# How does one determine the power consumption of a device/load in a circuit?

I have a question regarding circuit design, however i'm not sure whether i can express it in correct terms or if there's a fundamental flaw in my thinking.

• When using a device,how do we know exactly how much power it requires in order to start up/operate correctly?
• How do we calculate such requirements?

Let's say we have a circuit consisting of a power supply, resistor and some device like a chip or other microcontroller, or any other kind of load for that matter.

• How do we reason about the power supply needed for that device to work?
• Knowing that power is essentially voltage times current, how do I choose values for those components?

Forgive if my qeustion does make any sense Regards

• I just build several subsections and measure them. If testing them in their actual environment is tricky to do, I come up with a simpler environment that isn't so tricky but where I know it should produce about the same results. And then I measure. For example, if I'm going to use an ultrasonic sensor but only power it for 100ms every 10s, I can just power it and divide by 100. If I'm worried that the power-up and power-down are important, I can measure the entire power-up and power-down plus 10s, do the same but with only 4s, do the same but with only 1s, and use that data to bound it.
– jonk
Jan 30, 2021 at 22:22

I.e., this 74AHC00 chip specifies $$\20\mu\mathrm A\$$ of draw on the power supply -- but that's just for the chip. If you were using it to drive LEDs with $$\10\mathrm{mA}\$$ of current, you'd need to add in $$\10\mathrm{mA}\$$ for each LED.