Suppose I have an electric car with a battery capacity of N kWh, how many kWh of power do I need to recharge it? It can't be a precise match of N - some must be lost through heat, powering the charging unit, etc.
Is it a significant percentage or is it negligible enough to be able to use the capacity of the battery as a basis for the number of kWh I will need to charge my car?
This is more of an engineering question, but there is a bit of physics, so here is a very short answer. First, question of terminology, kWh is not power, it's energy. You have a capacity of N kWh, and charging is done at P kW for a duration of T hours. You question becomes if N = P T exactly. The answer is of course not. There are always losses. The losses depend on many factors. Usually, slower charging (smaller P) results in smaller losses. Also, losses are larger in the cold.
The question is more complex than it seems because the question is "efficient relative to what?" Do you start counting losses at the power station or when the electricity gets in the car? These two values will differ a lot. I own an electric car and my informal research when I bought it mentioned that charging efficiency is of the order of 90% for a home charger. It may be lower if fast chargers (high power) are used.
ElpinikiA postolaki-Iosifidoua, Paul Codanib, Willett Kemptonac, "Measurement of power loss during electric vehicle charging and discharging", Energy, Volume 127, 15 May 2017, Pages 730-742 available at https://www.sciencedirect.com/science/article/pii/S0360544217303730 has empirical data related to your question and a very detailed discussion of power losses.
