Home EV Charging Has A Hidden Cost. Here’s How To Reduce It And Lower Your Bill

• Home charging is the best way to replenish an EV’s battery, but not all of the energy goes into the battery.
• Drivers pay for energy losses every time they charge, so the lower the losses, the lower the energy bill.
• Germany’s ADAC tested several electric cars and found the outliers, plus a solution to keep costs low.

Everybody knows that electric cars are more efficient than combustion vehicles, but what some new owners might not know is that EVs sometimes have a hidden cost. When you plug in at home, which is usually the best and most cost-effective way to charge an EV, a portion of the energy coming out of the charger doesn’t go to the high-voltage battery.

In other words, let’s say the energy meter recorded that 30 kilowatt-hours of energy were used during a top-up. However, the car might say that it only received 27 kWh, which is also true, but the owner still has to pay for the full 30 kWh.

The difference between what the meter recorded and what went into the traction battery is known as charging losses, and some EVs are better than others at minimizing them, as Germany’s ADAC found after testing five different models from competing brands. The respected car club also found a solution to keep costs down that applies to all electric cars.

According to the ADAC, using a high-powered home charger can pay for itself over time because the higher output resulted in lower charging losses on all of the EVs it tested. At the same time, using a mobile charger that plugs directly into a household outlet might be good in an emergency, but the energy bills will be higher because using lower-powered chargers results in higher charging losses.

The club ran three tests for AC charging: one with a 2.3-kilowatt mobile charger, one that replicates solar charging at 4.1 kW, and one with a high-powered home charger that could output either 11 kW or 22 kW.

The candidates included the Mercedes-Benz CLA, Renault 5, Tesla Model Y, Volvo EX30, and Volkswagen ID.7. 

The ubiquitous Tesla Model Y had the lowest charging losses of all the EVs, but even in the American crossover’s case, using a 2.3 kW mobile charger resulted in 12.7% losses. Stepping up to an 11 kW home charger cut those losses in half, to 6.1%.

At the other end of the spectrum was the new Mercedes-Benz CLA, which lost a whopping 24.2% of the energy it got from a mobile charger. According to the ADAC, this happens because Mercedes-Benz imposes an 8-ampere limit on the on-board charger when using a mobile charger. That said, using an 11 kW wall charger lowers losses dramatically to 6.9%.

The French Renault 5 had the lowest charging losses of the group when hooked up to an 11 kW dispenser, while the Volvo EX30, which was sold in the United States for a brief stint, was the only EV to support 22 kW charging, though the gains are minuscule compared to an 11 kW Level 2 charger. On a 22 kW charger, the EX30 had 6.7% losses, while on an 11 kW charger, the losses were 7%.

To keep things fair, the charging sessions were recorded when the state of charge was between 10% and 90%. This way, the results should not be distorted by potential cell balancing that occurs at high states of charge. The battery temperature at the start of each session was between 68°F (20°C) and 86°F (30°C).

Why do charging losses occur?

Charging losses happen because the car’s electronic components need to convert the alternating current (AC) that comes out of a home outlet into direct current (DC) that’s needed by the high-voltage battery. The highest losses happen in the car’s on-board charger (OBC), but other components like heaters or coolers can also sip power. 

That’s why some automakers have switched from conventional silicon semiconductors, which are more affordable but less efficient, to silicon carbide (SiC) components, which are much more energy-efficient but also cost more.