Charging an electric car from the sun: how surplus goes into the car

Charging an electric car at home is a flexible load — the car typically sits plugged in for hours. If solar surplus flows during those hours, it can charge the car's battery instead of going to the grid at a buy-back price.

How much energy the car uses

A compact electric car consumes roughly 15 kWh per 100 km. The average daily distance usually fits into a handful of kilowatt-hours, which charge comfortably over a few hours at lower power.

Charging from surplus

HoumeOS watches production and consumption in real time and raises the charging current only by the amount of free solar flow:

• surplus beyond your own needs goes into the car;
• a minimum charging level is set, so you always leave with enough charge;
• if a cloud reduces production, charging slows or pauses — with no manual intervention.

Priority: battery, water heater, car

The same distribution logic decides which load comes first. First the home battery up to its reserve, then hot water, then the car — and only after that export. The car is last precisely because it is the most flexible and the slowest load.

Why you need an OCPP-compatible unit

To control charging from outside, the station has to speak the system's language. OCPP is the standard protocol through which HoumeOS reads the status and sets the power — which is why we recommend stations that support it rather than "dumb" ones that simply push current.

Scheduling and limits

Sometimes you want a guaranteed full battery for an early flight. Then a "charge to X by time Y" rule delivers what you need, even if part of that energy ends up being bought from the grid. The choice is yours, not the car's guesswork.

What to expect

On a property with an established solar surplus, charging from the sun covers a large share of the kilometres essentially for free — using energy you have already paid for. The real share depends on the system, your habits and the season, and we calculate it with your numbers, not with marketing promises.