Here are the specs for this computer: http://support.acer.com/acerpanam/desktop/2010/acer/Aspire/AspireX3200/AspireX3200sp5.shtml
The power supply is 220 Watts. Computers do not run at full rated watts. So allow a max of about 180 watts for the computer. The monitor (LCD/LED) are normally around 50 watts for the 17" screen; maybe even less. The keyboard and mouse power is included in the computer power.
The answering machine plus the 2 phones would be about 25 watts total.
This entire system as described would consume around 250 watts at peak; a lot less at idle (but ON).