Power Conversion – Converting Power/Watts in DC to Power/Watts in AC

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How can we convert Watts in DC to Watts in AC? for e.g A device needs 1A and 12V DC input i.e (1A x 12V = 12 Watts) 12 Watts DC. If we use an AC to DC adapter/converter to generate 1A,12V (12Watts) DC then What will be the AC Power/Watts that we have to input into the device's adapter/converter?

Best Answer

Summary: Watts out DC = 75% to 90% of AC Watts in, in most cases.

See below:

At 100% efficiency ADC Watts out = AC Watts in.
Energy is 'conserved" and energy = Watts x time. eg we often measure energy in Watt.seconds = Watts x seconds operated = Joules.

The efficiency of conversion depends on the technology. Power level and voltage also give an effect.

Using a conventional iron transformer the AC mains to AV low voltage conversion probably is > 95% efficient. Rectification efficiency depends on voltage and diodes used. As a silicon diode drops 0.6V or more, a single diode can account for (0.6V/(5+0.6)V) ~= 11% efficiency loss. If a bridge rectifier is used there are two diodes in the circuit and this can account for over 20% efficiency loss.

If a switching power supply is used the overall efficiency is liable to be in the 75% to 90% range. More than 90% efficiency is possible with special care.

So Watts out DC = 75% to 90% of AC Watts in in most cases.

Q: "you mean that for 90W or 75W DC out we will need 100W AC input?"


Rearrange these equations:

  1. DC_Watts = 75% AC_Watts so AC_Watts = .....

  2. DC_Watts = 95% AC_Watts so AC_Watts = .....


Watts_Out = Watts_in x Efficiency ( 0 <= Efficiency <= 1.0)

Then Watts_in = Watts_Out / Efficiency. ( 0 <= Efficiency <= 1.0)