Please forgive me if this isn't the right part of Stack Exchange or if my knowledge is poor. If you need to tell me about anything scientifically I encourage you to simplify it so me and other laymen can understand.

I notice that robots today need a lot of energy so I decided to run a little test in a hope to find solutions by seeing how much energy a new robot idea I had would need. This idea was a robot that, although completely non-biological, mimics the energy needs of a biological human to fix the aforementioned problem. I'm not sure how this could be done, though I assumed it's things like making the robot's brain (and the rest of the body) have similar wattage to a human's brain and taking inspiration from biology. I assume organisms can't purely run off electricity, so NO PART OF THE ROBOT IS BIOLOGICAL.

I tried to do some math with help and the answer we got is that a 1kg 700 watt-hour battery would run the robot for decades. That sounded so wrong to me but my friend told me that humans are just way way lower energy than even everyday machines. As a comparison, 2kg of those batteries can allegedly run a vacuum cleaner for only an hour! It looked to us like there was something fundamental about non-biological machines that makes them really consume a lot of energy and I really didn't know how to fix that.

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    $\begingroup$ I don't have a good reason to flag your question. However, it would be helpful if your specific question were asked (or repeated) in the body, so that what you want to know is clear to other readers who aren't necessarily invested in your problem. $\endgroup$ Jan 23 at 5:06
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    $\begingroup$ "I tried to do some math with help and the answer we got is that a 1kg 700 watt-hour battery would run the robot for decades." It would really help to show your math so we can point out where the error is. In addition, going through it and re-writing it in another form may make something stand out. A lot of times when I'm stuck on a problem and ask a friend to help me work it out I figure out the issue while explaining it to them. $\endgroup$ Jan 23 at 17:57
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    $\begingroup$ A 700 Watt-hour batteries worth of power. thats only 602.294 KCalories ('Calories') That would last a human less then 1/3 of the day. If the human was just sitting there doing nothing. $\endgroup$
    – Questor
    Jan 24 at 15:29
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    $\begingroup$ @Matthew That's fair. metabolic resting state of an adult human male is 1300 Calories, on average some people are higher some are lower. So its really more like half a day. But my point was "where is the 700 watt hours number coming from? $\endgroup$
    – Questor
    Jan 24 at 23:33
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    $\begingroup$ @DKNguyen A Human is also performing self repairs during that time. A robot isn't and a robots internal self maintenance/repair might be even more expensive then a humans. $\endgroup$
    – Questor
    Jan 25 at 16:02

3 Answers 3


You probably got confused between calories and dietary Calories, which are actually kilo-calories.

An average person is on about 2,000 kcal a day - which is approximately 8.37 million joules.

700 watt-hours is 2.52e+6 or 2.52 million joules.

For reference - that's about me doing a 35 minute jog on a treadmill at 9 kph.

On an equivalent of 2,000 kcal per day - your robot needs 2324.444 watt-hours of power.


There are things we humans have to do, that your robot doesn't - we consume power even when we are sitting down, we have to keep our body temp within a specific range etc.

And that actually uses a lot of energy - so realistically, you could probably halve it and use 2x 700 watt-hour batteries for a 24 hour period.

