Can your rig actually pull 1500W?? The PSU will only draw what it needs (plus lose a little to heat), so go off what your build will actually use at absolute max and add 25-35%.
If your rig can actually approach that 1500W (I hope you're mining or something!?), then you'll need a UPS rated at 1500W + whatever is 'lost' to heat by the power supply, could be 20%. That puts you at 1800W easily.