The charging efficiency is the ratio between the energy consumed by the charging process and the energy saved by the battery.

The EEVEE app shows the charging efficiency as percentage for every charging event in the History tab.

In this example, the charging event consumed 10,77 kWh and the battery saved 9,62 kWh of that energy. That means that 89% of the energy coming from the power grid is transported and saved in the battery. That also means that there is an energy loss of 11%.

Any loss of electrical energy comes back as heat. During the process of electric charging, the hardware components heat up and this means a loss of energy. Any additional losses are caused by the degradation of any hardware components, like for example the battery and the battery charger. A scientific research from the Department of Electrical and Computer Engineering, University of Delaware, USA in May 2017 shows that such energy losses during the electric charging process can go up to 36% and that the predominant losses occur in the electronics used for AC-DC conversion.

We know that the charging efficiency vary depending on the charge current and on the battery state of charge (SOC).