Overhealing is the concept of healing more than is required. For example, if a player has 3000 out of 5000 hit points, and the healer heals for 4000, that healer has overhealed for 2000, meaning that 50% of the amount healed was overhealing. Slight overhealing is always to be expected, usually ranging from 10-30% depending on the encounter and the spells used. Higher overhealing than that is generally undesirable, as the healer spent mana on healing that was not needed and which never actually restored health to the target, as it was already at full health. It is however worth noting that overhealing has no negative effects beyond the mana used, and that if mana is not a problem then neither is overhealing.
Overhealing and tank healing
In most high-end raid encounters, overhealing is standard practice on the main tank, as reactive healing is not enough - the only option is to be proactive and land heals all the time just in case they're needed. This tactic is generally of much less use in five man instances, as the tanks in such situations generally have much lower avoidance, higher health compared to how hard the bosses are hitting, and there will be no other healers healing the same tank.
One way to achieve proactive healing on a main tank while minimizing overhealing is to start casting a heal that could potentially overheal and then interrupt it by moving slightly, pressing escape, or using a /stopcasting macro just before the spell lands if the tank is at full health when the spellcasting is almost completed.
Overhealing and threat
Healing generates threat. However, overhealing does not generate any threat for any amount that does not actually heal. In the above example, the healer sent 4000 points of heal, but only 2000 points of healing were actually 'absorbed' by the target. The extra 2000 points of healing do not generate any threat at all.
This is important. Overhealing a tank does not make the tank's life more difficult or threaten the healer with aggro above and beyond the given situation. No matter how much healing a healer lands on a tank, only healing that heals damage actually taken by the tank will generate threat, and that is health that would have to be healed up anyway if the tank is to survive. This means that the only effective difference between a healer who constantly spams huge heals on the tank and one who heals just enough to keep the tank at full health is the amount of mana used.
Overhealing and healing meters
There are many addons that can track the overhealing done by the healers in a group or raid. This can be a very useful tool for a healer, as it makes it easier to keep track of performance when trying to improve your healing techniques. Such statistics are not always useful or accurate, however, and there are however a number of factors that must be taken into account when evaluating them:
- Overhealing is only a problem if mana is an issue. If a healer did not run out of mana during the instance then his overhealing was clearly not problematic.
- If multiple healers are healing the same target, one healer landing a heal just before another will cause the second healer to overheal. The time difference can be as low as milliseconds, and can potentially be caused by server latency and not player skill or reaction times.
- Heal over time spells (HoTs) will only heal if the target is not already at full health, but they cost the same amount of mana regardless of how much they end up actually healing for. Because of this, HoTs will almost always show as having done very little overhealing, which is not necessarily true.
- Smart-targeting heals like Chain Heal will only heal targets that are below full health. In the case of Chain Heal, the healer pays mana for a heal that is supposed to heal three targets, but if only one target has taken damage then the heal will only heal that target. In effect, the spell overhealed by an amount equal to the healing that would have been done to the additional targets, but as no healing was done this will not show up on overhealing meters.
- Reactive healing spells like Prayer of Mending and Earth Shield can only be placed on one target at a time and will only heal if the target actually takes damage. If no damage is taken by the target within the duration or if the healer casts the spell on a new target, the buff will expire without doing any healing. As the spell still cost the full amount of mana regardless of healing done, these spells will often show up as having done less overhealing than was the effectively case.