Example:
15 keV = 2.277856e-18 Btu (IT), Btu
1 keV = 1.51857e-19 Btu (IT), Btu | 1 Btu (IT), Btu = 6.585141e+18 keV
Conversion Steps:
15 × (1.602177e-16 / 1055.05585262) = 2.277856e-18