Smart charging for an electric car
Situation. The electric car is charged at home. The employer reimburses at the fixed CREG rate per quarter, while the actual cost follows the energy supplier’s dynamic hourly rate. Charging without thought can therefore cost money instead of saving it. At the same time the charging must not drain the home battery or overload the grid connection (peak power has to stay under control).
Approach. Home Assistant does not start the charging session itself, but controls the charge current and the battery’s lower limit. Charging is only allowed when the all-in price — the spot price converted to a consumer price including grid fees, levies and VAT — sits below the CREG reimbursement rate by an adjustable margin. If you choose “charge now”, an absolute price ceiling applies instead. A controller adjusts the charge current every minute so grid draw stays around the configured maximum capacity tariff. Any solar surplus is added on top automatically. The moment charging begins, the home battery’s current state of charge is fixed as the lower limit, so the car does not consume stored solar energy. If the car is full but still plugged in, that lower limit is released early. The full decision is recomputed every few minutes, so it recovers on its own after a restart or a missed event. Every session is logged with energy, cost, reimbursement and the difference.
Result. The car only charges when it is demonstrably cheaper than the employer’s reimbursement, within a set grid budget, with solar surplus as a free extra and without touching the home battery. A manual button to charge right now remains available, but always under an absolute price ceiling. Equipment involved: Alfen charging station, Sigenergy home battery, HomeWizard P1 meter, Nord Pool dynamic prices and the automatically retrieved CREG rate.
