Table of Contents

Document Scope & Audience

For anyone needing to calculate maximum server power consumption and/or BTUs

Information Needed -

The basics are wattage which is a basic of electrical understanding and thanks to Ohm's Law easy to calculate.

Additionally, the conversion factor between wattage and British Thermal Units per hour (BTU/hour) which is 3.41214

Theoretical VS. Actual -

This calculation is often performed assuming full server load as it not only much easier but accounts for worst case situations.

If the real numbers are needed, gathering the actual watts used requires some extra leg work. I would suggesting looking at the power history within IPMI if the system has this feature and history has been accumulated.

If this is being performed prior to building a solution, worst-case scenario would be best.

Calculation -

Theoretical -

Locate the Power Supply Unit (PSU) and find the wattage it is rated for and divide this against its efficiency. The efficiency should be listed on the PSU convert as needed.

ex. "80 plus Platinum = rated for at least 90% efficiency at 20% load, 92% at 50% load, and 89% at 100% load."

PSU = 3,200W/H

3,200W/H ÷ 0.89 = 3,595.51W/H

3,595.51W/H x 3.41214 = 12,260.69 BTU/H

Actual -

The difference between this calculation and the one above is the wattage value and how it is ascertained. For example, below are two different calculations illustrating the source and the calculation.

ex. IPMI states that for the past month the server has used the average of 1600 watts per hour.

1,600W/H ÷ 0.92 = 1,739.13W/H

1,739.13W/H x 3.41214 = 5,934.16 BTU/H

ex. A wattage meter plugged between the server and the wall outlet shows the system is running at an average of 800 watts per hour.

PSU = 800W/H

800W/H ÷ 0.90 = 888W/H

888W/H x 3.41214 = 3,030 BTU/H

Redundancy -

Most servers have redundancy os PSU and this article has only discussed calculating per PSU. This is always the case and then based on the redundancy of the system the calculated BTU per PSU can be multiplied.

If a system has a 1+1 redundancy that means the system uses the full power of one PSU at any time. Whether it be 50% across both PSU or 100% of one and 0% of the other. This means that only one PSU needs to be calculated.

Other systems may have a 2+1 redundancy, although 3 PSUs are present it can truly only lose 1 and therefore the base calculation performed will need to be multiplied by 2 for actual server usage.

Real World Example -

2 x 2,000W (100-127VAC input) 1+1 Redundant PSU

2,000W/H ÷ 0.89 = 2247.19W/H

2247.19W/H x 3.41214 = 7667.41BTU/H

1+1 Redundancy = 7667.41BTU/H

Related Articles -

Exxact System Power Requirements (Single System and Cluster Solutions)

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