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?

  • 3
    \$\begingroup\$ You'll find that most AC wall warts give what's known as RMS voltage and currents. This is a value that's scaled for the sinusoidal shape of the AC waveform to work like the equivalent value of DC. \$\endgroup\$
    – JustJeff
    Dec 17, 2011 at 14:24
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    \$\begingroup\$ @TheNoble-Coder, you are missing a spec, what is the efficiency of the converter? \$\endgroup\$
    – Kortuk
    Dec 17, 2011 at 14:35

2 Answers 2


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)

  • \$\begingroup\$ you mean that for 90W or 75W DC out we will need 100W AC input? \$\endgroup\$ Dec 20, 2011 at 6:37

So when converting your DC watts being used in AC watts, that would be 100 DC watts equal 75 to 90 AC watts. That is using the inverter and wiring efficiency of between 75% to 90% .

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    \$\begingroup\$ You have the question reversed. This is not about DC->AC, but us[ing] an AC to DC adapter/converter. \$\endgroup\$ Aug 28, 2016 at 15:21
  • \$\begingroup\$ OP asked "If we use an AC to DC adapter/converter ...". You've supplied the conversion for an inverter and answered a different question. It pays to read the question carefully. Welcome to EE.SE. \$\endgroup\$
    – Transistor
    Aug 28, 2016 at 15:22
  • \$\begingroup\$ I stated my answer/question the way I did, because I wanted to know if what I suppose is right using DC solar power, so that I know how many watts AC I can expect from 2000 watts DC at 24 volts output when it is inverted to 115/120 volts AC. I apologize for inverting your question, but this was the best explanation for me that I have so far found with my searches online. I am not an engineer or even an electricien. I am just a person trying to build my own off grid solar that needs a few answers. I thank you for your time. \$\endgroup\$
    – Goldken51
    Aug 29, 2016 at 2:11
  • \$\begingroup\$ Actually at the top of the page it does say DC -> AC Converting Power/Watts in DC to Power/Watts in AC? \$\endgroup\$
    – Goldken51
    Aug 29, 2016 at 2:30

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