Gravity powered lights, cheaper than solar

Simple things that we take for granted, like flipping a switch and having light suddenly appear, aren't so simple in developing countries. This fantastic gravity powered light may change that, if their Indiegogo project is successful.

The designers developed the project in their spare time over four years, while working at London-based design firm Therefore. They're expecting the light to cost less than $5 to manufacture at scale. Once a family purchases the light, they'll be able to keep it running at no additional expense.

(Thanks, Thom!)


  1. Neat. Misnomer, though. It’s not gravity-powered, it’s muscle-powered. “Wind-up” would have been more apt. Reminds me of my grandparents’ old cuckoo clock.

      1. Theoretically you wouldn’t need (or really want) a battery for this.  Just let the pull wait power the light until it runs out.  Batteries wear out over time, but this system should work for a good long while as long as they don’t overdrive their LED. 

        The big downside is that it is hard to extract much power out of a human liftable weight dropping only a few feet, so the amount of light you get out of this is going to be limited.

        1.  Avoid batteries when you can!  I have an antique Russian squeeze flashlight that has outlasted a good dozen fancy schmancy ones purchased by others in my family.  It’s got no batteries or capacitors and the bearings of the flywheel are lightly oiled wads of unprocessed sheeps’ wool.  Working great for around 20 years now, I keep it at the bedside.

          1.  I have an old one made of Bakelite that must be 50 years old – worked perfectly until I dropped it! My new one lasted about a year before the batteries died, so I tossed it.

        2.  The generator portion can be mounted as high as a human can manage to put it.  It can be hung from a tree limb 10m up or from a rock outcropping.  It just requires that the LED not be co-located with the generator.

          1. For that matter, using a couple of pulleys you can do as you describe, with the present LED-collocated-with-dynamo unit. And that way you don’t have to run wires to extend the electrical power to where you need — just a rope to extend the mechanical power.

    1. Yeah, I was thinking that somehow these worked by just being in a gravitational field, perhaps by having the hypothetical gravitons generating an electrical field or something.

      1. They actually do this … Some satellites are stabilized with a gravity gradient boom. Just the difference in gravity potentials, along with the frictionless of space, is enough to orient the satellite so that it antennas are facing towards the earth. Except, 50% of the time, the antennas point away from the earth so they have to do a “flip” maneuver.

    2. This is actually different from wind-up and muscle-powered. Wind-up uses mechanical potential energy stored in the chemical bonds in springs; traditional batteries use potential energy stored in chemical configurations in batteries. This light uses the potential energy stored in an object’s position in the gravitational field of the Earth. This is similar to a water turbine at a dam or a water wheel in a river.

      Yes you might need to use muscle energy to “charge” the gravitational “capacitor” (which stores gravitational energy and releases it slowly the same way an electrical capacitor does), but you could charge it any way you like…with a battery-powered motor or a combustion engine if you want. Whatever form of energy (“muscle”, battery, chemical) you use to charge your gravitational capacitor eventually you are storing things in a gravity battery and using gravitational potential energy to generate power.

      1. Yes, I agree.  In fact you could rig up your door thresholds so that stepping on them would pull the weight up a notch.  A very simple and robust mechanical linkage would suffice.  At that point, there’s no extra work being done by a human – they are walking through the doorway as an otherwise unrelated activity, and the force of gravity acting on the human body is driving the mechanism, and no additional food need be consumed or exertion made by the human to power the light.

        1. Two things about this: First, look at the video if you haven’t. The target “market” probably doesn’t HAVE door thresholds per se, they’ve got openings in their hut walls. Second, “no additional … exertion” isn’t strictly true. If it were, you’d have invented perpetual motion. Your “free energy” door threshold will be mushier and absorb some of the spring you’d otherwise have in your step. Whatever energy it delivers to the device, it saps from your step. I wouldn’t be surprised if people started unconsciously avoiding stepping on such a device.

