Your system supports the hot spare feature that significantly reduces the power overhead associated with power supply unit (PSU) redundancy.
When the hot spare feature is enabled, one of the redundant PSUs is switched to the sleep state. The active PSU supports 100 percent of the system load, thus operating at higher efficiency. The PSU in the sleep state monitors output voltage of the active PSU. If the output voltage of the active PSU drops, the PSU in the sleep state returns to an active output state.
If having both PSUs active is more efficient than having one PSU in the sleep state, the active PSU can also activate the sleeping PSU.
You can configure the hot spare feature by using the iDRAC settings. Dell.com/poweredgemanuals.