  • $\begingroup$ "we consume power even when we are sitting down" <- robots do this to. Yes, a robot can be turned off without dying which is nice, but a robot that is "awake" and idle consumes a lot more power than a human that is awake and idle. $\endgroup$
    – Nosajimiki
    Jan 25 at 22:24
  • $\begingroup$ @Nosajimiki - I gotta disagree - a computer that is on Standby mode uses 15 watts, a Human sleeping around 100 watts. A Human sleeping is about the same as an average laptop in use. $\endgroup$ Jan 25 at 22:52
  • $\begingroup$ A laptop uses 15 watts, yes, but not a robot. A robot in standby mode is still actively running an AI to process sensory input and make decisions as to if it should stop being on standby and start doing something. This uses a LOT more processing power than an idle laptop because of the nature of neural-networking software. It also takes power to hold a robot in a standing position (assuming humanoid features); so, just standing still so that you don't fall over consumes power. Boston Dynamics robots for example idle at about 1kw to just stand and observe thier environment. $\endgroup$
    – Nosajimiki
    Jan 26 at 16:37
  • $\begingroup$ @Nosajimiki the boston dynamics robot is not in standby when using 1kW, it is active, but unmoving. Real standby would get the humanoid robot into some metastable position, ideally lying down, but mayhaps just locking knees and setting the head onto the ground, and then power down the servos. Then the processing hardware goes into standby, meaning keeping the RAM powered, and possibly a watchdog routine on some subprocessor going online every 100ms for 1ms to check for some wakeup signal. 15W is actually rather wasteful for that. Still 24h*1000(error) is not 'decades', it's three years. $\endgroup$
    – bukwyrm
    Feb 1 at 21:39
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    $\begingroup$ @Nosajimiki this comment thread is already getting a bit longish. Again, we might have differing definitions of standby. But i, also, have robotics experience. And in the state that i call standby (no posture/position is held, robot will come back online without boot on button press) the power draw for multi-kw robots is in the single watt digits. Which makes sense, because nothing apart from some memory keep-alive and some temperature watchdogs is actually getting power. $\endgroup$
    – bukwyrm
    Feb 7 at 20:19

702 watt-hours is about 602 food/kilocalories, and an arbitrary average human needs about 2,000 per day.

If there's something fundamental to machinery in general that makes it consume more energy by weight than a human, than that's probably because machinery is often purpose-built for specific tasks where the task is more important than the energy usage, whereas humans (and living things in general) evolved over a very long time in a way which prioritized surviving to make babies and therefore lower energy usage — needing less food comes in handy. There is likely no way to fix this. If you have a humanoid machine, it's probably made of things which are denser than human tissue and therefore require more energy to move around.

If you need a power source I recommend an improved version of the Advanced Sterling Radioisotope Generator, which, with fresh plutonium-238 fuel bricks, can provide roughly enough power to run a human's base metabolism and is close to (if not quite) small enough to fit in a human-sized humanoid robot's torso. Presumably one designed with technology from perhaps twenty years in the future could be a bit more powerful and a bit less large. It will, however, be very heavy by human standards — 32 kilograms — and it will slowly degrade over time due to the radioactive decay of its fuel. Perhaps the robot gets a visit to the doctor, so to speak, every five years or so, for semi-spent fuel to be removed and fresh plutonium bricks to be inserted.

If you need an energy storage I recommend a flywheel, specifically one made of something very tough, rotating incredibly quickly, suspended by magnets, and inside a sealed-off vacuum chamber to minimize energy losses from aerodynamic drag on the flywheel. Again, the current state-of-the-art isn't really energy-dense enough or small enough to fit in what you seem to be talking about, but it's close enough, and flywheel storages have many advantages over chemical batteries.

I have no idea how much power or energy your robot actually needs (it's probably not going to be comparable to a human) because I have no idea how heavy it is, what type of actuators it uses, etc.

  • $\begingroup$ That sounds like a serious problem for infantry-sized future robots on months-long campaigns without replenishment. Both of your ideas are way too large and heavy and thus there seems to be no way around other than 'more advanced batteries'. Without those advancements, I suppose the takeaway is that organisms seem to be more the way to go. $\endgroup$
    – Terra
    Jan 23 at 4:21
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    $\begingroup$ @Terra when you understand how energy-efficient a biological organism is when compared to machines of the same weight and strength, its strategic advantages start to weigh against its tactical shortcomings. $\endgroup$ Jan 23 at 5:13
  • $\begingroup$ @Terra Also, even though orgamisms need continual replenishment, the way that cells and tissues function essentially means that the machine and the power source are two systems sharing the same structure, distributed across its mass synergystically. We don't build machines that way because we don't imagine them that way - we haven't had to before, we've been able to make do with naturally occurring denizens for millenia. $\endgroup$ Jan 23 at 5:18
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    $\begingroup$ @Terra Note that organisms are also useless "on months-long campaigns without replenishment" - make a soldier, or a horse, go days at a time without eating, and you'll soon run into problems. If an army marches on its stomach, a robot army would march on its batteries. In both cases, what you need is supply lines allowing the energy to reach the front in the form of food / fuel / replacement battery packs. $\endgroup$
    – IMSoP
    Jan 23 at 17:58
  • $\begingroup$ "machinery is often purpose-built for specific tasks where the task is more important than the energy usage" - in fact doing more work by expending more energy is more often than not the desired behaviour. You would rather have the vacuum cleaner's 2kg battery get drained in an hour than having to work 4 hours to do your cleaning. $\endgroup$ Jan 24 at 10:12

