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.
UPSs are not always necessary; all they protect against is data loss and corruption, which usually isn't a major issue. Worst case you have to reinstall windows. A good surge protector will do all that a UPS will against hardware loss.
A UPS that can run 1.5kW of hardware isn't going to be cheap...