I made this calculator while looking for detectors that were shotnoise limited. By inserting the Noise Equivalent Power (NEP) of your detector in the "Shot noise" field, the number in the "Power" field will be the minimum power for shot noise limited operation (\(\frac{\text{Shot noise}}{\text{Electronic noise}}>1\)).
Derivation
A more detailed derivation can be found in "Introduction to quantum noise, measurement, and amplification." (A. A. Clerk et al. 2010). The photon number fluctuations of a coherent beam with average photon flux \(\bar{\dot{N}}\) are given by the power spectral density \(S_{\dot{N} \dot{N}} = \bar{\dot{N}}\) (this is due to the Poissonian nature of photon counting statistics). The power spectral density does not depend on frequency and thus it's equal to its symmetrized version (the one detected according to Glauber's theory of photodetection). If we work in terms of power \(P=\frac{hc}{\lambda} \dot{N}\), then the fluctuations in power are:
$$S_{PP} =\frac{h c}{\lambda} \bar{P}.$$
This is the exact formula used to obtain the value in the calculator.