Lights and wipers use a negligible amount of power. For heating, my Leaf uses about 0.8kW to preheat followed by about 0.4kW to maintain temperature in the current conditions. The biggest loss in range doesn't actually come from the heating. It comes from the effect of low temperatures on the battery pack. If the car has a full battery temperature management system then the loss of range in winter is lower. With a 32.5kWh battery pack, the Mini only needs to achieve ~3 miles per kWh to hit 100 miles. That's a pretty low target. Summer could be 130+ without any bother.