PC power supply (PSU): how to choose wattage and efficiency

Choose a power supply (PSU, Power Supply Unit) based on the combined consumption of the processor and graphics card, with roughly 30% headroom and at least 80 PLUS Gold efficiency certification. For a typical gaming PC that means 650–850 W; for the most powerful builds 1,000 W or more. Don't skimp on the PSU: a cheap unit is the most common cause of instability and can damage other parts if it fails.

Summary

• The rated output (e.g. 750 W) is the ceiling of what the PSU can deliver to the components.
• Efficiency (80 PLUS Bronze, Gold, Platinum…) determines how much energy is lost as heat.
• For graphics cards with 12V-2x6 (12VHPWR) power, choose an ATX 3.0 or 3.1 PSU with a native cable.
• A quality PSU will last through several upgrades of your build – it pays to buy one with headroom.

What a power supply does

The PSU converts 230 V AC from the wall socket into DC voltages for the components. The main rail is +12 V – it powers both the processor and the graphics card, i.e. the vast majority of consumption. The +5 V and +3.3 V rails serve drives, USB and part of the motherboard. How much current the PSU can handle on each rail is shown on the label on its side.

How to calculate the wattage you need

1. Add up the consumption of the graphics card (TGP/TBP from the manufacturer) and the processor (maximum power draw under load).
2. Add roughly 100 W for the motherboard, memory, drives and fans.
3. Multiply the result by 1.3 – headroom for short graphics power spikes, PSU ageing and future upgrades.

Example: graphics 300 W + processor 150 W + 100 W = 550 W × 1.3 ≈ 715 W → choose a 750 W PSU.

As a rough guide you can also go by the card manufacturers' recommendations. NVIDIA, for example, recommends a 1,000 W PSU for the GeForce RTX 5090, 850 W for the RTX 5080, 750 W for the RTX 5070 Ti and 650 W for the RTX 5070.

Efficiency and 80 PLUS certification

Efficiency tells you how much of the energy drawn the PSU passes on to the components. The rest turns into heat. 80 PLUS certification guarantees minimum efficiency at various loads:

The values apply to the 230 V (EU) variant of the certification. Manufacturers sometimes quote them for the US 115 V, where they are slightly lower. PSU quality is also rated by the independent Cybenetics certification, which also measures noise.

How much a PSU actually draws from the socket

An 800 W PSU at full load and 80% efficiency draws 1,000 W from the mains (800 ÷ 0.8). At 90% efficiency it is only about 890 W. With the less efficient unit the 110 W difference is lost as heat – the PSU is noisier and you pay more for electricity. For a computer that runs many hours a day, a more expensive and more efficient PSU pays for itself within a few years.

What to look for when choosing

• ATX 3.0 / 3.1 standard: handles the short power spikes of modern graphics cards and has a native 12V-2x6 connector (the successor to 12VHPWR) for cards that use it.
• Connectors: enough PCIe cables for the graphics card, and 1–2 EPS (8-pin) connectors for the processor depending on the motherboard.
• Modularity: detachable cables make maintenance easier and improve airflow in the case.
• Size: standard ATX format, SFX for small cases.
• Warranty: quality PSUs come with a 7–10 year warranty – a good indicator of quality.

When a PSU is faulty

Typical symptoms are random restarts under load, a computer that won't turn on, or whining and a burning smell. We can test a faulty PSU and repair it as part of our computer repair service if the repair is worthwhile. If not, we will recommend a suitable new one.

How the PSU fits into the whole build is described in our guide How to choose a gaming PC.

What next

• Computer repair and service – PSU diagnostics and repair.
• Custom PC builds – a PSU correctly sized for the whole build.
• How to choose an SSD – another part of the build.

Sources

• 80 PLUS – PSU efficiency certification programme
• Cybenetics – independent PSU testing