          1.  Hmm, interesting point on the mushiness.  If you didn’t tune it right, it might feel like stepping in mud or sand.  But I wouldn’t worry about lack of thresholds – if you can’t make something that would work with a couple of sticks and some string, you won’t be able to feed yourself anyway.  Don’t assume the poor are stupid.

        2.  I was thinking a rocking chair or foot operated treadle would be a good way to incrementally lift the weight (on some kind of ratchet arrangement, assuming the supporting “cable” was a loop.

  2. While you can get meaningful low power light (like a night light) out of this, you’re not going to get anything that’s room filling (unless it’s very short lived).  Dan Rutter of dansdata published a basic analysis of a previous lamp that was intended to do similar:

    1. Those calculations look about right for this. Assuming that’s a 0.1 or 0.2 watt LED (it doesn’t look much brighter than that), you’d have to lift the bag about once every 15-30 minutes. That’s not too bad compared to not having any electricity at all (though it’s far from great, I agree).

      Does the project actually mention how often you’d have to lift the bag? It would be fairly underhanded if it turned out the inefficiencies were such that you had to lift the bag every 5 minutes, and they didn’t bother to mention that fact…

      1. Surely a solution would be to create an external (possibly) power generator along the same principles that could be hoisted higher and with a pulley system to effortlessly raise a much heavier weight. You could raise several weights at once, and have them drop one at a time as each one hits the bottom, or possibly provide more power by dropping them all.

        The system could be hooked up to several devices, or a series of low-power sockets.

      2. The video does say 30 minutes, but also suggests being able to charge things off of it.  A 10kg bag falling 1m provides about 100 watt seconds of power.  A typical cell phone battery holds about 5 watt HOURS or about 18000 watt seconds. Thus it would take lifting this 10kg bag 180 times (while waiting about 40 seconds between each drop assuming standard charging levels) to charge a cell phone over the course of about 2+ hours.  This assumes perfect efficiency of everything involved.  Reality would be closer to 50% efficiency after everything was taken into account.  I cannot describe this as anything other than absolute tedium.

        As a small task light (for reading and what not) it would be acceptable (assuming I could position the light as needed).  But by comparison, this would have the light output of a small keychain light.

  3. The name seems quite accurate and clear to me.  When I saw the headline and name, I immediately thought “Oh cool idea, a light powered by some sort of weight system” — which is just what it was.

    1. Yeah, I just figured the light itself would be the weight that you would lift and let drop. This is probably better though as it leaves the light stationary.

  4. neat.  so when can us first-worlders get one?   (for black outs, camping, and, of course, the coming apocalypse)

  5. Nice. In principle this is no different from a winding dynamo-powered torch, like the simple version at the bottom of this message. In exactly the same way, the muscle-energy you exert will power a light, and in both cases you will exert the same amount of energy to get the same amount of light.

    The difference is that this is a very simple design, easy to use, and you “store up” the energy which is then released over a much longer time period, which makes you feel like you’ve done much less work over all. I like it a lot and am funding.$523161231.jpg

    1. Easy to use?  You need something to hang it from, you can’t really move it around like a flashlight, you need a 10Kg weight(not included) and the light is pretty minimal.  A better solution is a windup device that stores energy in a spring or rechargeable battery and which already exist.  A “gravity light” sounds like a really cool idea and that’s why it will get funded.  People like “cool” ideas regardless of whether or not they’re practical.

      1. It’s senseless to complain that something that’s designed as a room light has to be suspended from something and cannot really be moved around. Have *you* replaced your home lighting with flashlights on those grounds?

      2. Think of the environment where it is intended to be used. No batteries, no kerosene, the included sturdy bag filled with local sand or stones, it will produce enough light to allow simple tasks. This isn’t intended to light a group of Hipsters sitting around complaining about how bored they are. It is designed to be inexpensive and cheaper than several days of fuel. There are one and one-half billion people in this world who don’t share the luxury of electricity. Rewatch the video and visit the site.

      3.  The video shows a peripheral flashlight-like object connected by wires.

        Rechargable batteries wear out over time and must be replaced, which would negate much cost-savings.

