Wallbox: How do I charge most efficiently?

Electric vehicles (BEVs) and vehicles with a plug-in hybrid drivetrain (PHEVs) charge their energy storage — the rechargeable batteries — through an external power source. On the one hand, BEVs and PHEVs have an internal charger, the onboard charger. This is a compact switching power supply in a robust housing and contains, in addition to the connectors (Type 2 or CCS connector), the charging electronics and various electrical components (rectifiers, converters, isolators, boost converters, etc.).

On the other hand, outside the electric vehicle, it also requires a charging connection:

• a public charging station / charging column or
• a private charging station in your own home or at the parking space — the wallbox or wall charging station.

Which of the two connections charges most efficiently depends on the combination of charging type, charging power, and efficiency — and ultimately directly on your price per kilometre. That is exactly what we look at in this guide, because when it comes to efficiency: The higher the efficiency, the lower the loss — and the lower the price per kilometre.

Why is a wallbox needed and how does it work?

A uniform, standardised definition of what a wallbox is does not exist in Germany. However, as a rule, a wallbox is understood to be more than a simple household socket. It is a dedicated charger, installed in a protected outdoor area such as a carport or in an indoor space (garage, underground parking bay, etc.).

The wallbox is connected to a separately isolated household wiring feed. It thus connects your own electric vehicle to the public power grid — and optionally also to your own PV plant or its storage.

Modern wall charging units differ mainly in their charging power and the scope of their equipment. A wallbox essentially consists of:

• a durable and waterproof housing (protection class IP54, 65 or 67),
• the charge controller, the “Electric Vehicle Charge Controller” (EVCC),
• the electrical safety devices (phase current detection, residual current device, etc.),
• the or the charging connector(s) (Type 2),
• the internal wiring, and
• optionally a Bluetooth, LTE or WiFi module, and a card reader.

What types of wallbox are there?

One wallbox is not the same as another — offered are single-phase and three-phase wallboxes. The different charging powers and equipment variants naturally also affect the price:

11 kW is the standard for most households and usually only requires a notification to the grid operator. The 22 kW variant charges faster but usually requires a permit from the grid operator — as well as a professionally installed installation.

When buying: the four decisive points

Efficiency: how efficiently does your wallbox charge?

Losses occur in the transfer of electricity between the grid and the battery — of varying magnitude depending on the charging type. A recent comparison from a practical analysis (engineering-explained.com / Autoblog):

Important: SAE J2954 is the international standard for contactless charging — its minimum efficiency of about 85 % is the lower limit, not the target value. A cable-ac charging via Level 2 (i.e. an 11 kW wallbox) is still the most efficient variant at ~94–96 % — the inductive air gap takes back a few percentage points of efficiency but replaces the cable in everyday life with comfort.

Does this also apply to other vehicles and wallboxes?

The specific percentage values come from a series of measurements on a Porsche Cayenne Electric — they are therefore a specific measurement point, not a universal law. What they indicate:

• Cable AC charging (Level 2) remains the efficiency standard — ~94–96 % is a good, realistically achievable value.
• Inductive charging (Level 2 equivalent) lies just below this at 89–92 % — the air gap between the base plate and the vehicle costs a few percentage points but compensates for the comfort advantage.
• Level 1 (Schuko) is clearly below both at ~60 %. Whoever charges at home does not get economical results with it.

If you are considering a wallbox, a look at the charging types in detail is worthwhile: To the charging-types overview (AC/DC/HPC) →

Conclusion: efficiency is cost-effectiveness

In the end, one factor matters most in charging: How much does the kilometre cost you — and how much of the electricity was unnecessary loss? Efficiency determines which share of the electricity taken actually reaches the battery and which share you “only” pay for the converters, cables, and charging electronics.

The rule of thumb is therefore: The higher the efficiency, the lower the loss — and the lower the price per kilometre. In practice, this means:

• Cable AC charging with a wallbox (~94–96 %) is the most efficient option for charging at home and beats both Schuko, as well as — narrowly — inductive charging.
• Level 1 on a Schuko connector (~60 %) leaves about a third of the electricity as loss: the most expensive charging kilometre in the whole guide.
• Inductive charging (~89–92 %) gives up a few percentage points for the comfort — a good compromise for occasional charging.
• Choose the charging power appropriately: 22 kW on the wallbox only offers an advantage, if your on-board charger permits more than 11 kW — otherwise you pay for power, that the vehicle can not use at all.

There is a second lever that has nothing to do with efficiency — but turns the same price/km knob: the electricity price itself. Whoever dampens the wallbox with cheap green power tariffs, PV surplus, or night power, halves the price per kilometre faster than any further efficiency gain in the technology.