Yeah, I'm sure they already thought of that at the design stage. And you might have noticed the MRT doesn't run at night.
Not in the grand scheme of things it isn't. Electric rail traction is extraordinarily efficient, and the EMUs currently in use have regenerative braking (whether this is being used effectively is a whole different issue, of course). Napkin calculation: let's say all 1200-odd EMUs are on the move at the same time (never happens) averaging 400kW each. That's "only" 500MW total, which is about 4km2 of PV panels or 1.5% of the area of Taipei city. Not suggesting that 100% PV power would be possible or desirable, just describing what it would look like in theory. More realistically, it's the output of a single common-or-garden coal-fired power station. That's the entire metro, at full capacity. The extra lines/services being added are a drop in the ocean compared to the rest of the capital city's electrical equipment sucking at the teat.
So why would they do that? If it were even necessary (which I just can't envisage) they'd shut down other things instead.