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I'm trying to get an idea of how many pi's I can run at the same time from the same wall socket. I've done some research and it seems to me that the peak power consumption is somewhere around 750mA. If that is the case, then in theory 20 Pi's can be connected to one 20A circuit at my home with no problem. (I'm thinking about connecting them all via 2 10-port, externally powered USB hubs, if there's an issue there, let me know). I just wanted to run this configuration by some smarter, more experienced people when it comes to power requirements.

  • Your calculation is wrong. Those 750mA are at 5V. That gives 31.25mA on the 120V side of each of your power supplies. And no, connecting them to USB hubs won't work, as those hubs usually do only supply 500mA@5V per port. – Janka Nov 9 '16 at 16:29
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    Besides calculations, you will in reality have a tough time finding a 10 port externally USB powered hub that actually supplies 750 mA to each port. You will most likely need to build that yourself, I suggest that you get a beefy PC PSU. – jogco Nov 9 '16 at 16:34
  • You could probably go cheaper than a PC PSU using closed frame switched mode supplies from eBay, but you'll have a little more work to do making it safe. – goobering Nov 9 '16 at 16:46
  • @Janka I think those hubs usually do not actually regulate per port. However, if there are 10 ports and a 5 A supply, you obviously won't be able to draw more than 10 A in total. – goldilocks Nov 9 '16 at 16:56
  • It depends. The USB spec requires it and all the hub chips I know of can do it. If the circuit designer actually uses that function - don't know. – Janka Nov 9 '16 at 16:58
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The instructions below Assume you mean 120volts at 20 amps which is the American standard. So we want to start by calculating the approximate wattage we could theoretically draw from the wall, which is volts x amps.

120x20=2400 watts

Now we are going to calculate the number of watts a Pi uses at peak load based on your research. We use 5 volts as that is the USB standard

.75x5=3.75 watts

Now all we need to do is subtract inefficiencies in the conversion from AC to DC which is approximately 10-20% for a good power supply, we will use 15% for our calculation. So now we multiply household wattage by efficiency(.85) to get the number of watts that are actually going to reach your Pi.

2400*.85=2040 watts 

And now we decide this by the number of watts a Pi uses to see how many pi's we could theoretically connect.

2040/3.75=544 pi's

There you have it 544 pis could be connected to a single 120 volt 20 amp outlet.

  • Thank you. This is great help! I'm a mechanical engineer and have forgotten my basic circuits calculations from undergrad. I can make you a box, but I have trouble remembering how to power things. – Danny Nov 10 '16 at 17:22
  • @Danny glad to help please click the check mark to the left of my question if it has helped you to let others know that this is a correct answer and to give me a few points – Mohammad Ali Nov 10 '16 at 17:23

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