How do I conclude the power requirements for Dell servers?

For example for Dell R730 servers, in the the technical specifications following are the ratings specified. I have three options to choose from. On what basis should I decide whether it's 495, 750 or 1100 Watts? 

AC Power Supply (per power supply)

Wattage: 495 W, 750 W, or 1100 W

Heat dissipation NOTE: Heat dissipation is calculated using the power supply wattage rating.
1908 BTU/hr maximum (495 W power supply)
2891 BTU/hr maximum (750 W power supply)
4100 BTU/hr maximum (1100 W power supply)