Tankless water heaters help the green movement because they are designed to save energy usage and therefore save you money.
A conventional “tank” water heater holds 30, 40, or 50 gallons of water (depending on the size) at all times. When you are ready for a shower, run the dishwasher, or do a load of laundry you expect hot water. This means your tank water heater must be ready for your demands at all times. Tank water heaters always need to be heating the water in the tank in order to meet your needs which uses energy (gas/electricity) all day long.
A tankless water heater fires up only when you need hot water by instantly heating cold water. This results in much less energy being used labeling tankless water heaters part of the green movement.