The specified Dell power supply is rated for 12 Amperes supply current (from the AC mains) for 110 Volt AC mains, and 6 Amperes supply current for 220 Volt AC mains.
This translates roughly to 1320 Watts of maximum power drawn from the mains (typically at start-up).
Efficiency rating at maximum load is stated as 87%, hence peak output power is no more than
0.87 x 1320 = 1148 Watts instantaneous / start-up power.
All of this has little or nothing to do with the 825 Watts continuous power rating of the power supply, as we will see.
How does the 825 Watt value fit in with the current ratings for the various output lines of the SMPS? Actually, it never does.
The data sheet states that:
- Each +12 Volt line supports 18 Amperes (7 lines) = 1512 Watts!
- The +5 Volt line supports 4 Amperes = 20 Watts
- The -12 Volt line supports 0.5 Amperes = 6 Watts
While it is tempting to simply add those values up to get an output wattage, that isn't how a PC power supply is expected to work. In reality, individual supply lines will have differing loads at various instants, with the transients being fulfilled by reservoir capacitors at these outputs. So long as each individual output is loaded within its specification, and the combination of actual loads does not exceed 825 Watts, everything runs smoothly.
Beyond that point, regardless of which specific outputs are most heavily loaded, the on board processor of the SMPS will throw an overload alert, and attempt to gracefully shut down the system.
For less sophisticated PC SMPS units, the "graceful shutdown" is a myth, but with server grade power supplies, there is typically control logic in there to notify the operating system of a power supply problem, and then downgrade or cut off specified outputs (look for the technical reference) until the load returns to the safe operating area. The server OS is in the meanwhile expected to raise appropriate alert flags, save machine state to disk, and shut down until manual intervention.