By saying “300-litre” we mean that the water tank volume of the solar water heater is 300l. Also, each vacuum tube takes 3l of water. A 300l SWT is equipped with 30 vacuum tubes so the system takes 390l of water but on the output we get only the water from the tank. But if we start an experiment and empty the tank we’ll get 10-20 litres less than the tank volume because there is always a small amount of water left in the bottom of the tank to keep the place of connection with vacuum tubes in water otherwise there will be a danger of evaporation of the water from vacuum tubes. If the water somehow is gone from the tubes don’t fill them with cold water until the sun is gone – otherwise the tubes can blow up. Shtigen offers 100, 150, 200, 250 and 300l solar water heaters made by the world leader SunRain.