        Perhaps they’ll include a spring in future models. As they make clear, this is only the first version.

  6. Why not use a wind up mechanism? What possible advantage could this system have over a self-contained system that takes up almost no space at all?

    1. The wind-up system needs a battery or a spring.  A big dumb weight is cheaper.  This is literally a small electric motor, some gearing, a weight, and a LED.  Mechanically it is very simple and robust.  No springs to break, no batteries to wear out, the only moving parts are the gears and the motor. 

      1. A spring isn’t that complex. Among the many advantages the spring has over a bag or rocks is that it won’t give you a back injury after winding it up. Oh, and it can’t fall on you, nor does it require any external support. And the spring powered light can fit in your pocket. After a quick search, I found there are many models of wind up systems that have lifetimes over an hour and have small weights (ranging from flashlights to full size lanterns). A light with a radio combination seems to be very popular for emergency use. Many models sell for less than $10.

        1.  Almost every hand-held wind-up-power-device I’ve seen or used is terrible, absolutely bloody terribly, to actually charge. If they had some sort of full sized handle and a high resistance, they’d be lovely, but having to use two fingers to turn a tiny little handle in a tiny little circle close to the actual body of the object means a lot of incredibly uncomfortable tedium, weak results, and (in my experience), a much higher chance of injury than this device would cause (although the injuries are generally slight).

          What I’d really like is is a “wind-up box” of some sort for these items, where I can stick them in, comfortably wind them up with the human-usable windup-up box, and then carry them around independently.

          1. “What I’d really like is is a “wind-up box” of some sort for these items, where I can stick them in, comfortably wind them up with the human-usable windup-up box, and then carry them around independently.”

            Fantastic idea!

          2. Yes, I’ve wanted to create (or, really, have someone else create) a nice generic dynamo with a solid, high-resistance winding mechanism and (maybe) some kind of gearing to allow you to produce more or less power without having to change your speed much beyond whatever is mechanically optimal for you. 

            Bonus if you could trivially hook it up to your stationary bicycle.

          3. The squeeze-powered, spring-based flashlights are quite widespread and require no fiddly motor movements.  In fact, squeezing them is incredibly satisfying.  Highly recommended.

        2. Now you’re just inventing problems. If you give yourself a back injury handling a 20-lb. weight, your survival chances in the developing world are basically already zero. Get a little girl to do it for you.

          1. If you give yourself a back injury handling a 20-lb. weight, your survival chances in the developing world are basically already zero.

            Please provide a citation showing how people that get back injuries due to repetitive lifting have no chance of survival. And also explain why you don’t think they deserve to live.

            Get a little girl to do it for you.

            Explain or delete this extremely crass comment.

          2. bzishi, you’re protesting too much, dude. If you find those horrid tiny plastic cranks easier to use than lifting 20 lbs once every half hour, that’s fine for you, and it’s pointless to argue over preferences. However most of us, I strongly suspect, would not agree with you.

            (And yes, most little girls can certainly lift 20 lbs (for various definitions of “little).)

          3. Between the comment about how we’re “spoiled” by bright light bulbs and the comments about how much we should be able to lift, I’m starting to think that old people, disabled people and children just don’t deserve to have light.

        3.  I would be willing to bet that you have never used one of these spring wound devices. If you had you would know how unpleasant these devices to charge. I would much rather move a 20lb weight every half hour (which would take about 5 seconds), than to endlessly crank one of those emergency flashlights, not to mention dealing with the handles breaking which has happened to both of the ones I have owned.

        4.  If you think a metal spring is not significantly complex and fragile compared to a sandbag, well, I deeply admire your smelting, casting, forging, and heat-treating skills.  If you think a nonmetallic spring would last longer and work better than a sandbag, I have to disagree.

    2. Advantage: lifting something heavy for two seconds takes considerably less time than extracting the equivalent amount of energy from a wind up device. 

      1) Its not hard to support 10kg from above. 
      2) If you’re getting a back injury from lifting 10kg then you’re lifting wrong.
      3) A spring is still capable of falling on you. (That point is just for giggles).

