hi
you can make use of the power calculator for calculating the requirement for specific platforms such as
Cisco Catalyst® 6500, Catalyst 4500, Catalyst 3750, and Catalyst 3560 series switches, and the Cisco 7600 Series Router.
http://tools.cisco.com/cpc/
for other devices like PIX 535 and 7513 the datasheets of the respective boxes will help u out to arrive an approximate power requirement.
PIX 535 :
POWER
Input (per power supply)
Range line voltage: 100V to 240V AC or 48V DC
Nominal line voltage: 100V to 240V AC or 48V DC
Current: 4-2 A
Frequency: 50 to 60 Hz, single phase
Power: 220W (dual hot swap power supply capable)
Output
Steady state: 135W
Maximum peak: 220W
Maximum heat dissipation: 750 BTU/hr, full power usage (220W)
Cisco 7513 :
http://cisco.com/en/US/products/hw/routers/ps359/products_data_sheet0900aecd800f5542.html
regds