How do I determine the required tank volume?

There are two cases to consider when dimensioning the volume of a hot water tank:

For tanks with an integrated backup heating system the tank volume should be 1.5 times the daily household DHW demand at 50°C.
For tanks with an external backup heating system the tank volume should be equal to the daily household DHW demand at 50°C.

For example, for a building requiring 500 liters of DHW per day at 50°C, a 750-liter hot water tank with integrated backup heating will be required or 500 liters in the case of a tank with external backup heating. Source: Planning and dimensioning solar water heating in single-family housing, July 2013, ADEME.