Based on the OBD wiki:
Foe is a scientific unit used to quantify the most energetic events in the Universe. 1 Foe equals 10^44 joules or 10^51 ergs (This being why it's called Foe or "[tenth power of] Fifty-one ergs"). This is also rather convenient because the normal Type Ia supernova usually releases energy equating between 1 and 2 Foe under just a few seconds.
It is also worth noting that Foe has no plural form. It is always in plural as this is an abbreviation referring to "fifty-one ergs".
A conversion from joules is trivial and it is also compatible with the common scientific prefixes. So when you see terms like "5.5 kilo-Foe", read it as:
5500 Foe or 5.5 x 10^47 joules of energy.
-GBE of the smallest star: 0.16 mili-foe
-Theoretical total mass-energy of the entire Earth: 5.4 mili-foe
-GBE of the Sun: 6.9 mili-foe
-Average power of a gamma ray burst: 0.5-1 foe
-Energy content of a normal Type Ia supernova: 1-2 foe
-Energy content of an average hypernova: 100 foe
-Theoretical total mass-energy of the Sun: 1.8 kilo-foe
-Estimated total mass-energy of the observable universe: 40 yotta-foe (4 x 10^69 J)