Dell Power supplies have combined 142 to 150W on the 3.3v/5v rails and at least 15W on the 5VSB Rail. This is the case all the way down to 305W units. Power supplies are not single 12v units with a single spec of watts. The other rails maximums count. on the 875W unit this spec goes UP to 225W on the 3.3v/5v rail.
CORSAIR TX650 works fine but CX650 does not for the same reasons.