Elon Musk's Speech at the Astronautical Congress 2017
There were so many sensations during this speech that one sensation was lost: the return from Mars with fuel generated by solar electric power.
Space travel is expensive because enormously expensive rockets are thrown away after a single use. The Falcon 9 managed to land the first stage 16 times in a row. With the new unified BFR - a rocket for all missions - the entire rocket should be able to be used again and again.
In 2022 two cargo ships and 2024 two cargo ships and two passenger ships are expected to land at Mars. Since there are 85 tons each spacecraft around 150 tonnes payload with empty tanks on Mars. 1100 tons of fuel per spaceship are missing for the return to earth.
To get back the six parked on Mars ships are needed 1,440 tons of methane and 5,160 tons of oxygen. These should be produced in just over 2 years until the next launch window, when Earth and Mars are in a favorable position.
The Martian atmosphere is thin but consists of 95% carbon dioxide. The rock should contain water. With energy input for raw material procurement and synthesizing, I estimate the electricity requirement at 45 GWh.
There must be something lighter! No problem with the moon, it has no atmosphere, so you could use a thin film of 10 W per g, 10 kW per kg, 10 MW per ton. But Mars is not the moon, it can be windy there. If you create 120 watts per kilogram of photovoltaic, a freighter would be filled only with photovoltaic.
54 MWh battery to continuously power the Power to Gas plant. With 360 Wh per kg battery, this is 150 tons of batteries. This would a cargo space vessel full again. This gradually makes it clear why in 2022 and 2024 four cargo spaceships are to fly to Mars: more than half of the cargo capacity is already needed for the energy supply.
But even if the system would occupy the entire 600 tons of cargo space, in just over 2 years to the next start window 11 times as much mass of fuel will be produced.