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What wattage PSU do I need for my server?

Something we get asked often is what wattage PSU is needed for a server.

One thing to immediately know is that a desktop/PC power supply is not as powerful or efficient as an industrial/server grade PSU, so I wanted to create a comparison chart between a desktop and server PSU to get started;

Desktop PSU

Server PSU

Low cost
Easy to replace
Readily Available – Most online PC retailers will stock them
PSUs usually have a peak capability for a short amount of time – an advertised 500W PSU could not sustain 500W peak for an extended period – It’s likely around 400-450W is the usual maximum sustained power output.
Some lower end models may have poor voltage ripple, which can cause stability issues if used with certain server-grade components such as motherboards and CPU.
Low MTBF
No redundancy capability
Not designed for 24/7 operation
Limited warranty

Higher price
Usually made out of a thicker steel
Sometimes harder to obtain in the event of a failure – we often keep a large stock though
Most do not have a peak-wattage and will be able to run at their advertised capacity indefinitely without degradation in lifespan or performance.
Low ripple/noise so works well in sensitive applications with server grade motherboards/CPU
High MTBF
Redundant PSU options available in dual, triple and quad configurations for full high availability.
Designed for 24/7 operation – hot-swappable PSUs in redundant PSUs allow continuous use without interruption.
Usually 3 year warranty on more premium brands.

Hopefully the above comparison helps you identify if you want a desktop or a server grade PSU. The most important element is to think about uptime. If you can afford downtime to replace a PSU in the event of a failure, which is more likely to happen with a desktop PSU and you have a limited budget then a desktop PSU is the way to go. If you cannot afford the downtime, then you should look at a server grade PSU and ideally a redundant model. I’ve written an article recently on Redundant PSUs here; https://www.servercase.co.uk/blog/article/tech-help---how-does-a-redundant-psu-work-and-what-are-the-benefits/

Now you know what type of PSU you need then let’s get down to choosing the right wattage. To help with this I’ve created a chart for common components and what power they use;

Component	Wattage
CPU	100W
RAM	5W
SSD	10W
Motherboard	30W
HDD (each) – 4 HDD’s	10W Each (40W Total)
Fans	30W
TOTAL	215W

So in the above example we’ve chosen a reasonably powerful CPU, that might end up being a Core i5 or perhaps a Xeon with a reasonable specification. RAM uses barely any power and neither does SSD’s. The motherboard has various components which will use a reasonable current and in my example, I’ve gone for 4x HDD.

In total our example uses 215W, which is fairly conservative – Under load the CPU will draw slightly more current, the fans will need to speed up and use more current and higher HDD load may increase the current draw on those too – so for this system you may be safe with a 300W maximum amount.

Today generally the smallest PSU you can buy is around 300W in desktop PSU grade and 500W in server grade. My suggestion for this setup would be a 500W PSU in desktop and 500W PSU in server grade. This will mean low stress on the lower-quality desktop PSU and add longevity and it will run very comfortably with a 500W server PSU.

To help even further I’ve made a small chart which I think will be helpful.

Load in Watts	Desktop PSU Recommended Watts	Server PSU Recommended Watts
300W	500W	500W
400W	600W	500W
500W	750W	600W
600W	850W	700W
700W	1000W	800W
800W	1200W	900W
900W	1300W	1000W
1000W	1400W	1100W