For water at standard temperature (25 C) the net cell reaction may be written:
Using Gibbs potentials ( kJ/mol), the thermodynamic voltage at standard conditions is
: 1.229 Volt (2 electrons needed to form H(g))
Just as the combustion of hydrogen and oxygen generates heat, the reverse reaction generating hydrogen and oxygen will absorb heat. The thermoneutral voltage is (using  kJ/mol):
: 1.481 Volts.