During charging, Tessie can measure the energy added to the battery, used by the charger and the overall efficiency.
Your Tesla tracks the energy added to the battery pack during charging. Tessie uses this number directly. It's the same as the +x kW value that can be seen in the vehicle while it charges.
Your Tesla measures the current and voltage from the charger, before any efficiency loss occurs.
Tessie monitors these values over time and calculates the power used over the course of the charge.
Charging efficiency is the energy added to the battery divided by the energy used by the charger.
Efficiency is affected by many factors, including power source, battery temperature and ambient temperature.
Charging via 120-volt outlets (the standard electrical outlet in use in homes in North America) will typically result in an efficiency of ~70%.
Charging via 240-volt outlets (a more powerful outlet used for dryers and EV chargers) will typically result in an efficiency of ~90%.
Charging via direct current (DC), like Superchargers, will typically result in an efficiency of 99-100%.