The big problem with home charging an electric car on off-grid power

South African households who want to charge their electric vehicle (EV) overnight at an off-grid home will have to spend upwards of R225,000 on their backup power system.

Driving an EV can be a bonus for those with solar installations, as they can refill their car with self-generated power.

EVs are powered by large battery packs, which can store more electricity than the average South African household consumes in a day.

While precise capacities vary, most cars with several hundred kilometres of range have packs between 40kWh and 80kWh.

MyBroadband has been testing the Volvo XC40 Recharge Twin for several months. It comes with a 75kWh pack.

Our driving during this period has resulted in a realistic range of 340km on a single charge.

This is based on an average energy consumption rate of 22kWh/100km, which we recorded when regularly using the aircon and primarily driving in town with some highway driving.

According to Numbeo, the average South African motorist has a daily return commute of 44km.

On the Volvo XC40 Recharge Twin’s consumption rate, the average commute would consume about 9.68kWh of energy from the battery pack.

The roughly 100km return trip between the popular suburban area of Pretoria East and Johannesburg’s business hub of Sandton would consume just about 22kWh.

Volvo XC40 Recharge Twin topping up at public charging station.

According to Eskom, the average South African household consumes about 30kWh of electricity daily.

Solar installer Solar Advice previously told MyBroadband that the typical household consumed 60% of this during the night.

Because there is no sunshine during this period, off-grid homes must use their stored battery energy when power is required.

Therefore, the average South African household consumes roughly 18kWh of electricity overnight.

Powering the average home and keeping an electric car charged for the average commute of 44km would require at least 27.68kWh of capacity.

That is more than two Tesla Powerwalls would provide.

In addition to the high home battery capacity, you will need a powerful inverter to support the typical 7kW charging speed.

Grid-tied homes can get away with a 5kW inverter because they can keep non-essential loads on Eskom’s supply.

However, off-grid homes might need an 8kW inverter because they must be able to support all your appliances.

If you want to use your other home appliances as usual while your car is charging, you would need to add an additional 7kW capacity for a total of 15kW.

The table below shows the estimated cost of solar power systems that would be sufficient to meet these specifications.

A house still connected to the grid is unlikely to need to tap into its battery to recharge an EV overnight.

Even under stage 6 load-shedding, households in cities with the longest load-shedding will only be without power for up to six hours over a 12-hour cycle.

However, this negates the cost benefit of using your own solar energy to run your EV, as you will be paying regular tariffs for every kWh of electricity pumped into the car’s battery.

When an EV makes sense

It should be emphasised that the calculations above assume the EV owner wants to start at full charge every day.

For the average South African commuter, it won’t be necessary to do so to travel between home and the office.

In fact, with a 44km commute, you could wait until the weekend to charge the car during the daytime hours when the sun is out.

However, there is no guarantee that you won’t have overcast weather.

In addition, you won’t be able to use the car for several hours if you want it to reach its maximum capacity on 7kW charging.

If your home is grid-tied and you only want to have the ability to charge your electric car for most of the time when there is no grid power while still having enough energy to power the house, your situation is slightly different.

Under stage 6 load-shedding, the on-grid 44km commuter would need a battery capacity of 18.68kW to fully charge the EV and have enough backup power to cover the power outages.

This is because the minimum six hours of power available from Eskom will provide up to 9kWh capacity for average usage.

For the 100km commuter, a 31kWh battery capacity will ensure sufficient backup energy to get the EV to full and cover the home with backup power during load-shedding.

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