  7. That is certainly an interesting amalgam of technologies but I’m not sure I would call the application of centuries old technology used to power the clocks of yesteryear a “radical” technology. Certainly wish them all the best :)

    1. Perhaps it’s radical in the sense that we’ve pretty much abandoned this technology except for fancy grandfather clocks. I’ve never before heard of this technology being used to provide light.

  8. you can play with the maths on wolfram alpha.*+1+meter+*+%28acceleratrion+due+to+gravity%29+%2F+1+W (if that url gets eaten try “10 kg * 1 meter * (acceleration due to gravity) / 1 W” )

    i assumed a 10 kG weight, 1m drop, and a 1W LED (less light than you would typically use to light a room, but probably brighter than the kerosene lamps) and perfect efficiency (should be pretty good). that only gives a 90 seconds. Any ideas of the better numbers to use.

    still a smart and cheap idea.

      1. Not really. A typical household lightbulb produces approximately 700 lumens. Car headlights vary greatly but are typically between 1K and 3K lumens (their reflective interior surface helps maximize the directional use of this light).

        The *best* performance I see right now for a 1w LED bulb is ~200 lumens. Good (and certainly more efficient than incandescent), but nowhere near as bright as a typical household light. Remember that that’s the best performance and a single bulb of that quality is more expensive than this entire setup. And dropping it to .1w would see a proportionate drop in output.

        1. You mean a “typical” *first world* household lightbulb, which are extremely bright by historical standards. Your 700 lumens are the equivalent of, what, about 50-60 candles?

          Everybody reading this now is spoiled for nighttime illumination.

          1. Uh, did you know that a 50 year-old needs twice as much light as a 25 year-old to see the same thing? So half the developed world can’t read a book with 700 lumens. I wouldn’t call that ‘spoiled’.

          2.  I’m in the age group and I need more light than I used to, but nowhere near double.  As for this light it looks like plenty to cook, dress, and care for the kids by.  Reminds me of camping lights, and we managed with those.

          3. I’m in the age group and I need more light than I used to, but nowhere near double.

            Your anecdote notwithstanding, an average 50 year-old needs twice the light.

          4. I think a number of commenters are missing the point that the standard to beat here isn’t a 60W incandescent bulb, it’s a kerosene lantern. I wonder if many of them have actually tried to read by the light of such a lantern. I have. A 60W bulb, it ain’t.

            Perfect is the enemy of good.

        2. Yeah, but a 1W LED is still far brighter than what they show in that video. My bike light takes a 1W LED, is blinding when viewed directly, and cost me $30. Granted, the LED itself only costs $2-3, but you still need to power that thing, and, as you say, it wouldn’t make sense with this.

          So, as OoerictoO says, it makes much more sense assume this is a 0.1W LED…

  9. I think the effort of weighting up the bag every 30 minutes could be lowered by having pulleys integrated into the design of the “lamp”.

    1. Nothing you can do, can get extra free energy.

      There are already pulleys/gears in it.
      Only thing you can do is double the weight so it will double the time/or watt as you will be doubling the gear ratio in response.
      But energy put in (by lifting the bag) is the energy you get out.

  10. I’m going to make a kerosene-powered machine that lifts the weight back up every thirty minutes and sell it for $30. I’ll make a fortune!

  11. A kerosene lantern puts out between 10 and 40 lumens, depending on the flame size.  Using a decent, modern neutral to daylight white LED with relatively common 100 or so lumen per Watt output, this means a .1 to .4 watt LED would be sufficient.  

    If we go with a bright lantern equivalent of .4 watts, then we have a time of operation of mgh/x=.4, where m is the mass, g is Earth’s surface gravity, h is the total drop, and X is operating time in seconds.  So, with a ten kilo bag and a one meter drop, so, rearranging that, we get 10*9.81*1/.4=running time in seconds, which gives 245 seconds, or about four minutes.  This is assuming the gearing, power generation, and power stabilization for the LED are perfectly efficient, which they won’t be.  I would estimate 80% efficiency if you do a good job of all that, so now you’re down to 3.5 minutes.     Going with a superior LED, from an efficiency point of view, you can get up to 170 lm/W, which improves things quite a bit. Your CRI will be lower, about like cheap fluorescent lighting, so it’s not the most pleasant light, but it would be effective from a utilitarian illumination perspective.

