Electrical – How does phone charger output current relate to input current

power

Typically 1 charger takes a 120V input, and produces an output of 5V and 2A, so its power consumption is 10W. Theoretically a single power outlet (20A limit on breaker) can supply 120V * 20A = 2400W, which is 240 chargers connected to a single power outlet to overload it.

But one single charger can already deliver 2A, and each power outlet can deliver at most 20A… I am confused how these add up, because it seems like >10 chargers can already deliver more current than a single power outlet.

Best Answer

In a switching converter (which 99.99% of phone chargers are), the power going in and power going out is nearly the same, less a little bit for efficiency. So in a buck-converter with 120Vrms input, and a 5V, 2A output, the output is 10W and the input is ~11W (for a 91% efficient converter, let's just use that as an estimate). Therefore, if the input voltage is 120Vrms, the input current is 11W/120V = ~92mA.

So, yes, 20 adapters can deliver more total current than a single wall outlet, but at a lower voltage, so power is conserved (and physics is happy).

As an aside, I don't know if you plan to plug 240 USB adapter into a single circuit, but I don't recommend doing that. Additionally, 92mA is wildly optimistic. Check your USB adapter for its AC current consumption. I happen to have one in front of me with the following specs: INPUT 100-240V~50/60Hz 0.2A, OUTPUT 5.1VDC 850mA. Which implies that plugging 100 of them into a single breaker is enough to cause trouble, among other things.