A power supply unit (PSU) is the computer component that converts alternating current (AC) from a wall outlet into the steady direct current (DC) power a computer's parts need. In a desktop, it's usually a box inside the case with cables that run to the motherboard, processor, graphics card, and drives. Its wattage limits how much hardware it can power, and its efficiency rating shows how much power it wastes as heat.
Power Supply Essentials at a Glance
A PSU converts wall power into the DC voltages computer components use
Wattage, listed in watts (W), is the maximum power the PSU can deliver
80 PLUS ratings show how efficiently a PSU converts power at different loads
Modular PSUs let you attach only the cables you need
Laptops, mini PCs, and many all-in-ones use an external power adapter instead
How Does a Power Supply Work?
A power supply takes AC power from the wall, converts it to DC, and splits it into the specific voltages computer components use, mainly 12V, 5V, and 3.3V. The 12V rail carries most of the load, powering the processor and graphics card. The PSU also smooths out fluctuations and includes protections that shut it down safely if something goes wrong, such as an overload or short circuit.
Types of computer power supplies
Type | Where it's used | What to know |
ATX power supply | Most tower desktops | The standard size, with the widest range of wattages |
SFX power supply | Small form factor desktops | A compact size for smaller cases |
Custom internal power supply | Many prebuilt desktops | Sized for a specific case and may not be swappable with standard units |
External power adapter | Laptops, mini PCs, and many all-in-ones | Converts power outside the computer, saving space inside |
Laptops charge through an external adapter, and many now use USB-C. Learn more about chargers and power.
Power Supply Wattage
Wattage is the maximum amount of power a PSU can deliver, such as 500W or 850W. A higher-wattage PSU doesn't force more power into your components; they draw only what they need. The wattage simply sets the ceiling.
How much wattage do you need?
Add up the power your components draw at full load, then leave headroom for peaks and future upgrades. The graphics card and processor use the most power, so graphics card makers usually publish a recommended PSU wattage for their cards.
Type of desktop | Typical PSU wattage |
Everyday desktop with integrated graphics | Around 300W to 500W |
Desktop with an entry to mid-range graphics card | Around 550W to 750W |
High-end gaming desktop | Around 750W to 1,000W |
Workstation or multi-GPU system | 1,000W or more |
These are general ranges, so check the requirements for your specific components. Running a PSU near its maximum all the time can make it louder and hotter, while a PSU running around half its capacity typically operates most efficiently.
Why PSU wattage matters for upgrades
In a desktop, the PSU often limits graphics card upgrades. A more powerful card may need more wattage and newer power connectors than your current PSU offers. Learn more about desktop upgrades.
Power Supply Efficiency Ratings
No power supply converts 100% of the power it draws from the wall. The rest is lost as heat. Efficiency is the share of input power delivered to your components, so a 90% efficient PSU delivers 90W for every 100W it draws.
80 PLUS ratings explained
The 80 PLUS® program, run by CLEAResult, certifies power supplies that meet minimum efficiency levels at 20%, 50%, and 100% of their rated load, with Titanium also tested at 10% load. Higher levels require higher efficiency.
80 PLUS level | 20% load | 50% load | 100% load |
80 PLUS (Standard) | 80% | 80% | 80% |
Bronze | 82% | 85% | 82% |
Silver | 85% | 88% | 85% |
Gold | 87% | 90% | 87% |
Platinum | 90% | 92% | 89% |
Titanium | 92% | 94% | 90% |
Figures are for 115V internal desktop power supplies. A newer Ruby level targets high-performance and data center systems. See the 80 PLUS program details from CLEAResult.
Does efficiency matter?
A more efficient PSU wastes less power as heat, so it can run cooler and quieter and may lower electricity use, especially in desktops that run many hours or under heavy load. For everyday desktops, Bronze or Gold ratings are common. Higher ratings matter most for high-wattage systems that run for long periods.
Modular vs. Non-Modular Power Supplies
Type | How the cables work | Pros | Trade-offs |
Non-modular | All cables are permanently attached | Usually the lowest cost | Unused cables have to be tucked away in the case |
Semi-modular | Essential cables are attached, while others are removable | Fewer loose cables at a moderate cost | Some cables still can't be removed |
Fully modular | Every cable is removable | The cleanest builds and easiest cable management | Usually the highest cost |
Modular cables make building and upgrading easier and can improve airflow by reducing clutter. Only use the cables that came with your PSU, because modular cable pinouts differ between brands and models.
Power Supply Connectors
24-pin ATX connector: Powers the motherboard.
8-pin CPU connector (EPS): Powers the processor, with high-end boards sometimes using two.
PCIe power connectors: Power graphics cards, using 6-pin or 8-pin connectors, or the newer 16-pin connector on many recent high-end cards.
SATA power connectors: Power 2.5-inch and 3.5-inch drives.
What to Check Before Buying or Upgrading a Power Supply
Wattage: Enough for your components at full load, with headroom for future upgrades.
Connectors: The right number and type for your graphics card and other parts.
Size: ATX, SFX, or a custom size that fits your case.
Efficiency rating: An 80 PLUS level that suits how long and how hard your computer runs.
Compatibility in prebuilt desktops: Some use custom power supplies, so check your model's service documentation on HP Support first.
Frequently Asked Questions About Power Supplies
How do you know what wattage power supply you need?
To find the right power supply wattage, add up the maximum power draw of your components, especially the processor and graphics card, then add headroom for peaks and future upgrades. Graphics card makers usually list a recommended power supply wattage, which is a good starting point.
Is a higher-wattage power supply better?
A higher-wattage power supply isn't better on its own. Components draw only the power they need, so extra wattage is useful only as headroom for upgrades or power-hungry parts. A quality PSU sized for your system is a better choice than an oversized one.
What does 80 PLUS Gold mean?
80 PLUS Gold means a power supply meets at least 87% efficiency at 20% and 100% load and 90% efficiency at 50% load, measured for 115V internal power supplies. It wastes less power as heat than Bronze or standard 80 PLUS units.
Can a weak power supply cause problems?
A power supply that can't deliver enough power can cause crashes, random restarts, or shutdowns under heavy load, such as during games or renders. Upgrading to a power supply with enough wattage and the right connectors usually solves the problem.
Related Components Topics
Computer components: the main parts of a computer and how they work together
Graphics cards (GPU): integrated vs. dedicated graphics and VRAM
Motherboards: chipsets, sockets, form factors, and expansion slots
Cooling: air and liquid cooling, thermal paste, and throttling
