With the weather we are having,about 3litres.

There would need to be room for variations in a formula like that.  You did say "average human" so it might not apply to chess players.

For instance,  I generate a lot more heat when I lose chess games than when I win (the face goes red, the breath gets short and the temperature rises) so if I'm on a losing streak I might boil as much as 35 litres a day, while if I'm winning, perhaps only 25.