Example:
15 Btu (th) = 4.393125e-9 GW*h
1 Btu (th) = 2.92875e-10 GW*h | 1 GW*h = 3.414426e+9 Btu (th)
Conversion Steps:
15 × (1054.349999974 / 3.6e+12) = 4.393125e-9