Total power consumption is about 7:1 when comparing states of moderate physical labor or 9:1 when comparing idle states.

A Boston Dynamics Atlas robot is a humanoid robot with a human like body mass of 190lb (85kg). It reportedly can run for about 1 hour using its 3.7kW/hr battery doing normal physical exertion including walking, standing, use of tools, and other movements or it can idle for about 4 hours.

According to this source, a human in this same weight range would consume about 450 kcal per hour performing similar activities which translates to 0.52 kW/hr or about 85 kcal/hr (0.099 kW/hr) when awake and doing basic sensory processing like watching a TV show or reading. This means that if you were to replace all of the systems on a current Atlas android with something as efficient as biology, you could get approximately 7 hours of work or 37 hours of idle time out of the battery packs we already use.

So no, you will not get decades of battery life (you definitely had a math error here), but you are correct that biology is significantly more energy efficient than modern robotics.

Why muscles are so much more efficient than cybernetics

Even though the electrical motors that we use to move a robot tend to be over 75% efficient in terms of converting electrical energy into kinetic energy, there are a lot of sources of inefficiency outside of the actual motors.

When it comes to making a motor that turns electricity into rotational energy, like turning the wheels on a car, mechanical engineering is pretty reliable, but moving something android shaped means that you need to perform a lot of contractions and rotations in ways that don't leave room for motors at the point of articulation; so, to engineer an android using electric motors, it means that we need to use a lot of hydraulics, belts, gear systems, axils, and other intermediate systems to translate the energy from the motors into the kinds of articulation that we need.

However, when we compare this to a muscle, the entire mechanical system between 2 points IS the motor; so, there is no wasted mass and resistance from intermediate mechanical systems.

But Why Do Humans Idle Better Too?

A human sitting around is doing all sorts of complex metabolic stuff, so the idea that robots, without metabolisms, consume so much more power than we do when just sitting around sounds counter-intuitive, but it is true.

For starters, just standing or sitting will still consume some energy. It takes energy hold an electric motor in place against the resistance of gravity. It takes energy to perform the micro adjustments to rebalance your body when trying to stay upright against changes in the environment. It takes energy for your robot to run all of the cameras, microphones, gyros, pressure sensors, and switches required for it to understand its environment and position in space.

There is also all of the power lost to induction currents. Electricity does not just stop flowing because you turn off a switch, the switch creates an air gap that makes the electricity flow much more slowly. Even if you turn an average robot completely off, if you do not unplug the battery, it's charge will run out within a few weeks as electricity jumps the gap creating by the power switch.

There is also the problem with robots that they don't think nearly as efficiently as we do. The human brain a true neural network, but modern robots use linear digital networks to simulate neural networks which is a grossly inefficient way of doing it. So, the power it takes a robot to process it's inputs is higher than that of a human brain. So, if you want a robot to be physically in standby, but still processing its environment, it will use a lot more thinking power than a human does.

