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It's not free. The energy extracted means that the rate of flow through your 'generator' is slightly reduced, depending on the ratio of conversion. Either way, the water company must then increase the pressure to maintain flow because of your power drain on the system - granted, this may only be a small amount, but it is a drain nonetheless. Anyway - long term view is, if everyone starts doing it the Energy the water company will need to push water will be greater, they will have to invest in stronger materials and joints to cope with the backpressure now powering everyone's generators and yes you guessed it, that fee we pay for the water will grow exponentially. We don't get free 'leccy, just a higher bill. - so you're better off building a water tower to collect rainwater and create a mini hydropower-station. Then you've also got some free water for the washing machine, garden etc.