Charging your car on your own solar power

On a sunny day your panels often generate more than your home uses at that moment. That surplus normally flows to the grid, for a payment that keeps shrinking. Surplus charging sends that power to your car instead: the cheapest kilometres there are, because your own generation is already paid for.

The idea fits a broader approach: shifting consumption to the moments you generate. That principle, and what else it delivers, is covered in smart home energy steering.

A smart charger gets its information from the fuse box: a metering module on the connection sees second by second how much power flows in or out of the house. When your panels generate more than the house uses, the charger raises the charging power; when a cloud slides in front of the sun, it throttles back. The car charges along with the weather.

Most smart chargers offer several modes for this. Solar-only: the car charges exclusively on surplus and pauses when it disappears. Solar-plus-grid: surplus comes first, but the grid tops up to a minimum speed so the car is full in time. Which mode suits you depends on how far and how often you drive.

Here is the detail missing from many explanations. The international charging standard that cars and chargers follow (IEC 61851-1) specifies a minimum charging current of 6 amps per phase. On a 230-volt phase that is about 1.4 kilowatts: below that, a car simply cannot accept power. If your surplus is smaller, charging pauses or the grid tops up.

If your car charges on three phases, that same lower limit sits around 4.2 kilowatts. For pure solar charging a single-phase setting is therefore often the better fit: the threshold is lower, so charging continues on half-cloudy days too. Many smart chargers switch automatically between one and three phases and pick the setting that matches the surplus.

How much surplus you have over a year depends on your system and consumption; make a first estimate via solar panel yield.

Surplus charging is about charging as cheaply as possible; load balancing is about charging safely. That function watches the total load on your connection and throttles the charger as soon as the rest of the house demands a lot, for instance when the oven and washing machine kick in together. The main fuse stays within its limits without upgrading the fuse box.

In practice the two belong together: the same metering module that sees the solar surplus also guards the total load. We configure both during installation.

The benefit grows over the years. The Dutch net-metering scheme is being phased out and more and more suppliers charge feed-in costs on power you send to the grid. Every kilowatt-hour of surplus that lands in your car instead of on the grid skips that whole circus: you neither feed it in nor buy it back later at a higher price.

Honesty also demands: it mainly works from March to October. In winter there is rarely a surplus, and charging during the cheap hours of a dynamic energy contract is the logical alternative, outside the evening peak, in line with what net-aware home charging (Dutch) advises. A smart charger combines both: sun in summer, cheap hours in winter.

A common question: cannot the home battery charge the car at night with the day's sun? Technically yes, but do the maths: an average home battery stores 5 to 15 kilowatt-hours, while a car battery is easily 50 to 80 kilowatt-hours. The home battery is there for your household evening use; the car charges most cheaply straight from the afternoon surplus.

The order we advise: first the house on its own sun, then the car on the remainder, and the battery as a buffer for the evening. That way the devices do not compete for the same kilowatt-hours.

We install chargers with surplus charging and load balancing on board as standard, and tune them to your panels and fuse box during installation. If you do not have panels yet, we include the combination in the same quote; that saves a second installation round.

Curious what fits your situation? Put it to us or see the charger page and solar panels.