First, take a look at the energy source for the water heater. Some of them run on electricity while other consumes gas for optimum use. If you have a gas stove in the kitchen and you have installed the necessary chimney system, you may want to get a gas water heater because it is much cheaper as compared to an electric one.
Remember it depends on your requirements on how you want to get hot water. You can either purchase an instant water heater or the type that stores hot water in a tank. In the first case, the cold water from the supply pipe passes through the cylinder and you get hot water. In the second case, the tank is filled with water and then gradually heated.
When considering the option of buying, you have to take into account the dimensions of the area where it will be installed. Flow-through devices are much smaller and they can be hung on the wall. The storage water heater, on the other hand, consumes more space. In some cases, they are often installed outside. They are both vertical (i.e. hung on the wall) and horizontal. Some models of horizontal machines can be equipped with a sink.
When buying a water heater (gas or electric), pay special attention to its protective coating. Porcelain and enamel is cheaper and does not corrode. However, enamel is sensitive to temperature changes. Water heaters with tanks made of stainless steel and coated with titanium are more expensive but also very durable.
Electric instantaneous water heaters consume a lot of electricity, as it is designed to instantly heat the water. To wash the dishes and take a shower, 8 kW equipment is good enough. For hot tubs you might want to buy a more powerful device rated at around 13 kW.