Dual PSUs: accounting for A/B inputs and redundancy

With dual PSUs, account for server energy through non-overlapping A and B inputs while describing redundancy separately. Equal load sharing is not universal, and two supplies do not guarantee full performance after losing one. Preserve wiring, platform mode, and meter scope. Accounting can be checked through observation and documentation without unplugging production cables or performing an incidental fault test on a working node.

Separate capacity from consumption

A PSU rating describes supported delivery capacity under specified conditions, not the server’s current watts. Doubling two nameplate ratings does not produce measured consumption. Dell’s redundancy discussion distinguishes Full Redundancy and Fault Tolerant Redundancy for relevant platforms, including different consequences of redundancy loss. This is platform-specific policy rather than universal vendor terminology. Establish the actual server mode and supported configuration before stating availability or performance requirements. Physical PSU count alone cannot define those guarantees or establish how capacity is reserved.

Map power inputs

Identify PSU A, PSU B, PDU outlets, and upstream meters. A meter already covering both inputs must not be added to one outlet again. Energy from independent AC feeds can be summed over matched intervals. Preserve meter identities and physical connections. A/B labels in different dashboards can refer to a rack, circuit, or outlet, so similar names do not establish equal coverage. Verify the map before calculating costs or inference efficiency. Wiring changes need a new map rather than silent reuse of earlier assumptions.

Create an accounting table

Make rows for input A, input B, and upstream total. Record coverage, units, interval, and inclusion in the final sum. Select either non-overlapping A/B values or the corresponding total, never both. This is an accounting template without invented watt readings. Do not infer the second input by copying the first because supplies match and appear healthy. Actual sharing can depend on PSU policy. Keep observed contributions separate from redundancy capacity and from a hypothetical behavior after an input failure.

Keep component zones distinct

The powercap framework describes CPU regions and hierarchies, not PSU AC-feed meters. Use it as a separate component view rather than a replacement for A/B readings. The difference includes other consumers and measurement effects, so it cannot be assigned entirely to losses in one PSU. If only component telemetry is available, label incomplete coverage honestly. Ordinary energy accounting does not establish fault behavior; redundancy needs platform evidence and its own agreed operational validation scenario.

Preserve acceptance evidence

Keep wiring map, redundancy mode, supported configurations, meter boundaries, and matched intervals. Distinguish normal-operation observations from documented platform properties. A planned fault test requires a separate approved procedure; this accounting task does not require removing power from production. Recheck after PDU changes to avoid counting neighboring equipment. Do not promise unchanged performance under every failure merely because two supplies exist. The useful record explains both energy totals and availability limits through actual capacity and platform policy rather than connector count.

Power and cooling: the overlooked conditions of GPU operation; GPU Colocation Power Budget: Check the Site Limit.