Creating electricity from mains water pressure. Could someone cleverer than me help me out?  
I had a crazy thought going through my head the other day and I can't lay my mind to rest until I get an answer.
Q. How much energy could be produced by using mains water pressure to turn a generator? And would it be feasible to install a system to feed whatever is produced back to the grid?  Assuming that the system would be installed in a building where a constant water supply is needed so the generator would be turning continuously, and a rough water pressure of around 3-4 bar. 
Thanks in advance for any help
 A: You could certainly make electricity this way, it just wouldn't be cost effective.  3-4bar would be the same pressure as a 30-40 meter hydroelectric dam.  The energy per time unit depends upon the flow rate (which depends upon the 4th power of pipe diameter). 
potential energy = pressure X volume 
I wouldn't want to see your water bill!
A: A generator converts mechanical power to electrical power; pressure alone is insufficient.
Assuming the flow in = flow out and a constant flow, the power output of the generator would then be proportional to the pressure difference between the inlet and outlet.
Thus, subtract the minimum pressure required by the building from the mains pressure and multiply that by the flow to find the potential power available for conversion.
Since generators aren't 100% efficient, the actual electrical power generated will be less.
A: We pay 3 dollars per cubic meter for water where I live. At 400 kPa  (60 psi) that's 400 kJoules per cubic meter maximum theoretical power. But that's only about a tenth of a kilowatt-hour, which costs about a penny at 10 cents/kW-hr. So it's not that good a proposition.
A: 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).

A: A domestic cold water tap full on could produce enough power to run a big TV assuming 100% efficiency. 
Currently working on the idea of using tap water energy to lift well water to a storage tank in the top of the house for use in supplying the loo and garden. Once most of the energy has been extracted from the tap water then there could be enough left to deliver the water for normal consumption.... best wishes Physics teacher
