Working Doc

Purity Levels

Metallurgical grade silicon is 98% (one nine?) [1]
Solar grade silicon is 6-8N [2]
Microchip silicon (Electronic Grade Silicon) is nine nines pure, whereas power transistor silicon is 6 nines. [3]. Solar grade silicon is same purity as transistor grade - 6 nines. [4].
3 nines pure by acid leaching [5]
Czochralski silicon is 11 nines pure [6]

Microchip silicon is nine nines pure, whereas power transistor silicon is 6 nines. [7]

See this reference for more detail [8]
And this discusses poly si vs mono si [9]
embodied energy of 165-375 kWhr/kg [10]
- Good news - 65% of all energy goes into the wafers, as opposed to everything else (glass + All + EVA) according to same link
- Similar for amorphous silicon [11]
Compare to steel at 20 MJ/kg [12], and 200MJ/kg more than this for aluminum [13]
How many grams of silicon are needed for 1kW of PV panels? 2 kg [14], based on 2g/W.
5% of weight of PV is silicon [15]. 76% is glass, 10% is aluminum, and 10% is EVA.
Embodied energy of EVA is about 80 mega joules per kilogram [16]

600 kWhr for 2 kg of silicon
3kWhr per kg for glass, or 90 kWhr for 30 kg glass. [17]
About 40 kWhr per my of EVA/aluminum x 8 kg = 320 kwhr
Total 1010 kWhr
- if the low estimate of 165 kilowatt hours above is true and this is 65% of the energy required then we have around 300 kilowatt hours of energy for wafers, and total around 500 kWhr for 1 kW of new PV panels - meaning energy payback of around 1/4 year!
The case here would be easy for open sector PV production
365*6kWhr~2100 kWhr/yr
Payback is thus 1/2 yr with only the above.
This matches results from Fraunhofer 2021, close enough - .7-1.4 year payback time. [18]