    Now you’re doing six minutes per run, with a drop of only one meter.   If you’re hanging it higher, and letting it fall two meters, you’re doing 12 minutes, and, if you scale back the output to make it more of a medium lantern output instead of maximum lantern output, you’re getting 24  minutes per run.   So, that’s starting to look not too bad. 

    A very cool feature would be a transmission so that you can trade off drop rate for brightness, but that would add to the cost, even if it were something simple like a stack of different sized pulleys with a drive belt and spring loaded tension idler to take up the slack.

    I think it’s a viable idea, but meeting the target of cost, decent run time, and durability will be a challenge.  I could easily see making something like this for $20-30, but getting it down to $5 will be tricky.

    1. Actually, that makes me wonder: they show a brightness dial on the machine, and mention it in the video. Are they changing the gearing with it, or just dimming the LED and wasting the extra energy?

      1. They have to be changing the gearing.   They say that when it’s dim it goes for 30 minutes, if you want it bright it doesn’t go as long.   I can’t think of any way to make this happen other than selecting different gear ratios.

  12. It will end up selling for $50 on Sharper Image or from some company that makes $500 “ecologically friendly” briefcases from left-over joints. (Remember how hemp was going to change the world by being cheap and available? Ever price hemp goods?  Yuppy luxuries.)

    Stuff designed for use by the extremely poor rarely actually gets there since it’s hard to make money off them and there’s usually four layers of corrupt government between you and them that has no interest in them becoming better off unless they get their squeeze.

    1. Assuming is not flat-out lying, your hemp example is pretty terrible, since there are severe legal barriers to growing hemp. (You have to have a DEA permit.)

    2. The hemp example is terrible. Hemp used to be a cheap and well used crop. Then it was made illegal virtually everywhere, now growing hemp is highly restricted and as a result, demand outstrips supply. If hemp was perfectly legal to grow you’d expect hemp goods to be far cheaper.

    1.  They say in the video that they hope to eventually get the price down to $5.

      People will be able to spend what they would have spent on kerosene on other goods. Perhaps a local shopkeeper will stock less kerosene and sell more food, medicine, etc.  It’s not like their kerosene is refined/produced locally.

      And this wouldn’t be a complete replacement of kerosene lamps. If someone needs portable light outside the home, for example.

    1. Well, the NiMH battery has a constantly decreasing effectiveness that eventually requires you to pay to replace it. The whole point is to remove the ongoing cost of kerosene, not simply replace it with a slightly cheaper alternative.

        1.  Maybe, but grandfather clocks have the same kind of moving parts, and they seem to run for an awfully long time.

        2. Not necessarily; it depends on build quality.  I have a thirty year old electric tractor (DC) and a 1959 table saw (AC) with the original motors; they’ve both been in constant use and still work fine.  The DC motor has had several brush replacements.  A good electric motor or generator will outlast any small battery by decades.  If it’s brushless, even longer.  And you can replace low speed motor bearings with indigenous materials – the arbor vitae bearings in big Francis turbines have been known to last a hundred years in heavy use.

          Now, BIG batteries can last as long as a good electric motor, but you won’t want a half-ton iron chlorine cell in your shanty.

        3. You’d be surprised – and more importantly, an owner/community can be taught to repair and maintain a mechanical system themselves. They can’t be taught to be batteries.

          1. Sorry but I think thats a little bit condescending. Are the users of this device necessarily better at understanding 19th century technology than 20th century technology?

          2. No, I just think they’re as equally incapable of literally becoming NiMH batteries as the rest of us. 

    2. Did you watch the video? It addressed your statements directly

      PV cells will only gather power during daylight hours, limited by cloud cover, etc and have no opportunity for recharging without sunlight. They also cost more than any part in this system. Batteries wear out, and require replacement every few years. They’re also relatively expensive.

