In a Down’s cell, the mixture of molten NaCl and CaCl2 is used wherein the sodium ions are reduced at the cathode to liquid sodium metal while at the anode, liquid chloride ions are oxidized to chlorine gas.

The reactions are:

Na+(l)+e−→Na(l) --------       E0 = - 1.36 V

2Cl−(l)→Cl2(g)+2e− -----       E0 = -2.71 V

So, in theorey, it requires minimum 4.07 V for this reaction to occur.

The industrial conditions are however bit different which needs to consider the production capacity of the cell.

Cell voltage: 6.5 - 7 V, Current : < 45 kA, Temp: 590-610oC, current efficiency: 80-90%, energy consumption: 9.8 - 10.5 kWh/kg of sodium.

ref:

https://books.google.com.au/books?id=0XmiTX6e_9YC&pg=PA534&lpg=PA534&dq=downs+cell+voltage+current&source=bl&ots=n6w2zKZQZA&sig=JhAei6SzlvIx1JViSrEPRFuCPZ8&hl=en&sa=X&ved=0ahUKEwjNl4zNi7rMAhVB56YKHS56DOgQ6AEILzAD#v=onepage&q=downs%20cell%20voltage%20current&f=false

For design purpose, however, current density (amps/m2) is important, which is typically: 7 kA/m2.

ref:

http://www.thyssenkrupp-electrolysis.com/fileadmin/documents/brochures/Chlor-alkali_electrolysis_plants.pdf

For an electrolytic cell, it  is important to provide an enormous amount of current to obtain the desired production rate. In industrial scale, this is achieved in combination of transformers and rectifiers.

In theory, solar cells could be connected together in the parallel fashion to recharge a wet battery which can then produce high current output to operate the electrolytic cell.

Along with the production of sodium metal, is calcium metal produced also?

In theory, the reaction Ca2+ -2e --> Ca  has higher redox potential E0 = -2.87 V and compared to Na (E0 = -2.71 V), and therefore it should not form, if appropriate voltage is applied.

However in practice, applied voltage is higher than the theoretical requirement, and possibly Ca is also produced in little quantity at cathode but later could be separated applying gravity separation method due to signiicant density difference between Na and Ca.