How to work out PSU wattage
Add the peak power of the processor, the graphics card and about 50-100 W for the rest of the system. Multiply the total by 1.3 or 1.4 for headroom and pick the next common PSU size: 450, 550, 650, 750, 850 or 1000 W.
| System | CPU / graphics / rest (W) | Load (W) | PSU with 40% headroom (W) |
|---|---|---|---|
| Office PC with integrated graphics | 65 / 0 / 50 | 115 | 300 |
| Mid-range gaming | 125 / 220 / 75 | 420 | 650 |
| High-end gaming | 180 / 320 / 100 | 600 | 850 |
| Enthusiast or workstation | 250 / 450 / 100 | 800 | 1,200 |
Calculated examples; rounded values.
Why you should not cut it close
A power supply runs most efficiently and quietly between 40% and 70% of its capacity. Modern graphics cards also have millisecond power spikes well above their rated power: a tight PSU can shut the PC down mid-game even when the average is fine. Headroom also covers component ageing and future upgrades.
Where to find the power of each part
For the processor, do not look only at base TDP. Intel publishes maximum turbo power (PL2): a Core i5-14600K reaches 181 W and a Core i7-14700K 253 W. For AMD the figure that counts is PPT, about 142 W on Ryzen chips rated at 105 W TDP.
For the graphics card, use total board power (TBP or TGP) from the maker spec sheet. As a guide: RTX 4060 about 115 W, RTX 4070 about 200 W, RTX 4080 about 320 W and RTX 4090 about 450 W. Factory-overclocked models may draw a little more.
The rest of the system adds little: motherboard and RAM about 30-50 W, each SSD about 5 W, each hard drive about 8 W, and fans and lighting a few watts each. For a typical PC, 75 W covers it comfortably.
How much headroom to leave
Around 30% is enough for office PCs or modest graphics cards. 40% is the reference for gaming because it absorbs graphics card spikes. If you plan to upgrade the card within a year or two, calculate with the card you would buy: replacing the PSU later costs more than getting it right now.
If your graphics card maker recommends a minimum PSU wattage, follow it even if the calculator gives less: that figure already allows for power-hungry processors.
80 PLUS: efficiency, not wattage
80 PLUS certification shows how much of the power the PSU draws from the wall reaches the PC. At half load, an 80 PLUS Bronze unit is around 85%, Gold around 90% and Platinum around 92%. The bill only changes noticeably if the PC spends many hours at full load, but a better rating usually comes with better components.
Be wary of uncertified units or very high wattage at very low prices: they often fail to deliver what they promise on the 12 V rail, which feeds the processor and graphics card.
ATX 3.1 and the 12V-2x6 connector
Many recent NVIDIA graphics cards use a single 12V-2x6 connector, the revised version of 12VHPWR. ATX 3.1 power supplies include it natively and are designed to tolerate the short spikes of current graphics cards. If you buy a new PSU for a modern card, choose ATX 3.1 and avoid adapters.
Also check the PSU length and whether it is modular: a modular unit only uses the cables you need and makes building in small cases easier.
Common mistakes when choosing a PSU
Buying watts instead of quality: a cheap 750 W unit can perform worse than a good 650 W one. Ignoring connectors: check it has the PCIe cables your graphics card needs and enough SATA connectors for your drives. Forgetting the warranty: good power supplies offer 7 to 10 years, a clear sign of how much the maker trusts its components.
Above all, do not reuse a very old power supply with a new graphics card without checking its real 12 V capacity and condition: a failing PSU can damage other parts of the PC.
Frequently asked questions
Does a bigger power supply use more electricity?
No. The PSU only delivers what the PC asks for. An 850 W unit powering a 300 W PC uses practically the same as a 550 W one; the difference is price and efficiency at very low loads.
How much does my PC really use?
Less than the calculated maximum: while gaming, processor and graphics card are rarely both at 100%. To know the real draw, use a plug-in power meter and work out the cost with the electricity consumption calculator.
Will the same PSU work if I upgrade my graphics card?
Only if the new load plus headroom stays below its rating and it has the right connectors. Run the calculation again with the new card before you buy it.