this post was submitted on 17 Sep 2025
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How come we can't design energy storage that lifts something heavy when there's excess power, and lets it fall to generate electricity when needed?
We have, pump storage hydropower: https://old.reddit.com/r/askscience/comments/chm70g/askscience_ama_series_were_from_the_pacific/
Basically they store water up high to act as a battery. Some combine this with a solar lens and turbine (can be sourced from old tvs, it'sa Fresnel lens for a solar death ray) and boil the water with the sun/ray to get it to evaporate and then condensate in the elevated position.
Potential energy (in joules) is mass (in g) times height (in meters) times 9.8 m/s^2 .
So in order to store the 30 kWh per day that the typical American house uses, you'd need to convert the 30 kWh into 108,000,000 joules, and divide by 9.8, to determine how you'd want to store that energy. You'd need the height times mass to be about 11 million. ~~So do you take a 1500 kg weight (about the weight of a Toyota Camry) and raise it about 7.3 meters (about 2 stories in a typical residential home)?~~ (this is wrong, it's only 0.001 as much as the energy needed, see edit below)
And if that's only one day's worth of energy, how would you store a month's worth? Or the 3800kwh (13.68 x 10^9 joules) discussed in the article?
At that point, we're talking about raising 10 Camrys 93 meters into the air, just for one household. Without accounting for the lost energy and inefficiencies in the charging/discharging cycle.
Chemical energy is way easier to store.
Edit: whoops I was off by using grams instead of kg. It actually needs to be 1000 times the weight or 1000 the height. The two story Camry is around a tablet battery's worth of storage, not very much at all.
There seems to be an error in your calculation: Up to the 11 000 000 kgm required it is correct. However the Toyota Camry with 7.3 m provides only 11 000 kgm. So you miss a factor of 1000. You would need 1000 cars lifted the height of your home. For just one day (or a few days in more efficient home)
You're absolutely right.
I don't know why I thought to use grams instead of kilograms. I knew kg was the base unit for these conversions but just slipped for some reason.
Honestly that is way, way more reasonable than I was expecting. This isn't half as bad of an idea as I thought it would be
Sorry whoops I was off by a factor of 1000 because I used grams instead of kilograms. The Camry needs to be raised 7.3 km. Or you need 1000 of them in one house.
Pumping 1500L of water up into a tower doesn't seem difficult or expensive.
Actually, yes. Lifting the weight of a Toyota Camry 2 stories seems reasonable for a day's worth of energy storage for a house.
I'm not sure how expensive the lift and generator will be, but the weight itself can be anything that's sufficiently heavy.
You say chemical energy is way easier to store, but is it really easier and cheaper to store the energy needed for a home in a chemical battery?
Actually I was off by a factor of 1000. That Camry needs to be raised to 7.3 km. Or you need 1000 of them. Or some combination of increased weight and height.
And not just that, but safety and cheapness akd accessibility of the materials. Water is pretty cheap and common. If it spills, probably no big deal (flooding notwithstanding) unlike battery acid. Not as likely to explode (sometimes steam explodes stuff).
Yes. A 5kwh battery is about 50kg and smaller than a carry-on suitcase. String 6 of them together and you've got 30 kWh stored with no moving parts. Anker has that for about $15,000, maybe $30k installed.
How much does a 3-story elevator cost? What about one that can capture the stored potential energy on the way down, and not break down?
Hmm… this might be easier to do with an electric car. Put it on an inclined track, and then drive uphill to store energy, and go downhill to release the energy.
You would just use the car wheels itself if you're doing all that - how do you think it would store energy driving uphill?
Well you want some weight which is why I’m suggesting the whole car but sure if you want some custom solution you can build something better.
It's an idea that's been played with a few times, but there are many energy loss points in such a system, as well as logistics for keeping the "stack" from falling over. The best so far is pumping water up to an artificial lake, but that's still not very efficient.
1 Watt is the equivalent of moving 1Kg 1 metre in 1 second.
If you want a kilowatt - you need to move 1,000Kg 1 metre in 1 second. Or, I guess, 1Kg a Km.
Plug the numbers together and you'll see that you need a massive physical load and a huge distance in order to store a useful amount of energy.
You got your units confused.
1 Watt = 1 J/s = 1 N m/s = 1 kg m^2 / s^3
Just moving things horizontally changes does not take energy (except for friction). But when we move something upwards, we move it against the surface acceleration of earth of g = 9.81 m/s^2. So we can say:
1 W ≈ 0,1 kg m/s
This means to store 1 kW, we would need to raise e.g. 1 ton with 0.1 m/s. So 1 minute of medium power cooking (1 kW), corresponds to lifting 1 ton approximately 6 meters.
This seems like a way different conclusion than the car * 7.3m / day guy
The secret ingredient is gravity!
Hmm
The energy math doesn't make sense for grid scale applications with solid objects.
https://youtu.be/iGGOjD_OtAM
However if you can get water between two places it can work quite well. You need to live close to a big change in altitude and do a bit of geoengineering to create the upper and lower reservoirs, which can be destructive to local ecology, but not as much as a dam.
https://en.m.wikipedia.org/wiki/Pumped-storage_hydroelectricity
You can also use pumped air underwater with higher energy losses than pumped storage hydro because of compatibility of air.
https://electricalindustry.ca/changing-scenes/1785-world-s-first-utility-scale-underwater-compressed-air-energy-storage-system-activated-in-lake-ontario/
It's usually called hydroelectric and implemented with dams and turbines.
Adamsomething covered this a while ago.