Sustainability Mobility Energy18 August 20261 min read
Solar surplus charging, explained without the jargon
Charging "from your own roof" sounds simple until a cloud passes. Here is what the good implementations do about it.
Surplus charging means one thing: send the car only the electricity your roof is producing beyond what the house is already using. Boil a kettle and the car should take less. Cover the sun and it should take less again.
The problem is that a three-phase charger has a minimum. Below roughly 4.2 kilowatts it cannot deliver at all, and a typical domestic roof spends a lot of the year producing less than that. A naive implementation therefore charges in bursts: on, off, on, off, which is hard on nothing in particular but delivers very little energy over an afternoon.
The better implementations do two things. They drop to single-phase, which takes the floor down to about 1.4 kilowatts and keeps the car charging gently through a cloudy afternoon. And they read a dynamic tariff, so once the sun is genuinely finished they schedule the remainder into the cheapest hours before you said you needed the car.
One last practical note: this needs a meter that sees the whole house, not just the charger. Surplus is a subtraction, and you cannot subtract what you cannot measure.
