Solar grade silicon can be formed by vapour deposition onto electrically heated silicon rods at temperatures around 1373 K. As the rods heat up with the passage of current, the centre of the rods will become hotter than the surface which is maintained ideally at 1373 K. The electrical conductivity of silicon is a strong function of temperature and such temperature gradients will cause variations in electric conductivity and current density through the rods. For large diameter rods the centre temperature may approach the melting point of silicon. Computer modelling of the behaviours within the silicon rods during production has been carried out. The applied voltage has to be decreased and the current increased as the rods grow in diameter. The power required to keep the rods hot should be minimized to decrease the specific energy consumption. The energy consumption decreases with increasing silicon deposition rate and decreasing heat losses. (C) 2009 Elsevier B.V. All rights reserved.