The Homeowner’s Guide To Solar And Electric Cars

Last Updated: 3rd Oct 2019

By Finn Peacock – Chartered Electrical Engineer, Ex-CSIRO, Founder of SolarQuotes.com.au

solar panels and electric car charging

Almost every day, I field questions from homeowners about solar power, batteries, and how they tie in with electric vehicles (EVs).

As I’ve witnessed the growth of the solar industry in Australia over the last 10 years, one of the “final frontiers” of solar energy, in my opinion, is having a home that is fully integrated with both energy storage and electric vehicles.

So, to try and keep all of my answers about this subject in one place, I’ve put together the following guide.

If you have any further questions that you feel this guide doesn’t answer, shoot me an email here and I’ll update this article.

First things first – how many panels do you need on your home to support an electric vehicle?

(If you don’t understand the difference between a kilo-watt [kW] and a kilo-watt hour [kWh] I strongly advise reading this first)

Just like the “L per 100 km” statistic for petrol cars – the amount of litres of petrol that a car will use, on average, to travel 100km – electric cars also have a similar statistic: “kWh per km”.

This varies from EV to EV – but, on average, most electric cars will get about 6km of range from 1kWh of electricity in their battery pack.

For the average Australian driver who drives around 50km per day, an electric car will need about 8kWh of electricity to recharge what they use.

1kW of solar capacity (around 4 panels) will produce, on average per day over a year, 4kWh of electricity – less in the winter, and more in the summer.

This means you’d need to add around 2kW of solar panels to your roof to offset the charging of an electric car that is driven ~50km per day.

Charging your electric car with solar power

The simplest way to charge a car using rooftop solar panels is to plug the car into your house during the day when the sun is shining.  If the amount of solar electricity being generated is equal to or greater than the amount charging the car, no grid electricity will be needed.

However, if the amount of solar energy generated is less than what is required by the car, grid power will be used to make up the shortfall – keep this in mind for when you want to charge your car on a cloudy day with only a small solar power system!

Round trip efficiency, and why 1kWh of solar energy doesn’t equal 1kWh of EV charge

Due to inefficiencies between solar panels, inverters and the batteries in your car, you should expect charging losses of over 10%.

What this means is – if your solar panels generate 1kWh of energy, only 900Wh of that will end up as energy in an EVs battery pack.

Because of this, you may want to put on more solar panels than you think you need to compensate for these charging losses.

You don’t need battery storage to charge your electric car at night

While it is possible to charge a home battery system during the day with solar power, and then charge an electric car in the evening with this stored energy, this is very unlikely to be cost effective compared to selling your excess solar generation to the grid for a ‘feed in tariff’ and using a controlled load tariff to charge your EV from the grid.

I go into detail about the economics of battery storage and feed-in tariffs here.

From an environmental perspective, charging with off-peak electricity at night and sending solar energy into the grid during the day is an environmentally sound option, because the excess solar energy that you send into the grid directly offsets fossil-fuel generated electricity.  

I’m building a new home – should I put in single-phase or three-phase power?

If you are building a new home, consider three-phase power – this will allow you to put in a much larger solar array (over 6kW) that will be able to reliably supply the ‘home of the future’ that has high electricity needs due to a household battery and one (or more) EVs.

10kw solar power system on a roof

A house with a huge 10kW system

Home charging, and the difference between Level 1, 2 and 3 charging

Electric car chargers are divided into three levels, where ‘level 1’ is the slowest form of charging and ‘level 3’ is the fastest. 

Level 1 charging is the slowest rate that’s normally possible and uses a standard power point. This will allow around 20 kilometers of range to be added to an electric car per hour – but this can vary from vehicle to vehicle.  

So, if you park for 10 hours overnight, a ‘level 1’ charger will add around 200 kilometers of range to your car. 

Level 2 charging involves installing a specialised EV charger in your home, which can add over 40 kilometers of range to an EV per hour. There are many brands of EV chargers available, and as a ballpark figure expect to pay ~$2,000 to get one installed. 

Be warned that your EV manufacturer may require a level 2 charger to be installed where the car is kept in order for it to stay in warranty.

All in one - solar inverter and EV charger

A ‘level 2’ SolarEdge EV charger. Image credit: SolarEdge 

Level 3 charging is rapid charging using dedicated public chargers. Probably the best known example of a ‘level 3’ charger is the Tesla SuperCharger:

tesla supercharger

A ‘level 3’ Tesla SuperCharger. Image credit: Tesla

These types of chargers are most likely to be used by people traveling long distances, or those caught short of charge, as they can add around 400km of range per hour. 

An important thing to keep in mind is that not all EVs are able to fully utilise the charging capabilities of a ‘level 3’ charger. If you plan on taking your EV on long trips with the help of a ‘level 3’ charger network, you’ll want to make sure that your chosen car will be able to make use of them. 

“Range anxiety” and charging when away from home

A typical car petrol tank can be filled at a petrol station in a couple of minutes. You’re never too far from a petrol station, so being caught out low on fuel is usually something that only happens to people travelling long distances in rural areas.

In comparison, even a ‘level 3’ EV charger will take around an hour to completely charge a car (even longer if you have a larger battery pack), and far longer if you’re charging at ‘level 2’ or even ‘level 1’ rates.

The problems around the slow charging speeds of electric cars and the (current) rarity of ‘level 3’ fast EV chargers is commonly expressed as ‘range anxiety’ – the fear that your car has insufficient charge to reach its destination, or will be unable to charge fast enough to reach a destination on time.

Personally, I think that ‘range anxiety’ fears are overblown, as anyone who understands their own driving habits and has the ability to plan ahead will almost never be caught out with a flat EV battery.

Many households these days have two cars – so a solution to range anxiety can be to have a traditional petrol car for long trips or for use when an EV is flat, and the electric vehicle for shorter trips around town.

If you find yourself out and about with low charge and there’s no public ‘level 3’ rapid public charger available, caravan parks have 15 amp power points (providing ‘level 1’ charge), and you can also use three-phase sockets at some businesses and showgrounds.

Some of these locations are publicly advertised EV charging points – others are ones that you must ask nicely if you can charge your car there!

The final word

When it comes to sizing a solar power system for an electric vehicle, there’s a number of things you need to think about:

  • The make/model of an EV and its specifications
  • Average distance driven and the driving habits of those who will be using the car
  • The energy usage of your household, and whether you plan to add home battery storage or more EVs in the future.

The average Australian driver will only need about 2kW of solar power on their home to offset their daily driving consumption.

But because of factors such as household energy storage, I firmly believe that a household should install as much solar as they reasonably can. A ‘house of the future’ with a large battery and multiple EVs could easily gobble up the output of a large 10kW system.

If you’re thinking of installing solar panels for your home and adding an EV in the future, or if you already have an EV and want to add solar, SolarQuotes can help you get quotes from high-quality installers quickly and easily:

About Finn Peacock

I’m a Chartered Electrical Engineer, solar and energy efficiency nut, dad, and founder of SolarQuotes.com.au. My last “real job” was working for the CSIRO in their renewable energy division. Since 2009 over 360,000 Australians have used my site to get quotes for high quality solar from pre-vetted solar installers.

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