We’ve had our XC40 PHEV 21 months now. You can leave the car to its own devices and it will mix battery and engine use as best it can. Your pattern of use will determine if you can improve on that. We keep the battery topped up with the 13A plug charger. Daily use is usually less than the battery range so we don’t use any petrol. Journeys longer than the battery range, I’ve taken to using the battery on the slower parts (usually beginning and end) and using the engine for the open road parts, having looked the cost ber mile of each – petrol at low speed costs more, electricity at high speed costs more.