Does anybody know how an unit XP value is calculated?

I do not know the formula, but there was a stage when casting Wither on an enemy reduced the amount of xp one got for killing them, so their attack value is probably included in the xp calculation somehow.

Wow! that sounds like a prize reduction for using unfair techniques. LOL!

So it seems like the formula uses the killed unit stats to calculate the power and then multiply by the factor in ElementalDefs.xml. It would be very nice to know which stats are used and how the are weighted. In other words, to know the exact formula

Experience is straight up collected from the enemy's Combat Rating, which is a hidden value. You receive a very rough estimate when you look at an enemy and it tells you whether they are Strong, Lethal, Epic and so on. Units that fight for AI factions also give significantly less experience.

But other than that, generally it goes that the stronger and more dangerous enemy, the more experience you'll get. Since there is such a vast amount of variations of each monster (even a single black widow can come in some 400 minor variations) the devs probably thought there was no point assigning units individual experience rewards.

Thanks for the answer. I get the general idea The doubt that remains is how are abilities taken into account. For example, the ability that gives an attack boost but only again lower level creatures, how is that rated as combat rating? Or maybe it's just as simple as a flat value per each ability.

Thanks also for pointing that there is a way to know the calculated value. By modding those rough estimations in lower ranges it could provide a lot of information about how the game perceives the combat rating.

Some traits for monsters do have combat ratings, but I don't know if the value is added flat or something else.

Thanks again. I saw what you mentioned. For example the overpower ability has

<CombatRating>300</CombatRating>

So I think it's safe to assume that to combat rating comes from "basic" stats (Attack, Defence, Accuracy, HP...) plus those "manual" adjustments.

Once the total combat rating is resolved for all the units in the army it is then multiplied by 

<CombatRatingToExpMultiplier>0.075</CombatRatingToExpMultiplier>

which is then split up depending on whether the unit is a champion or trained unit, henchmen fall under odd rules.  After that it just depends on who lives and dies as to whether they receive experience.  That line of code is hidden within the ElementalDefs file.  You can also adjust how much experience total is given to trained units within the same file search for the line,

<ExpMultiplierOnDesignedUnits>0.5</ExpMultiplierOnDesignedUnits>.