The spatial variations in upconversion luminescence from NaYF4:Er3+,Yb3+ nanoparticles embedded in PMMA on Au nanocavity arrays are investigated over a wide range of excitation powers, spanning the nonlinear and saturation power-dependence regimes. Spatially resolved upconversion spectra on these arrays show a minimum of ≈3× luminescence enhancement compared to the adjacent smooth Au surface under high-intensity excitation, with progressively higher enhancement ratios, up to 30×, at excitation intensities below 100 W/cm2. It is found that the average upconversion luminescence enhancement, obtained by spectroscopic imaging and far-field measurements, can be almost entirely accounted for by an effective multiplicative shift in the excitation intensity, Peff = F·P, which is robust over 5 orders of magnitude variation in excitation intensity. We reconcile this constant excitation enhancement factor, F = 4.46, with the wide range of observed luminescence enhancement factors, ranging from 3× to 30×, using an an...