Effect that S/N in the optical gr-bands has on the estimation of the NIR peak magnitudes. The grey lines represent ΔT_max multiplied by the gradient of the NIR light-curve template bands as a function of phase, for each simulated SN from Sect. 5.2. We note that the shape of the templates depends on the value of s_BV, which is obtained from the fits of the gr-band light curves. The average NIR peak magnitude offset is shown as a red line for J (top panel) and H (bottom panel) bands. The 1σ scatter in NIR peak magnitude (red shaded region) is ≲0.02 mag for J band and < 0.01 mag for H band.