array when you own an electric car is a great idea. If you do then you could legitimately say that your EV is, at least in part, “Solar Powered” and that is quite a claim.

However, getting the most from a solar PV installation when you also own an EV is not quite as obvious as it might sound. Your first and perhaps obvious thought might be that it would be best to charge the EV using the solar generated power when the sun is shining and whilst this might well be true it is not always so.

There are a lot of factors that must be considered to determine when you should charge the car to get the best from your solar PV system and to reduce your costs. You must consider your own priorities as what is most important to you might be less important to others and so when you decide to charge your car may be at different times to someone else.

These are some of the factors involved:

Saving CO2 and Emissions

If saving CO2 and emissions is your priority then it is always better to charge your car at night even with solar.

To understand why this might be so it is important to realise that daytime electricity generation generally creates more CO2 than at night. In the day the demand is higher and so the dirtier forms of generation are needed, such as coal and gas, to satisfy demand. At night the demand is much less and so they can shut down those dirty power stations and leave running the cleaner ones of nuclear and renewables.

So, if your priority is to reduce CO2 and you are less concerned about cost you are better not using your own solar generated power to charge the car. Instead, allow your clean solar power to go out to the grid and be used by someone else there by reducing the amount of the less clean daytime power that needs to be generated.

This may seem rather perverse and counter-intuitive at first but by always allowing your clean, solar power to go into the grid you then allow someone else who would have used the less-clean, daytime power, to use clean solar-generated power. This only works if you can time-shift your own use of power to the cleaner time overnight and this is easy with an electric car as you probably charge overnight anyway.

The more solar power you can export to the grid and the more of your own electricity use you can shift to the night-time the greater the impact your solar PV generation will have on CO2 and emissions and for this reason it is often considered that the best arrangement for CO2 and emissions reduction is to have a Solar PV array combined with a Dual-Rate or Economy 7 tariff and always charging your EV at night. This is not always the best arrangement for many as priorities and lifestyles vary.

Saving Money

If saving money is your priority then it is becomes quite complicated.

Factors such as:

  • Do you normally charge at night?
  • Are you on a Dual-Rate tariff (Economy 7)?
  • How much power are you generating?
  • How fast are you charging (6A, 10A, 16A, 32A)?
  • Is the sunshine continuous or are there some clouds about?

All these factors combine to determine the cost of charging in the daytime and so calculating how to best benefit from your solar generation is a difficult task especially as the solar PV output can vary considerably from minute to minute as the clouds roll across the sky… the situation can be constantly changing for the better or the worse!

However, if you are not on dual-rate or Economy 7 then it will always be cheaper to charge in the day if you have surplus power. Rather than exporting to the grid plug in the car and use it yourself there by reducing your electricity costs.

If you are dual-rate or Economy 7 then the calculation is complicated as you need to determine at what point it becomes cheaper to charge in the day over the reduced costs of charging at night. This is a complicated calculation and as the solar output can vary so much, not just from day to day, but from minute to minute, it can be almost impossible to determine, but as you have cheap rate electricity at night anyway why not just charge the car at night regardless. It might cost you a little more than in the day if it is sunny and a little less if it is cloudy. Either way you are benefiting the grid by making it cleaner because your surplus will go out to the grid reducing CO2 and making the grid cleaner.

Charging Strategies with Solar

As you can see, if your priority is reduction of CO2 then the best strategy is to always charge at night.

If you are looking to save money on your electricity bill then if you do not have a night time dual rate tariff always charge in the day if you have surplus solar generated power. Otherwise charge at night.

If you have a dual rate tariff then if you do not want the complications of calculating the switch over point at which it becomes cheaper to charge in the day then just charge at night and accept that overall that is the better policy.

As you can see, in most cases, it is nearly always better to charge your electric car at night and so if you are unsure and you have the option – charge at night.