Energy reservoirs (aka batteries) are distinct supplies of magical power used to create magic. Mage smiths have spent a great deal of effort in understanding how energy can be stored and how they can be constructed.
The easiest reservoir possible is a mage or talent tapping into their personal energies to provide the power for their magic. The energy storage usually comes from physical sources, burning through sugars and fats before eating into muscles and even bones.
Like heavy exercise, this can result in fatigue and exhaustion. Unlike physical activity, the rate and efficient of the cannibalization happens at greatly distinct rates that are specific to the individual. This means some mages are physically fit and use their personal energies not unlike working out, but others are able to convert their stores of fat more efficiently and may have less muscular bodies.
While many consider the mental exhaustion to be part of the energy storage, the mental component comes from the shaping and adjusting of the magic for ambient situations.
Almost any physical object can be used to store energy however solids are more conducive to storing energy while liquids are less effective, followed by gas. It is nearly impossible to store energy within plasma.
In solids, crystals are the ideal form for storing energy. The more complex and dense crystalline structures are the best, which is why diamond is considered one of the best crystals for storing vasts amount of energy. In theory, lonsdaleite may be a more suitable matrix for creating artifacts but no one has been able to craft or find any in quantity.
Other solids can also be used, but their fragility makes it harder to handle the stress of feedback and will cause deterioration and eventual destruction.
A common medium for runes is paper, which provides a miniscule amount of energy but usually enough for a single one-shot effect.