In Median XL, direct damage spells gain a damage bonus from energy. This bonus has diminishing returns: the damage gained per additional point of energy is lower if you already have lots of energy to begin with.
The energy factor bonus on some caster items modifies this curve, causing each point into energy to provide a significant benefit until much higher levels of energy. Energy factor is particularly effective for high level characters with most or all points into energy.
The formula used to calculate the bonus from energy to spell damage is:
This rare modifier can only be found on some legendary items and the druid's Force of Nature skill. It functions the same way as energy factor, but is based on strength instead.