In this cool video of a presentation about SpaceX's GPU-based computational fluid dynamics, there is a slide early on about making rocket fuel on Mars - specifically using water from the ground and carbon dioxide from the atmosphere to make oxygen and methane:
2H2O + CO2 -> CH4 + 2O2
Going in that direction requires energy, and I'm guessing solar power in some form or other would certainly be one way.
There may be multiple ways to use solar power to drive endothermic chemical reactions. There is photocatalysis, concentrated solar thermal catalysis, photovoltaic for thermal catalaysis and/or electrolysis just for example. "What is the best method..." would be a great question, but it's not this question.
This question: Ballpark figures: if seven people wanted to get in some capsule (could be the same Dragon that got them there or not) now sitting on a methane/LOX burning booster, blast off and get back to Mars orbit to rendezvous with a waiting vehicle to go home, and for some reason they had to make the propellants from water and carbon dioxide, roughly how many square kilometer years of solar power on Mars would be necessary to synthesize those propellants?
Assume food and water and vehicle for the return is waiting for them in orbit, but they have to make the propellants to get to orbit on Mars as shown in the screen capture.