Low-flow shower heads use 5/minute instead of 10-20 liters/minute. Savings with a 10 min. shower is around 10min. x 10l = 100 ltr, in a year that is 36m3. Each m3 of hot water, heated at 45 °C, takes 52 kWh to heat, and annual heat saving is 1900 kWh.
Cheapest 10-20 EUR (DK)
See for instance https://www.smartwatermark.org/WELS/showers/