So imagine this, an hourglass that is the size of a two storey house in height but is wide as an average sedan car that can trickle sand for 5 hours, and at the middle there's a gear which is turned as the sand trickles through it (thus creating electricity **just guessing here), on the outside of the hour glass in the middle, there is a turning point to allow the hour glass to be turned when there's no more sand in the upper part of the hour glass. That turning point will be turned by a motor that gets power from an inverter, which is connected to a battery that is charged by solar.

  1. Will this be able to create electricity continuously or not (since there are two sources of energy, radiation from the sun and gravity from the earth)

  2. If not please tell me why and be gentle about it, still young :).

  3. I assumed that it would take less energy to turn the hourglass itself than to actually lift the sand inside upwards (am I wrong about this, if so, why? )

  4. I also heard someone say you cannot get more energy that you put in (now to me this was out of context because I didn't really understand it ,please try to simplify this to me)

TL,DR : I'm a teenager who wants to generate electricity by trickling the sand in an hour glass ,and turning the hour glass using a motor ,connected to a battery ,recharged by solar, is it possible or feasible for one household?

  • $\begingroup$ Point 1 is partially incorrect - gravity isn't an energy source. Point 3 is completely incorrect - you are still lifting the sand. It's like thinking it will take less energy to lift 1kg if you are wearing gloves. $\endgroup$ May 12, 2023 at 22:27
  • $\begingroup$ it would make more sense to forget the hourglass and the motor, and use the solar panel to charge the battery $\endgroup$
    – jsotola
    May 13, 2023 at 16:49
  • $\begingroup$ You might be interested in a modern invention called Gravity Light, that can be used in remote locations away from the grid. Solar could never power a modern home alone though (unless it used a whole field of panels), which explains why we are still on traditional power almost everywhere $\endgroup$
    – RC_23
    May 14, 2023 at 22:07

2 Answers 2


The initial potential energy is given by $$E = mgh$$ where $m$ = mass of the sand, $g$ is the acceleration due to gravity and $h$ is the height above the turbine. All the potential energy between the turbine and the bottom of the hourglass will be lost as the sand free-falls there without energy conversion. This matters greatly because the obvious place to locate the turbine in your scheme is in the neck of the hourglass.

You'll also have to consider the turbine efficiency. Let's say it's 90%.

Then we have to rotate the hourglass and lift the sand from the bottom of the glass back up to its initial height. The energy required this time is $mgH$ where $H$ is the height from the bottom (not the turbine as before) back to the storage height. This again will have a motor and gearbox efficiency. Let's say that's 80%.

If we say, for example, that $H = 2h$ we can recalculate that energy as follows: $$E_{out} = mgh\times 0.9 = 0.9mgh$$ $$E_{in} = mgH \times 0.8 = 2mgh \times 0.8 = 1.6mgh$$ That means that the system will consume $\frac {1.6}{0.9} = 1.77$ times as much energy as it generates.

In this situation it would be far more efficient to just use the solar. The only reason not to would be if your sand battery was a cheaper solution to energy storage than a chemical battery.

In practice there are better ways of doing this. Hydro pumped storage is the most common and water would be much easier to manage than sand. You could also have a look at "gravity battery" concepts that are being trialled but they seem a bit daft as they are replacing water with concrete.


Of course this will work, you are putting energy into the hourglass to move the sand up. This is essentially how hydropower works, we let the sun do the work of evaporating water and raining it upstream so it can be dropped through a water turbine.

Unless you have an unlimited supply of elevated sand, you will have to carry it to the top of the hourglass, or use energy to turn the hourglass over. You could use solar energy to do the turning if it's during the day. This is the concept of a physical storage battery, such as a Pumped Storage Hydropower. It's also how grandfather clocks work - you physically raise a weight that then makes the clock work. You are storing energy via elevating mass.

Rotating the hourglass would be no easier than raising the sand any other way. A gear-driven mechanism might make the force required easier, but work is force times distance, no matter how the work is done (you up the stairs or a gear spinning the hourglass).

  • $\begingroup$ The content of your answer is fine, but saying "this will work" as the opener is misleading since the original concept of using gravity as an additional energy source will not work. $\endgroup$ May 12, 2023 at 22:28
  • $\begingroup$ @JonathanRSwift I don't know what the original concept was - I answered if one could use sunlight to raise the sand back up. $\endgroup$
    – Tiger Guy
    May 13, 2023 at 3:39

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