The problem with home solar and electric cars

Electric vehicles (EVs) can offer an affordable means of transport — bar the high cost of buying an electric car — when travelling short distances and charging at home.

However, as with most emerging technologies, they present their own unique set of challenges. For example, those with a solar system at home may need to upgrade their equipment significantly to meet the demands of EV wall chargers.

I recently took Volvo’s XC40 Recharge Twin on a trip from Johannesburg to Durban and back and soon realised that charging at home is easily the most cost-effective way of topping up an EV’s battery.

For those with solar, day-to-day charging costs are effectively zero. However, this is assuming the homeowner has sufficient battery and inverter capacity.

Volvo’s “wallbox” EV chargers provide up to 7kW of charging capacity for those with a single-phase system and up to 11kW for a three-phase system.

Typical inverter sizes in South Africa can range from 3kW to 11kW. While the latter would be sufficient to manage your household usage in addition to the draw from the 7kW charger, it wouldn’t be enough for the 11kW charger.

For homes with lower-capacity inverters, they will likely have to double up to meet the requirements of their household usage and the 7kW charger’s draw.

For those with three-phase power, 16kW of inverter capacity — or two 8kW units — will likely cover your household electricity draw and the charger’s 11kW of constant electricity usage.

This assumes the solar panels installed produce more than 7kW and 11kW during the day, depending on the charger.

It should be noted that most wall chargers have adjustable charging rates. However, even with the 11kW unit at full power, it would take more than seven hours to charge the battery completely.

This is more of an issue for those with single-phase power, as the 7kW charger would take more than 11 hours to charge the 78kWh battery.

Charging the Volvo XC40 Recharge Twin through solar power at my Durban destination

If the car is charged overnight, this presents another challenge — backup battery capacity. Solar generation falls away at night, meaning the inverter will start to pull power from battery storage to charge the car.

This means a Volvo XC40 Recharge Twin owner would need more than 78kWh of battery capacity to charge the car from zero to 100% overnight and still manage their typical household electricity usage.

However, most electric cars allow you to set an upper limit for charging, which could be useful for those with short daily commutes.

EV owners can still charge their cars using a charging cable that plugs directly into a 230V wall outlet, although this process is very slow.

Volvo provided a 2.3kW charging cable, which takes over 34 hours to charge the battery from zero to 100%.

This is because charging is significantly slower than the rated output from 80% to 100%. According to Octopus Electroverse, this helps protect the car’s battery health.

Based on my experience, it takes around 37 hours to charge the Volvo XC40 Recharge Twin to full using the 2.3kW charging cable.

Charging at public chargers is expensive

Even those without solar power installations will save money by charging their electric car at home.

This is because the chargers installed on major routes and at points of interest, such as malls in South Africa, charge R7.35 per kilowatt-hour (kWh).

Therefore, charging the 78kWh battery from zero to 100% using a public charger costs around R573.

The Volvo I tested averaged 25.4kWh/100km — equivalent to roughly a third of the car’s battery capacity per 100km travelled. This works out to a range of approximately 312km.

With these figures, the cost of charging an EV at such charging points works out to around R1.84 per km.

Around town, the car averaged around 17kWh/100km, giving it a range of about 450km and bringing the cost of charging at a public charger down to R1.27 per km.

However, EV owners can charge their cars at home in the evenings, significantly reducing the price per kilometre.

For example, the City Power’s electricity tariff in Johannesburg is R2.41 per kWh for homeowners who use more than 350kWh per month.

This works out to around R188 to charge the 78kWh battery to full.

Considering the car will likely be used for short commutes, such as to and from work when charging at home, this works out to around R0.47 to R0.42 per km based on a range of 400km to 450km.

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The problem with home solar and electric cars