Does the temperature of the body of an aircraft increase its efficiency? I am working on a prototype for an airplane. I was wondering, can the temperature of the body of the plane make it more efficient? If the structure is at 25°C, does it need less energy to maintain a steady state flight than a structure at -20°C? 

I had this idea about excess energy that could be collected during the day, and was wondering if there is any chance that when the batteries are fully charged and there's still excess power being generated, I could use it in order to increase the performance of the aircraft.
As your speed increases, your temperature will increase due to fluid friction of the air. When metal gets heated, it becomes weaker and more malleable. Think about when a blacksmith is hammering on a piece of iron over a hot forge. Due to this, your Vne should be reduced so your wings don't fall off. 

As far as your initial question, if temperature affects efficiency; not really, unless your body gets so hot and you are going so fast as to alter the shape of the aircraft (which could happen, I suppose.)