New Delhi, Oct. 29 -- Every winter, the same scene plays out in a lot of Indian homes: someone stands under a shower waiting for the water to warm up, someone else is already knocking on the door, and a quiet argument begins about whether the geyser is too small. If you are buying a new one, you will quickly run into the big divide: instant geysers on one side and storage geysers on the other. They look similar on a shop shelf but behave very differently once they are on your wall. Here is what actually separates them and how to pick the one that fits your bathroom, your family and your wiring.

An instant geyser has a small tank, usually somewhere between one and three litres, and heats water as it flows through. Open the tap and the ele...