The Achilles' Heel of any solar power system. Energy storage. You store the energy you collect in batteries, which provide a reservoir of electrical energy. Eventually those batteries can't hold a charge, just like your car battery when it gets old or, in the case of my good old Nissan Sentra, when it gets old and gets periodically frozen in near-zero temperatures. Spirit (and Opportunity, if she makes it) will have it worse, having to brave nighttime temperatures around -150 deg. F. or so. The Warm Electronic Box (WEB) helps on that score, but eventually the WEB will freeze up as well.
They could have helped themselves on this score by going with an RTGs. That kept the ALSEPS on the Apollo missions and the Viking Landers going for years, and have done quite nicely on the Voyager and Galileo and Cassini spacecraft, but the wackos didn't like it because of that "nasty nuclear material" (plutonium). So you go with the PC solar panels and storage batteries, and end up with a 90-day mission (if you're lucky).