    3. Well, for one thing NiMH batteries can’t be fixed by poor people.  Neither can LEDs or solar panels, but those are both significantly more robust.  But then again PV sucks hard during dust storms, and blizzards, and week-long rains; you can keep a sandbag and dynamo indoors and not have to expose yourself to harsh weather.

  13. for a stationary light, wouldn’t a small FLYWHEEL make more sense? use an old bike wheel, fill the tire with water or sand or something – spin it up for five seconds and it should be good for hours – heck, just provide a low friction bearing and hub that can connect with the “chain” part of this light and let people make their own “wheel” out of whatever, all it has to do is balance and spin

    1. Getting a very low friction bearing cheap enough to be bought and used in the third world is probably going to be the hardest part of this.

      1. a marble? a pointy chunk of metal? a wad of Russian wool as mentioned above? 

        wouldn’t need to be *very* low friction, just low enough not to be a burden

    2.  Essentially you would be replicating an old style bike generator. If you have ever played with one of those, you would see the drag from the generator slows the wheel to a stop in less than a minute. Even if you added weights to store more power and made the generator drag half as much, it is unlikely that the light would last over a few minutes.

  14. reminds me of the world in “the windup girl” – an entire economy running on stored kinetic energy devices

  15. Technically speaking, that’s not gravity power; it’s bacon sandwich powered (or whatever the operator ate to provide the energy to raise the weight). which I suppose makes it solar powered, or essentially the fusion of hydrogen, as that’s what powers photosynthesis and is therefore the source of (amost) all biological power. The gravity bit is just an interim store of potential energy…

    1.  As your comment subtly hints, the very same thing could be said of the bacon sandwich, the pigs and wheat from which the sandwich is made, the photons emitted by the sun, and human beings themselves — and just about everything else in the universe.  What makes this device unique is the fact that its “interim store of potential energy” is in the form of a big rock being worked on by gravity.  Hence “gravity-powered light”.

      1. Also, the fusion that runs the star that emitted the photons that grew the wheat that fed the pig that you ate to get the calories to lift the rock comes from a gravity well so steep that it fuses hydrogen into helium.   So, gravity light.   Of course, by that definition all lights are gravity lights…

    2. Everything is solar powered.  Coal comes from dead plants created with solar energy.  Wind is caused by solar energy. But in the end it all comes from thermonuclear fusion.

      1. Well, there’s fission power. Yeah, the fissile materials are also a fusion product, but not from the sun, so not “solar”, strictly speaking. And at this level of hair-splitting, we’re definitely speaking strictly.

  16. For more robust devices, a second pulley and weight would allow someone to lift the weight by pulling down on the rope like a bell ringer, which would be several fold more efficient using body weight instead of arm strength. 

  17. It’s not powered “by gravity”, you ignorant klutzes, any more than we’re fed by Sun. Yes we are fed by SUn, you’d say. Yes, we are, but not through our solar antennae but by eating produce and animal flesh which was fed by…..which grew in Sun. Similarly, this “invention” is powered not BY gravity but directly by a dynamo wound to speed by falling weight. A far cry from your sensationalistic “gravity powered light” at zero cost. Especially if (WHEN) it breaks and family has to buy a new one (and couldn’t afford even the first one to start with). Whatever!

  18. Seems like a good idea, ie. freeing up funds that were normally spent on kerosene.  It would be interesting to see how if it could be used to say re-charge a cellphone battery. Cell phones have led to big changes in Africa for instance, and many people use them to send money and banking as well as business opportunities to re-charge peoples phones etc.  

    This also reminds me of that project (merrygoround water pump)  that was really popular a few years back but didn’t really succeed for various reasons)  I still wonder why they didn’t use the windmill style pumps that were pretty much on every farm up until the 50s’.

  19. Big muscles work better than the small muscles that power typical hand-crank flashlights.    Or I wonder if this could be powered by rain in rainy climates.

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