On May 2, 2017, Tom Mueller, propulsion CTO at SpaceX, conducted a speech/interview with some members of the New York University Astronomy Society.

The Mars rocket is meant to be completely reusable. Both stages, ship and [inaudible] can lift hundreds of tons in a single flight; it can go all the way to Mars and back, and you’ll have to fuel it on Mars; **we’ll need to make about 1000 tons of propellant on Mars** over a two-year cycle; bring it back; and that’s a tall order. You need about half a megawatt of energy to [inaudible] that much propellant [inaudible]

https://zlsadesign.com/post/tom-mueller-interview-2017-05-02-transcription/

1000 tons (or tonnes) of propellant can get you the water needed as follows

Assume stoich (a SWAG) and no other losses (again a SWAG) and integer atomic weights (a fair approximation at this accuracy level) so the atomic ratios are :

1 CH4 + 202 ( -> 1 CO2 + 2 H20 )

12 + 4*(1) + 4*16 or 80 of which the hydrogen is 4 or 1/20

So you need 50 tons (or tonnes) of hydrogen

The water is going to be (2+16)/2 more than that or 9x or about 450 ton(nes)

Check this, how much CO2 is needed

12+2*16/80 = 44/80ths or 550 ton(nes)

Yaay it balances.