How hot and cold temperatures affect power efficiency in solar panels and their power lines:
The law of thermodynamics when applied to electronics and the transfer of energy states the more heat you have the less efficient the transmission of power is due to the increase of resistance.
The heat that originates from hot summer days generates resistance in the power lines by pushing molecules away from each other that are needed to allow for the flow of electrons.
This heat causes a back up in the flow of energy which is released in the form of more heat. This causes more resistance and thus more heat. That means less power makes it to the battery storage.
Also the smaller diameter the power line from collection source to deposit source creates larger resistance levels thus generating heat and reduction of energy transfer. Again less power makes it to the storage battery.
In winter there is still heat generated from the resistance in the power lines however cold outdoor temperatures keep the line cool and the molecules close together thus reducing any resistance created from heat. That means more power getting to the storage battery.
Here is a great article that gives a lot more information on this topic:
How the Weather Affects Conductors