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I'm trying to calculate how long my UPS will power my cable modem and router.

I know that together the two draw 2.25 amps at 12 volts (27 watts), but this is on the DC side of the power brick. Does the wattage increase linearly with the voltage increase?

Basically, how many watts will my devices draw from the wall?

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  • \$\begingroup\$ Watts is "energy per unit time". Energy cannot be created or destroyed, so the power into the system is the same as the power out of the system (including waste heat power), unless it has some way to store energy. \$\endgroup\$ – endolith Oct 3 '11 at 19:28
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In the ideal case it will draw 27W from the 120V too, but the conversion isn't ideal: you have heat losses in transformer, rectifier and voltage regulator, so your mains power will be higher. How much higher depends on the design quality of your power supply, but it can be easily 50% higher, so you could draw 40W.

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  • \$\begingroup\$ Ok, so ignoring the losses incurred by the power brick, the wattage remains the same, that's where I was confused. Thank you! \$\endgroup\$ – ChiperSoft Oct 1 '11 at 6:19
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Energy cannot be created out of nothing. So the watts out will be no greater than watts in. In practice, the converter isn't 100% efficient, so you will lose some power as heat in the conversion.

A good guess for 27 watts out might be 40 watts in, or 50 watts if the brick gets really hot in use. Or you can avoid guesses by measuring the 120V current to the devices.

50 watts / 120V is 0.42 amps.

(Power factor is out of scope for this answer.)

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