Lanthanide-doped upconversion nanoparticles (UCNPs) are promising single-molecule probes given their non-blinking, photobleaching-resistant luminescence on infrared excitation. However, the weak luminescence of sub-50 nm UCNPs limits their single-particle detection to above 10 kW cm⁻², which is impractical for live cell imaging. Here, we systematically characterize single-particle luminescence for UCNPs with various formulations over a 10⁶ variation in incident power, down to 8 W cm⁻². A core–shell–shell (CSS) structure (NaYF₄@NaYb_1−xF₄:Er_x@NaYF₄) is shown to be significantly brighter than the commonly used NaY_0.78F₄:Yb_0.2Er_0.02. At 8 W cm⁻², the 8% Er³⁺ CSS particles exhibit a 150-fold enhancement given their high sensitizer Yb³⁺ content and the presence of an inert shell to prevent energy migration to defects. Moreover, we reveal power-dependent luminescence enhancement from the inert shell, which explains the discrepancy in enhancement factors reported by ensemble and previous single-particle measurements. These brighter probes open the possibility of cellular and single-molecule tracking at low irradiance.