Operating mode

Charging on the power that would otherwise be exported

The whole point of surplus charging sits in the gap between what an imported kilowatt-hour costs and what an exported one earns. Making it work means knowing second by second how much is spare, and tracking charging power to exactly that.

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The floor nobody mentions

An electric car cannot be charged arbitrarily slowly. The standard's smallest step is about 6 amperes — roughly 1.4 kW on one phase, 4.2 kW on three. With 800 watts spare on a March morning there is nothing to put into the car: below the step there is no “charge a little”, only on or off.

Everything else follows from that. Control worth having must not only track the surplus but be allowed to decide: wait until enough accumulates, top up a little from the grid, or drop to one phase to lower the floor.

Smallest step: ca. 6 A
One phase from: ca. 1,4 kW
Three phases from: ca. 4,2 kW
Measuring point: grid connection
Inverter: connected over Modbus TCP
Control group: charge points, heat pump, battery

Three charging priorities

Only sun, or full by morning?

Surplus charging is not a switch but a trade-off between price and deadline. With aCharge it is set per access medium — the person driving to a client tomorrow can hold a different priority than the one whose car stands until Monday, at the same wallbox.

Solar only

Charges only when there is enough surplus. No grid draw, no guarantee about the state of charge — the cheapest and the most patient setting.

Normal

Surplus first, grid when needed. The everyday setting: sun is used, but the car is not empty in the morning.

Maximum

As fast as the connection allows, regardless of the sun — for when a journey starts in two hours.

From the field

Where surplus charging fails

Four cases where everything is installed and grid power still ends up in the car.

The inverter is what gets measured

Then control sees generation instead of surplus and charges along whenever something else in the building draws. The measuring point belongs at the grid connection.

The house battery is quicker

Battery and wallbox reach for the surplus independently, the battery wins and the car gets nothing. Both belong in one control loop with a defined order.

Clouds switch charging on and off

Without hysteresis and a minimum run time the session starts and stops constantly — bad for the vehicle and for billing. Control has to be allowed to be sluggish.

The wallbox cannot switch phases

A device that only does three phases needs about 4.2 kW of surplus before anything happens at all — on a small array that means off for most of the year.

Depending on what you are working out

Technology

The control behind it

Surplus charging is one operating mode of the same controller.

Regulation

Section 14a EnWG

The same installation, the other setpoint — from the grid instead of the sun.

Hardware

Can my wallbox do this?

Phase switching and Modbus — the list says what runs.

Frequently asked

PV surplus charging — answered briefly

What is PV surplus charging?

Charging with the part of your solar generation the building does not need and that would otherwise be exported. Since the export tariff is well below the price of an imported kilowatt-hour, every self-charged kilowatt-hour counts twice.

Why is the car not charging although the sun is out?

Because the surplus is below the level at which a car accepts current at all: the standard's smallest step is about 6 amperes, roughly 1.4 kW on one phase. Below that there is nothing to regulate down to.

What has to be measured?

The point where power leaves or enters the building — the grid connection, not the inverter. Only there do you get generation minus consumption.

One phase or three?

One phase starts at about 1.4 kW and suits a small array or the edges of the day; three phases start at about 4.2 kW but scale to 11 or 22 kW. Ideal is a wallbox that can switch between the two.

What about the heat pump and the house battery?

They draw on the same surplus, so they belong in the same control loop — otherwise the battery fills while the car waits, or both draw at once and the remainder comes from the grid after all.

Is it worth it for apartment buildings or company car parks?

Often more than at home: a roof array over an underground garage or a yard generates most exactly when vehicles are parked there. The question becomes less whether there is surplus than whom it belongs to.