Some tankless water heaters are unregulated and the outlet temperature depends upon the amount of water being drawn and some are regulated providing a uniform temperature rise within specified water flow rates. Energy savings over storage type water heaters. Small in size, can be located at the point of use. A minimum flow rate is required to turn on. The higher the flow rate the lower the temperature rise.
Electric units require heavy gauge wiring and large circuit breakers.
Gas units require larger flue pipes and gas supply lines than storage type heaters. They are more complicated and harder to get parts for.