According to Wikipedia, 125 watts when it was launched, down to about 100 watts in 14 years.
The JPL guys claim that the RTG is really just a trickle charger for the batteries that actually handle the load -- which likely has transients that the thermocouple in the RTG couldn't handle.
I don't know the lifetime of the lithium-ion battery pack but I'm guessing it'll degrade way before the RTG power decreases below the point where it can effectively charge the battery.
There's a LiPo technology called "thin film lithium" which is claimed good for 10,000 cycles - assuming one charge/discharge cycle per day (and incorrectly using earth-days for simplicity), that'd give you ~27 years lifetime from the battery. (I wonder if this'll end up with the same "over engineered" reliability that let the Spirit rover last over 6 years when intended for a 90 day mission?)
The JPL guys claim that the RTG is really just a trickle charger for the batteries that actually handle the load -- which likely has transients that the thermocouple in the RTG couldn't handle.
I don't know the lifetime of the lithium-ion battery pack but I'm guessing it'll degrade way before the RTG power decreases below the point where it can effectively charge the battery.