If someone were to invent a device that uses the hydrodynamic energy contained within the city water supply to generate enough electricity to reduce their monthly electric bill (however unlikely this would be), would it legal to do so?

In other words, are there any existing federal/state laws that specifically state that a home owner cannot use any device that generates electricity from the hydrodynamic energy contained within the city water supply?

I have an idea for such a device but I don't want to invest the time and money to construct it if say sometime in the future my local water company finds out about it and then sends one of their lawyers to my home informing me that I have to stop using it or face heavy fines.

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    $\begingroup$ Welcome to Engineering Stack Exchange! While this may be more of a legal question then anything else, what-if.xkcd.com/91 explains in detail why you wouldn't get enough energy to make it worthwhile. I'll be voting to close this question as out of topic since it's more of a legal question, but if you have design ideas you want to discuss, feel free to ask questions. $\endgroup$
    – Mark
    Sep 6, 2018 at 14:27
  • $\begingroup$ If your water meter has a check valve then you won't see much (if any) dynamic pressure changes unless you're using water at the same time. $\endgroup$
    – brhans
    Sep 6, 2018 at 14:34
  • $\begingroup$ @Mark, my idea is to basically use the city water pressure to lift a hydraulic jack--a jack that using water as the hydraulic fluid--to lift up a heavy weight, and then that heavy weight would be used to power a gravity-based electric generator. Since the water pressure in most city water supplies is around 60 psi, a lot of weight could be lifted into the air. If a garden hose were to be attached to a 12" hydraulic cylinder/jack, this means 113 sq inches. 113 sq in x 60 psi = 6780 lbs. Once this weight descends back to its starting point, the hydraulic jack will push it back up again. $\endgroup$
    – user17424
    Sep 6, 2018 at 14:44
  • $\begingroup$ @Mark, you would have to turn a valve to release the water from the hydraulic jack so it could descend back down. You would also need a valve to allow water to enter the hydraulic jack. $\endgroup$
    – user17424
    Sep 6, 2018 at 14:47
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    $\begingroup$ @FanofComets - while an interesting idea, from a first law of thermodynamics perspective it will gain no more energy than the water has in itself. At the normal flow rates of a typical garden hose and the 60 psi of pressure, the water has 200 W of power, or 0.2 kW-hr of energy for every hour of operation. Over the course of the month, you'll generate about 150 kWhr, which at 12 cents a kWhr, is only about $18 every month. It's not worth it. $\endgroup$
    – Mark
    Sep 6, 2018 at 15:34

2 Answers 2


Interesting problem.

Let’s see, if we have a cylinder 12 inches in diameter and 8 feet high that would be

(pi*(0.5f)^2*8f)*7.48 g/ft3=47 gallons per operation.

At 3 GPM, that would be about 15 minutes to fill the cylinder all the way.

If it were used to lift a 6780 pound weight, 8 feet up, that would be 8*6780 foot pounds of work, in 15 minutes. That would be

(8 feet*6780 pounds/(15 minutes*60 seconds/minute) = 60 foot pounds per second.

Converting that to watts, we get

60 fps*1.36w/fps=81.7 watts in 15 minutes.  

Converting that to watt hours, which is how the electric company bills, we get

81.7W*0.25 hours=20.4 WH or 0.0204 KWH.

Where I live, we pay about $0.12 per KWH so you are generating

(0.0204KWH*$0.12/KWH)=$0.00254 of electricity.

To do that, you need 47 gallons of water. In my area, we pay about $0.008 per gallon of water. That comes out to


If you use this 100 times, you will use 38 dollars of water to generate 25 cents of electricity.

But what if your water is free? In that case, if you run this continuously, it will cycle 96 times in 24 hours (15 minutes per cycle*24 hours) and use 96*47, about 4500 gallons of water to produce about enough to power to run a few LED light bulbs. If you can get the equipment for free and nobody is going to notice if your water consumption goes up to like 135,000 gallons per month, go for it. In the meantime, I suggest that if you start using that much tap water, somebody is going to notice and if it was not illegal when you started, it would be after a short time.

  • $\begingroup$ actually, the 12" cylinder would definitely be overkill for a device designed for home use. I used a 12" cylinder just to show how much weight could be lifted. A more ideal size cylinder for this type of device would be a 3" cylinder\hydraulic jack, which would enable you to lift a weight up to 420 lbs (7 sq inches x 60 psi), or if you just wanted enough electricity to keep your mobile phone charged, a 1" cylinder could be used, which could lift a 50 lb weight up to a considerable height and would use much less water. $\endgroup$
    – user17424
    Sep 6, 2018 at 18:45

Someone already came up with this idea - but working with the water companies and it was fitted as part of the pressure regulation system so that any over-pressure situations were controlled and something useful generated. Can't find a link at the moment though.

As for you doing it privately, depending on the country and legal statutes, you may well be charged with waste of resource, misappropriation of resource or somesuch.

  • $\begingroup$ I was thinking that once the water enters into my house, it is mine to use as I see fit. For example, the electric company can't tell me how I can use the electricity once it enters my house. $\endgroup$
    – user17424
    Sep 6, 2018 at 14:52
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    $\begingroup$ If you use electricity in a way that interferes with other customers' use of it, the electric company can most definitely "tell you how you can use it" (in court, if necessary). The same is true of the water supply. But if you do the math you will discover your idea of a gravity powered generator won't do anything useful, if it has to be small enough to fit inside your house. $\endgroup$
    – alephzero
    Sep 6, 2018 at 15:50
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    $\begingroup$ How much do you pay for water? The cost of wasted water should be included in your analysis. $\endgroup$
    – Mark
    Sep 6, 2018 at 15:54
  • $\begingroup$ @alephzero, that is true, it would have to be built outside your home. Ideally, this weight would be lifted higher than your house, say to a height of 50 feet and then it would slowly descend down say over a five minute period. $\endgroup$
    – user17424
    Sep 6, 2018 at 16:36
  • $\begingroup$ @Mark, I have thought about that. One idea I have is to save the water exiting the hydraulic jack into a large container, or perhaps into an empty swimming pool and then use that water to water your grass, garden, etc. $\endgroup$
    – user17424
    Sep 6, 2018 at 16:41

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