# Using "h" or "hbar" in the calculation for generation rate

Hi Ahsan,

According to the whitepaper and In this link: https://kb.lumerical.com/en/solar_cells_methodology.html

You mentioned that the denominator for g is hbar instead of h, this would mean that you are off by a factor of pi.
I saw that this is also the case in the script of solar charge generation rate of FDTD Lumerical.