I have a metal box with some electronics inside.

This boxis going o be placed in a place where in winter temp. can be very low.

The volume of the box is about 2 liters and I want to calculate how much energy is going to be needed for the internat temperature not be below say of 5 degrees when the outside may be -20 Celsius.

A big heatsink with a power transistor is the possible solution the heat the box, but how many watts are needed to do that in relation to the temperatures?

Ioannis