the point is that you can actually live inside the rockets when they first land, you don't need to build extra housing immediately. this significantly simplifies the mission requirements, at least for the first few years.

tank volume is about 1000 m³, which stores about 5000 kg of oxygen assuming 5 bar of pressure, which lasts a crew of 10 for about half a (martian) year, or about as long as the winter sand storms threaten to reduce solar panel production, so you have enough oxygen to breathe through this time.

it is a myth and misleading to assume that energy supply has to be constant. it is good enough if you can intermittently replenish your live support resources.