I need a little help from the shower users amongst you all. How much water in gallonage or litres per second terms is dropping through your systems

I am told by Engineers (Yale/USA) quite simply that there is no free lunches (technical term). " You cannot generate power from the output of a pump and end up with extra energy. Power will always be lost in such an arrangement".
As a group and through other threads on koiquest the primary objective has been to develop a solar/wind heating system. To that ends can a cheap effective design be developed to raise and maintain the water temperature to a constant during the colder winter months, without paying for it directly.The concept of harvesting solar, wind to this means is relatively simple, though tricky! This hydro concept is merely another potential add on. We all readily accept that our pumps feed our mechanical filters and of course there is a direct cost. No free lunch is very apt. It is the secondary stage i.e. water returning to the pond, cleaned under pressure 24/7, that I hope can be harvested without upgrading a pump. Currently the water is simply returning directly to the pond under pressure.
I know many of you use showers, where water enters a system at approx 2 metres head and feeds through circa 66 litres a second (circa 3,000 UK gals). According to wet calculations and allowing for variation in set ups; Even at 60% efficiency this suggests 777 watts can be produced using a pico hydro turbine?
This is all theme tunes in my head at the moment. I might need a lie down, have an executive moment, a damp cloth on my forehead an get in touch with my forefathers

Oh, declaration - I haven't got a shower so I am not biased or conflicted.
Cheers,
TicToc