Example:
15 eV = 2.277856e-21 Btu (IT), Btu
1 eV = 1.51857e-22 Btu (IT), Btu | 1 Btu (IT), Btu = 6.585141e+21 eV
Conversion Steps:
15 × (1.602177e-19 / 1055.05585262) = 2.277856e-21