Noise equivalent power of thermopile sensor?
Usually Johnson-Nyquist noise is dominant in thermopile detectors.
So NEP =  Vn/Rv where Vn is the J-N noise voltage per root-Hz and Rv is the responsivity of the detector in volts/watt, giving the result in W/root-Hz. That’s for the sensor alone, the amplifier noise will add to it (typically it’s uncorrelated noise so it adds in quadrature).
Johnson-Nyquist noise follows the formula Vn = \$\sqrt{4k_BTRB}\$ where kB is Boltzman’s constant, T is the temperature in Kelvin, R is the electrical resistance and B is the bandwidth - calculated per sqrt 1 Hz in this case.
You might want to check out some of the publications of the esteemed British scientist R.V. Jones, who did a lot of the pioneering work in this field, in support of the war effort in WWII.