Most 172watt pelts I've seen have been 24Volt, 15v would almost be in the sweetspot(15~18V). 220watt pelts I've seen have been 15Volt(max)~24Amp(max), so 12v would be near their sweetspot. At 75%(i think, very near that anyway
) TEC's are at their optimum, over that they throw out ALOT more heat for little extra cooling power...
Go 220watt for sure!...