How to make an Android as efficient as a human

To make an android able to do human things as efficiently as a human, you first need a fibrous material that contracts when electrically stimulated. There has been a significant amount of research in recent years studying Electroactive polymers (EAPs) and Shape-Memory Alloys (SMAs) to replicate the way that muscle fibres contract, but these technologies are still mostly in the experimental phase of implementation. But if you want to write a story about androids with human levels of efficiency, saying that they use EAP or SMA based synthetic muscles, is going to be very plausible.

Next, you need to address the efficiency of computers. If you want to really make your robot approach human energy efficiency, part of your design will need to be a whole new kind of computer processor designed specifically for running AIs on much less power than modern computers use. Such proccessessors are already being designed, but as of yet, they are still theoretical. The idea behind them is that modern computers are all designed for linear mathematics which makes them really inefficient at hosting the neural network programs used by AIs; so, if you were to design a whole new kind of computer chip designed for massively parallel processing more like a human brain, you could get your idle efficiency as good, if not better than a human.

The last efficiency issue is the battery, but this is more of a non-issue than most people give it credit for. People like to talk about how much more efficiently bio-chemical energy can be stored than using batteries, but batteries come with some meaningful advantages to. Most notably: they don't need a digestive system to support it. So, going back to our 190lb human, you are looking at about 25-35lb (11-16kg) of digestive organs and feces before you even get to any actual energy storage whereas all an android needs to recharge is some kind of power socket. Add in about 45lb (20kg) of body fat at an average healthy BMI, and a 190lb human body devotes 70-80lb (31-36kg) to its power system.

Once you have a good synthetic muscle to make your android out of, you can eliminate a lot of the wasted weight caused by our current clunky mechanical systems I previously brought up. This means you can focus more internal space/weight on power storage making a 70-80lb (31-36kg) lithium ion battery far more reasonable than in most android designs to date. This gives you about 22-25 kW/hr of power storage meaning that the android could actually perform about 2 days of continuous activity, 10 days of idle activity, or a couple of months fully powered down but with the battery in.

  • $\begingroup$ How does this work out? Because motors can be 90%+ efficient. So where is the 7x reduction in power consumption coming from? $\endgroup$
    – DKNguyen
    Jan 25 at 4:04
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    $\begingroup$ Because batteries are dead-weight with bad [specific energy/energy density], and even worse, it is constant regardless if your battery is full or empty (compare to human stomach). E.g. you typical lithium-ion battery might have specific energy of 0.8 MJ/kg, while body fat has 38 MJ/kg. Of course you could make it run on gasoline (46 MJ/kg) or even liquid hydrogen (120 MJ/kg!), but those require bulky, heavy and needy combustion engines (cooling), so don't work well for human-sized robots (but truck-sizes robots are much better match). $\endgroup$ Jan 25 at 4:31
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    $\begingroup$ @DKNguyen also, electric motors are heavy, due to being mostly metal. Just because they have relatively good mechanical_motion/heating ratio (i.e. efficiency), does not mean their mechanical_motion/mass ratio fares as good - they are made mostly of metals, and metals are heavy... Also while you can use more of human-alike sugar/fat burning and energy transport mechanisms to avoid bulky and heavy rechargeable Li-ion batteries, but then your robots becomes being more like human replicants with few prosthesis enhancements than "real" robots... $\endgroup$ Jan 25 at 4:41
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    $\begingroup$ @DKNguyen MN is correct about the mechanical systems. I coach my daughter's robotics team, and the amount of power loss the kids see for every gear, belt, and axel they add to thier designs is very significant, and engineering something as complex as a hand can not be done with a bunch of direct motor connections because they would not all fit where you need them to fit. I've expanded on my answer to cover this. $\endgroup$
    – Nosajimiki
    Jan 25 at 15:46
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    $\begingroup$ @DKNguyen Muscles are bad at doing motor things, and motors are bad at doing muscle things, you lose efficiency either way when you try to use one to do the other's job. Also, horsepower is a bit of a misnomer. It is the linear power of a walking horse. most horses max out at 10-14 horse power iirc. $\endgroup$
    – Nosajimiki
    Jan 25 at 17:02

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