Science Questions

How long to charge a human by USB?

Sat, 25th Jul 2015

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Aaron Fuller asked:

If a human could be fed with energy via USB ports rather than food (like the synths in Channel Four's 'Humans) how long would it take to fully charge an average human with, say, 2000 calories of energy?


Chris Smith plugged in to answer this question...

Chris - Those 2000 Calories are "big" calories because an average person uses between 2000, 2500 Calories a day. So I thought this is quite interesting. USB flash driveWith USB 3 you can draw up to 3 amps. Given that itís at 5 volts, that means that the rate of energy delivery from a USB is a maximum of 15 watts Ė in other words, 15 Joules per second.

Now, one "big" Calorie - like is in food - thatís equivalent to 4200 joules. If you want someone to consume 2000 big Calories in a day, which is your average, thatís 2000 times 4200 joules. Thatís about 8 million joules.

To work out how long it will take in seconds to get that much energy into someone with a USB port then you have to divide 8 million by 4200. So, thatís about 53000. In other words, it takes you 6 days to get one dayís worth of energy - equivalent to one dayís food ration - into you. Probably not going to be a very sustainable means of nutrition unless we can up the wattage; a person runs at about 2 watts per kilo. So, on average, a 50 or 60-kilo person is running at about 100-watt. Therefore, weíve got to find a way of delivering energy at that sort of rate. I think Iíll stick to food. Food is nicer.

Ewen - Why donít you use more plugs, you could have several plugs to plug one person...?


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2000 Calories (capital C = kilocalories) is about 10 megajoules.

If you USB port delivers 5 volts then it will take 2,000,000 ampere-seconds to deliver 10 MJ. A 10 amp USB charger will thus require 200,000 seconds or 56 hours to recharge a human.

However most of the 2000 Calories you eat each day, is used to keep your body at its working temperature. If you were replaced by an entirely electrical system to do the same amount of useful work, you could be recharged in 2 or 3 hours. alancalverd, Wed, 29th Jul 2015

I think that should be 8 million divided by 15 (not 4200)? BobonToast, Wed, 29th Jul 2015

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