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When reading about the electrical needs of various electronic devices and/or appliances, I often see a requirement for only one of the various measures related to electricity. For example, it will say "requires 120 volts" and not mention anything about amps. Or it will say "takes 10 amps" but not mention anything about volts.

Here is just one example from Lifehacker, in an article about the Raspberry Pi:

A power supply: The Raspberry Pi is powered by a micro USB, much like the one you’ve likely used for your phone. Since the Pi 3 has four USB ports, it’s best to use a good power supply that can provide at least 2.5A of juice.

Isn't a statement like that useless? There is no mention of required voltage. 2.5 amps can be delivered in many different ways - i.e. by 5 volt pressure, or 240 volt pressure. I suspect this can produce very different outcomes.

So why are volts and amps often given as though they are completely independent? Why would there be an explicit requirement for one and not the other?

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    \$\begingroup\$ If it's got a mains plug, it's designed to run at mains voltage, if it goes into a car's cigarette lighter socket, it's designed for 12V, if it's USB powered it's designed for 5V because each of these outlets is intended for only one specific voltage. However, if the product has some generic connector (like a barrel jack) then it's considered poor form not to have both V and A ratings (the only exception is if it needs so little power that the amps rating is irrelevant, probably <50mA I'd say) as barrel jacks are used for a whole slew of different voltages (mainly anywhere between 3 to 24V) \$\endgroup\$ – Sam Jan 1 '17 at 20:45
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You are right in that both voltage and current must be specified for a DC power supply. However, sometimes one or the other is omitted because it is implied.

In the case you quote, note that it mentions "The Raspberry Pi is powered by a micro USB". That implies 5 V. The only remaining question is now the current.

As another example, consider a device that has a power cord attached ending in a wall plug compatible with outlets in your locality. Brief documentation for that device might only say that it draws 1.5 A. Since it obviously runs from line power, it might not explicitly say "120 VAC, 60 Hz" (or whatever the line power is in your locality).

More thorough documentation someplace should specify the voltage, current, and any other relavant parameters.

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  • \$\begingroup\$ I think you mean current, not voltage, in your second paragraph. \$\endgroup\$ – Barry Jan 1 '17 at 21:04
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It's going to depend on exactly what specifications are given in each case.

In the example you quoted, when they said it is powered by USB, that implies the voltage is 5 V.

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