Converting Power/Watts in DC to Power/Watts in AC?
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?"

A: 

Rearrange these equations:


DC_Watts = 75% AC_Watts so AC_Watts = .....
DC_Watts = 95% AC_Watts so AC_Watts = .....


i.e.

Watts_Out = Watts_in x Efficiency        ( 0 &lt;= Efficiency &lt;= 1.0)

Then  Watts_in = Watts_Out / Efficiency.          ( 0 &lt;= Efficiency &lt;= 1.0)