How do I use the I2T rating of a fuse?
The resistance for energy dissipated in the fuse is the internal resistance of fuse itself, which is already included in the I^2.t figure.

But from "user" point of view the useful energy is dissipated in the load connected in series with fuse, so the useful energy fuse can pass is determined by load impedance.

Nevertheless for fuse itself it is only current what makes difference regardless how much voltage will be there over the load. So you need to know how current peak flowing through your load looks (not power peak) to determine suitability of particular fuse.