According to https://www.watersafe.org.uk/advice/common_plumbing_questions1/pressure/what_is_the_minimum_water_pressure_that_a_water_supplier_must_supply/ , in the UK the domestic water supply should be expected to provide a minimum pressure equivalent to a 10m head, and a 9l/minute flow rate at the kitchen tap.
Then at https://www.renewablesfirst.co.uk/hydropower/hydropower-learning-centre/what-is-the-minimum-head-and-flow-i-need/ there is some information on micro-hydro schemes. That reckons a 380l/s flow rate with a 10m head should generate 25kW.
So assuming(!) the same efficiency could be achieved, the amount of power which might expected to be generated by the domestic supply's 0.15l/s flow is 25000*0.15/380 = ~10W.
Equivalent of a couple of old (pre-"rapid charging") USB3 ports (900mA * 5V = 4.5W).
Hard to imagine this being of any practical use, even for those with an unmetered water supply. 10W solar panels are dirt cheap and far less wasteful of a precious resource like treated, potable water.
Update: It's been bought to my attention that items such as this
https://www.amazon.co.uk/Yosoo-Water-Turbine-Generator-Charging/dp/B00ZCBNNOC/ exist (Amazon has many other almost identical looking and probably made in the same factory items, but this one has the most interesting reviews).
I note it claims to produce 10W power - pleasingly in line with my 10W estimate above - however there seems to be no more detailed information given specifying power output in terms of flow and pressure.
- One review claims to have used one to power some LED lighting at an "alpine hut".
- One review claims to have used one in a working model of a hydro-electric plant!
- One review reckons the claim of a 10W output is rather optimistic and might need a 50m head to achieve. (Which might indicate the assumption in my calculation that the efficiency of a "pro" 25kW generator could also be achieved by a 10W "toy" one is rather dubious).