[rsynnott]

Watts and watt-hours are generally used internationally for heating; the Btu is largely a historical oddity in most places outside the US.

Also it is a mess, as is traditional for US customary units:

> A Btu was originally defined as the amount of heat required to raise the temperature of one pound of liquid water by one degree Fahrenheit at a constant pressure of one atmospheric unit. There are several different definitions of the Btu that differ slightly.

(Best one is still the fluid oz, tho. A US customary fluid oz is about 29.6ml, a US food labelling fluid oz is exactly 30ml.)