we have a 160-liter hot water tank for our house and the wife also likes to take baths sometimes. so far the water has never gotten cold. if you want to install solar or something similar on the roof, you should choose at least 300 liters. 160 liters are easily enough for 4 people to shower. more is really only necessary if you have special cases where a lot of hot water is needed at the same time (e.g. giant bathtub). it also doesn't matter whether you use it often or rarely. even if you only use it at Christmas, the bathtub has to be able to fill up with warm water. otherwise, it's a one-time event and the expensive large bathtub just gets in the way...