Assemble a Computer

What Is a Power Supply Unit and Why Do We Need One?

All of this fantastic electronic tech you’re assembling needs a power source, also called a power supply unit (PSU).

PSUs come in various standardized sizes, but not all will fit every case, and there are some crucial parameters to consider, as you’ll see below.

How to Choose Suitable Power Supply Unit

For most things, you get what you pay for, which applies to PC PSUs. The suppliers listed in chapter 1 will sell only reliable brands. You might find cheaper units for sale on other sites, but it’s generally best to stick to the mainstream PSU brands.

Wattage

This indicates the maximum power the PSU can deliver. If the wattage is too low, the computer won’t start or will keep switching off. If the wattage is over-specified, the PSU will cost more than necessary. However, you might opt for a larger wattage unit if you plan to expand the PC later by adding more HDDs or a second graphics card.

Typically, a general desktop PC will work fine with a power supply rated for around 500-650W, but high-end gaming systems with multiple graphics cards might require a unit rated for 900W or more.

Efficiency

The power supply converts the incoming AC mains electricity to a range of lower-voltage DC power feeds. Power supplies are graded according to how well the conversion works - mainly in how much power is wasted through heat loss and the sophistication of the power control circuitry. Higher-efficiency power supplies have more complex electronic designs. They are, as expected, more expensive, but the efficiency may be necessary to you (and your business) from a running cost and ecological point of view.

Power supplies are graded as follows (this is a simplified explanation):

The PSU efficiency ratings are usually shown on the unit like this:

Physical size

Most PSUs are described as ATX size, but sizes can vary, so it’s vital to re-check PSU choice once you select a case and know what PSU size it supports. You may find that another PSU brand has managed to make a smaller unit that delivers the same wattage as a larger one. There’s also a smaller format, known as TFX, for slim and (some) cube cases.

Motherboard Power Connection Types

A PSU has two categories of power connector:

• A connector for the motherboard.

• Connectors for add-ins (disks, graphics cards, etc.). These are subdivided into Molex, Mini-Molex, SATA, PCI-E 6, PCI-E 6+2 (usually for high performance, high wattage graphics cards).

The latter is very standardized, but motherboard connectors have changed over time, so you need to check the manual for what you’ll need. The main types are:

• 20/24 ATX

• ATX P4 (EPS) 

The above image shows the following power connectors - not all will be present on your power supply:

1. Floppy disk drive

2. Molex (Some HDDs, optical disc drives, and fans)

3. SATA (Modern HDDs, SSDs, and optical disc drives)

4. 6 or 8-pin graphics card

5. 6-pin graphics card

6. Motherboard 8-pin

7. Motherboard 4 or 8-pin

8. Motherboard ATX-24 or 20-pin (connector can be split)

Always check what type of connector your motherboard requires. Count how many of each type you’ll need for the add-in connectors, and make sure your PSU provides enough. If not, you might need to choose again or buy some splitter/adapter cables.

Some power supplies come with a range of detachable cables. These are known as modular PSUs, and they allow you to configure the unit to meet your connector needs:  

Checklist

Here are the main points to remember when choosing a PC power supply unit:

• Is the PSU wattage suitable for the intended use and future expansion? Make sure your PSU has headroom for future computer upgrades.

• Will the PSU fit your case? Make sure yours will fit, or keep it in mind when picking the case.

• Is there a preference for PSUs that meet a specific energy efficiency rating? The top-rated PSUs have lower energy consumption and running costs over time.

• Does the PSU have enough power cables for all devices? If not, you may need some splitter cables or a different model.

Your Turn!

🎯  You need to go online again and choose a power supply.

⚙️  Identify a power supply that’s suitable for your needs (remember, you’re building a general PC for the office). Also, choose a less suitable option. For both cases, note the reasons for your choice.

✅  Once you’ve finished, see how your selections compare with mine in the Example Solutions document: .docx / .odt.

Let’s Recap!

• A PSU provides power to all the components inside your PC.

• The PSU needs to provide sufficient power without being overloaded - the power rating is measured in watts.

• Make sure that your PSU fits in your intended case.

• There are several different motherboard power connectors, so make sure your PSU has the right one.

• Make sure that the PSU has enough power cables for all the internal devices - or buy splitters.

We’re almost ready to hit the power switch! But you need a case for your PC parts!