Server Power Supply Replacement: Matching, Wattage and Redundancy
A server power supply is one of the parts most likely to fail, and one of the easiest to get wrong when replacing. This covers matching a replacement and what redundancy actually gives you.
Three things a replacement must match
Getting any of these wrong means a supply that does not fit or does not work.
The chassis and generation. Power supplies are specific to a server family and frequently to a generation within it. A supply from a Gen9 does not fit a Gen10 even where both look similar. Our power supply identification guide covers reading the label.
The wattage. All supplies in a machine should be the same wattage. Mixing ratings is either refused outright or disables redundancy, because the machine cannot guarantee either supply alone can carry the load.
The input type. Most are AC, but DC and high-voltage variants exist and are not interchangeable.
The safest method: read the part number off the existing supply rather than working from the server model.
What wattage do I need?
Not the maximum the chassis supports, and not the smallest that boots.
The load depends on what is actually fitted: processor count and power class, memory quantity, drive count and type, and expansion cards. A machine with one processor and four drives draws far less than the same chassis fully populated.
Two rules.
Size for the configuration you will have, with headroom. Adding a second processor, filling drive bays, or fitting a GPU all raise the draw, and a supply sized to today’s configuration may not carry tomorrow’s.
Supplies are most efficient in the middle of their range. A heavily oversized supply running at a small fraction of its rating wastes power as heat. Our TCO guide covers counting that.
Most vendor configuration tools will calculate the draw for a given configuration, which is more reliable than estimating.
What redundancy actually protects against
Two supplies protect against one supply failing. That is genuinely valuable, because power supplies fail more often than most server components.
What they do not protect against, unless you do more.
A feed failure, if both supplies are plugged into the same PDU or circuit. This is the most common way redundancy is defeated — two supplies, one feed. Plug them into separate PDUs on separate circuits. Our PDU guide covers arranging this.
A UPS failure, if both feeds come from the same UPS.
An undersized configuration. If a single supply cannot carry the full load, losing one takes the machine down anyway. Redundancy requires each supply to be capable alone.
Our redundancy sizing guide covers this in more detail.
Signs a power supply is failing
Four.
A fault light or management alert. The obvious one, and the management controller usually reports it before anything visible happens.
Fan noise from the supply changing or becoming erratic. Supply fans fail before the supply does, and an overheating supply follows.
The machine restarting under load but running fine when idle. A supply losing capacity fails when demand peaks.
A burning or ozone smell. Stop and replace it. Do not wait for the alert.
Our monitoring guide covers routing supply alerts somewhere a person reads them, which matters because a failed supply in a redundant pair produces no symptom the user notices.
The failure nobody notices
Worth its own point.
In a redundant pair, one supply failing is invisible without monitoring. The machine runs normally on the remaining supply. Nothing is slow, nothing restarts, nobody reports anything.
Then the second supply fails, or the remaining one is asked to carry a load it cannot, and the machine goes down with what looks like a sudden failure but was actually two failures weeks apart.
Check supply status on every machine periodically, or alert on it. This is one of the highest-value checks in a routine inspection.
Replacing a supply
Hot-swap supplies can be replaced while the machine runs, provided the other supply can carry the load. Three steps: confirm the remaining supply is healthy, remove the failed one, fit the replacement and confirm both report as good.
Non-redundant machines need a shutdown. And in both cases, each supply needs its own power cord — our power cords guide covers the types.
Frequently asked questions
Can I mix power supplies of different wattage?
No. All supplies in a machine should be the same wattage. Mixing is either refused or disables redundancy, because the machine cannot guarantee either supply alone can carry the load.
What wattage power supply do I need?
Enough for the configuration you will have, with headroom for a second processor, more drives or a GPU. Not the chassis maximum, since supplies are most efficient in the middle of their range. Vendor configuration tools calculate the draw reliably.
Will a power supply from another server model fit?
Usually not. Supplies are specific to a server family and frequently to a generation within it. Read the part number off the existing supply rather than working from the server model.
Does redundancy mean I am protected from power failure?
Only from one supply failing. If both supplies are plugged into the same PDU or circuit, a feed failure takes the machine down regardless. Plug them into separate PDUs on separate circuits.
How do I know if a power supply has failed?
In a redundant pair, you do not without monitoring. The machine runs normally on the remaining supply and nothing looks wrong until the second one fails. Check supply status periodically or alert on it.
Can I replace a power supply without shutting down?
On hot-swap supplies, yes, provided the remaining supply can carry the load. Confirm it is healthy first, then swap. Non-redundant machines need a shutdown.
Send us the part number from the existing supply and we will match wattage, generation and input type exactly.
