Solar and Electric Vehicles: Getting the System Size Right

How much electricity does EV charging use?

It depends on how much you drive, but the average Australian drives around 40 kilometres per day. A typical EV uses roughly 18 to 20 kilowatt-hours per 100 kilometres, so 40km of daily driving requires about 7 to 8 kilowatt-hours of electricity.

That's significant. The average Australian household uses about 18 to 20 kilowatt-hours per day without an EV. Add a car, and you're looking at 25 to 28 kilowatt-hours per day. That changes your system size recommendation considerably.

Can you charge entirely from solar?

In theory, yes. In practice, it depends on when you charge.

Most people plug their EV in when they get home from work, which is 5pm to 7pm, when solar panels have stopped producing. If you charge overnight on grid power, you're not saving much compared to just drawing from the grid.

The smarter approach is either to charge during the day (if you're home, or if your workplace has chargers), or to pair your solar system with a battery large enough to cover the evening charging session.

A 10kWh battery would cover roughly 50 to 55km of EV range per night. If you drive more than that daily, you'll still draw some grid power, but your bills will be dramatically lower than charging from the grid alone.

What system size do you actually need?

As a rough guide, add 2kW to your system recommendation if you have an EV you charge at home. Add more if you're a high-mileage driver or if you charge a second car.

So a household that would typically need a 6.6kW system should look at 8.5kW to 9.5kW once EV charging is factored in. This is why Sunlytics includes EV ownership in the refinement questions, and why it adjusts the system size accordingly.

What does it actually cost to charge from grid versus solar?

At Sydney's current tariff of around 34 cents per kilowatt-hour, charging 8kWh per day from the grid costs approximately $2.72 per day, or roughly $993 per year.

If that same 8kWh comes from your solar system, the cost is effectively zero during daylight hours, or the opportunity cost of not exporting it at around 5.5 cents, which is $0.44. Your annual savings from solar-charging your EV are in the range of $800 to $900, just from the car alone.

Across the country, households with solar and an EV are saving $1,500 to $2,500 per year on combined electricity and fuel costs compared to grid-charging or petrol.

Smart charging tips

Set your EV charger to run between 10am and 2pm if your car is home during the day. This is when solar production peaks and grid demand is lowest.

If you have a battery, set your car to charge from the battery between 6pm and 10pm, which covers most typical daily driving without touching grid power.

Look for EV chargers with solar integration, which can communicate with your inverter and only draw power when your panels are producing more than your home needs.