When a house’s solar array is generating more power than the house consumes, have the car use the excess energy to charge the batteries. This means the charge rate will vary depending on how much excess energy is being created. The timer would still be used to ‘top’ off the power in non-peak hours.
This feature is useful in regions where the electric company either provides no compensation for excess power generation, or more commonly, a far lower rate of compensation than rates charging in non-peak hours (i.e. midnight to 6 am).
This system would have some outside costs – as the solar and main house power must be monitored, and a means (likely WiFi) to communicate with the car. Retail products that perform similar actions typically cost under $500. The car itself would require some additional software, but no hardware changes.