This Guide tells you all about weight, and how it affects your stats in Celtic Heroes
Weight is a reflection of the physical weight of your gear, and as such your Energy is reduced the more weight you have. Warriors tend to have a lot of weight, Rangers have what is considered average weight, and Druids and Mages have very low armor and consequently low weight, which is fortunate as they're Casting Classes and as such need a lot of energy, while Warriors, Rangers, and Rogues are a combination of base attack and casting.
Each point of "Equipped Weight" (only things you are wearing) will decrease your energy by an amount. As weight goes up, it seems that less energy is subtracted for each point in weight so it is non linear. For example at weight = 5 it subtracts energy at 6.7 energy per point of weight, but at 15 weight, only an average of 5.26 energy is subtracted per point in weight.
The equation for weight is currently unknown, but could be found out with enough time and data points.