We first consider their results for monocrystalline silicon cells. For the 1989 situation they estimated that an input of 20.5 megaWatt hours (MWh) of conventional energy was required to produce each peak kilowatt* (kWp) of photovoltaic power station capacity, leading to a payback period of approximately 86 months. Under the 1994 conditions the embodied energy was found to be reduced to 12.2 MWh/kWp and the payback period to approximately 51 months. With polycrystalline technology the production energy was 20.0 MWh/kWp and the payback period was 84 months in 1989.

For the 1994 case, in addition to the changes mentioned above, the cell thickness was assumed to be reduced from 0.45 mm to 0.2 mm and the embodied energy and payback time reduced to 9.0 MWh/kWp and 38 months. For amorphous modules, figures of 13.3 MWh/kWp and 56 months were predicted to be able to be reduced to 7.5 MWh/kWp and 31 months. A more recent (1992) study [4 ] used data from commercial production lines for polycrystalline silicon and amorphous silicon cells.

They neglected the so-called "balance-of-system" components such as inverters and support structures and argued that their energy costs could be reduced to an insignificant level. The polycrystalline cell factory they considered used silicon waste from the electronics industry as its feedstock and there is no obvious methodology for the estimation of its energy content - What is the energy content of material which would otherwise be wasted? The manufacture of electronic grade silicon used approximately 200 kWh/kg while metallurgical grade silicon consumes only one tenth as much energy.

In choosing for their analysis a figure of 20 kWh/kg Palz and Zibetta argued that solar grade silicon had been produced elsewhere with energy content of less than 50 kWh/kg and that a reduction towards 20 kWh/kg was expected. The resulting payback times for the 12 per cent efficient polycrystalline modules was calculated to be in the range of 1.6 to 2.7 years, depending on the choice of European location in which they were used. The corresponding payback times for 6 per cent efficient amorphous modules was estimated to be 0.9 to 1.6 years.