If you land a healing spell which takes somebody over their maximum health then the additional healing is wasted. This additional healing is known as “Overheal”.
Many people seem to think that overhealing is the sign of a bad healer, and they’re wrong. They think that because your overheal isn’t effective, isn’t actually replacing lost health, that it is a waste. Waste in the sense of wasted mana, and wasted time taken to cast the spell. The reason they are wrong is because, many times, exactly the opposite is true. Overhealing will save you time and save you mana.
Before I go any further, let me just dismiss a common myth some people have with overhealing. If you overheal a target, the additional healing causes no threat. You only get threat from effective healing – healing that actually increases somebody’s health. If you healing isn’t effective, it isn’t producing threat. So threat is not important to our thinking about Overheal.
Now there is obviously a difference between 100% overheal on a spell (i.e. you cast Greater Heal, it landed late and you did no healing) versus casting five spells with 20% overheal each. You are wasting the same amount of healing, but in the first case that Greater Heal was essentially pointless, whereas in the 20% case you’ve topped somebody up to full health which might be important. The reason it might be important is that sometimes the fastest, or the cheapest, way to heal somebody may involve some overhealing.
Sometimes time is short and overhealing somebody is faster than not. For example it may be faster to cast a Penance and overheal for 30%, than to cast two flash heals. Or you may find that casting Prayer of Mending as soon as it is off cooldown will mean the first couple of jumps will overheal by a lot, but that may be better than waiting for more raid damage to occur if you are hard pressed to heal the tank.
At other times you may be thinking more about your mana. Overhealing is wasted healing, but that doesn’t mean it is wasted mana. For example, casting Prayer of Healing when only four people are damaged will mean at least 20% overheal. However, whilst prayer of Healing is expensive, it is cheaper than casting four Flash Heals. So given the option of 20% overheal on Prayer of Healing versus 0% overheal on four Flash Heals, the Prayer of Healing is the more mana efficient option – and by quite a long way.
So don’t be afraid of overhealing. What is important is knowing when you have the time to avoid it, and when it is cheaper not too.