Hot water tank: How much shower water do you have available?
The number of liters in the tank is not the same as the number of liters of water at shower temperature. The hot water is mixed with cold water. Here you can calculate an idealized mixed amount and see an estimate of the reheating time.
Worked example
- Tank volume: 26.4172 US gal
- Tank temperature: 140 °F
- Cold water: 50 °F
- Use temperature: 104 °F
- Heating power to the water: 3 kW
Mixed water at the use temperature
44.0287 US gal
Heating 5.8139 kWh · about 1.938 hours
An idealized energy balance. Stratification, mixing and losses can give less usable water.
The temperatures determine the mixing ratio
Enter the volume of the tank, the temperature in the tank, the cold water temperature and the desired outlet temperature. The calculator finds the theoretical amount of mixed water from an energy balance.
The actual usable amount can be lower. Stratification of the water, mixing in the tank, pipes and temperature control matter. A product can therefore have other documented capacity figures than the simple worked example.
Guide to showers and reheating
Use the actual flow of the shower
Divide the calculated amount of water by liters per minute to see a theoretical shower time. If several showers are taken one after another, it is also relevant how quickly the system can add new heat.
Example with a 100-liter tank
With 60 degrees in the tank and 10 degrees in the cold water, 100 liters can ideally be mixed to about 167 liters at 40 degrees. This assumes the simplified model without further losses or limitations.
Reheating is calculated from the heat demand and the power actually delivered to the water. The electrical power of a heat pump is not necessarily its heating power. Temperature settings must also follow the instructions for the system and hygiene requirements. The calculator is used to understand capacity and to plan, not to lower the tank temperature automatically.