Charging an EV with home solar and battery could cut annual energy costs by 40 pct

Homeowners who charge their electric cars at home during the day from a solar panel-and-battery combination could save up to 40 per cent in annual energy costs compared to petrol car ownership, a new study suggests.

University of South Australia (UniSA) researchers say record-low prices for rooftop solar panels combined with falling battery costs can create sizeable cost savings for EV owners, compared to owners of petrol cars who don't have solar.

The team, led by smart grid engineer Professor Mahfuz Aziz, used data from South Australia where more than 40 per cent of homes have solar panels.

They compared EV charging demand, PV solar panel installation cost, battery degradation and limitations on exporting power to the grid, and energy consumption during the peak hours of 5pm-9pm, and assumed average daily power use of 17 kW/day with a daily drive of 36.7km.

"For motorists with private car spaces, home charging is the most convenient option, but for those still totally reliant on the electricity grid for their energy, the costs could mount significantly," Aziz says.

"In a basic case, all energy is imported from the grid where there are no solar panels, batteries or electric vehicles.

"When solar panels are added, about 20 per cent less energy is imported and with batteries this is reduced by around 83 per cent. When electric vehicles are added, consumed energy rises significantly, but imported energy can be reduced by around 89 per cent of total consumption.

"Our results demonstrate that households with petrol-based cars can reduce their annual energy costs by 6.71 per cent using solar panels, and by 10.38 per cent with the addition of a battery system.

"Replacing petrol-based cars with electric vehicles can reduce annual energy costs by 24 per cent and 32 per cent respectively. The most significant reduction (39.6 per cent) can be achieved with off-peak charging."

The modelling used a petrol price of $1.43 for the base scenario and for the EV scenarios did not factor in vehicle-to-home or vehicle-to-grid charging, Aziz told RenewEconomy.

It also took into account the ways EV batteries are charging in the real world when charging power is reduced progressively once it reaches 80 per cent until the battery is fully charged. It included the replacement cost of a new EV battery after eight years or 160,000km, salvage revenue of $80/kWh for EV and home batteries when they’re replaced, and export limits of a nominal 5 kW and the 1.5 KW limit for locations in South Australia which are overloaded with solar.

The UniSA team is not the only group to advocate for midday charging.

A Stanford University team found demand for energy from the grid in Western US states will actually increase by 25 per cent if charging takes place at night. Moving charging to daytime hours makes use of the region's huge capacity for solar and wind generation and will require fewer batteries.

The potential strain on electricity grids of a significant new source of power demand is a worry for regulators and infrastructure owners alike, as Australian EV drivers look to charge their vehicles at home.

But many drivers are already picking the times when a charge is cheapest. The Electric Vehicle Council and Tesla Owner's Club of Australia ran a survey in 2022 that showed a majority of Tesla owners charged during the nightly off-peak of 12am-2am, and the middle of the day (10am-2pm).

Demand for EVs is also accelerating quickly.

Data collated by The Driven shows some 40,000 EVs have been sold in Australia to date, and sales of new cars have finally breached the 2 per cent of all car sales mark.

By 2030, about half of all new cars sold in NSW alone are expected to be electric with other states set to follow this trend, according to Drive.com.

This is despite potential buyers being held back by a lack of affordable options, and unhelpful state government policies such as Victoria's EV road tax.

The federal government still doesn't have a firm policy to support EVs yet either, with submissions to its National Electric Vehicle Strategy discussion paper open until the end of October.

Supporting that boom is a new wave of demand for solar panels and, increasingly, battery storage in the wake of high petrol and electricity prices.

Solar retailers and battery manufacturers have told The Driven sister site, One Step Off The Grid, that demand for home storage systems has grown exponentially in the second quarter of the 2022 calendar year.

Victorian installer RACV Solar said in July – usually a quiet time, in mid-winter – that it had sold 85 home batteries over just three weeks, compared to 400 for the whole of 2021.

Rachel Williamson is a science and business journalist, who focuses on climate change-related health and environmental issues.