PC Power Supply Wattage Calculator: What Size PSU Do You Need?
Work out the PSU wattage for your PC build: CPU and GPU power (or pick an RTX 50/40/30 or Radeon RX 9070 card), the rest of the system itemised or as one figure, headroom, the card maker’s minimum PSU, load %, wall draw by 80 Plus rating and a PSU size table.
How it works
System draw = CPU + GPU + rest of system · Recommended = draw × (1 + headroom %) · PSU = next common size ≥ recommended (and ≥ the card maker’s minimum, if you pick a card) · Load = draw ÷ PSU size · Wall draw = draw ÷ 80 Plus efficiency at that load
A power supply has to cover the highest sustained draw of every part at once, plus headroom. The calculator adds the CPU’s maximum package power, the graphics card’s rated board power and the rest of the system, either as one figure or itemised (about 50 W for the motherboard, 5 W per RAM stick, 5 W per SSD, 8 W per hard drive and 5 W per fan). It then adds your headroom, 30% by default, and rounds up to the next common retail size. Headroom matters because modern CPUs and GPUs draw brief spikes above their rating, supplies lose a little capacity as they age, and a supply running around half load is quieter and close to its best efficiency. If you pick a card from the list, the calculator also checks the maker’s minimum PSU (NVIDIA’s “required system power”, AMD’s “minimum PSU recommendation”) and never recommends less. Under More options, choose the 80 Plus rating to see how much power the PC pulls from the wall at full load and the heat it puts into the room; efficiency comes from the 80 Plus 115 V test points (for example Gold: 87% at 20% and 100% load, 90% at 50%). Use the CPU’s PL2 or PPT rather than its TDP: AMD chips can draw about 1.35 times their TDP, and Intel’s unlocked chips well over their base power.
Worked example
A 125 W CPU, a 250 W GPU and 90 W for the rest of the system draw 125 + 250 + 90 = 465 W. With 30% headroom that is 465 × 1.30 = 604.5 W, so the next common size is a 650 W supply. At full draw it runs at 465 ÷ 650 = 72% load, and a Gold unit (about 88.7% efficient at that load) pulls about 524 W from the wall, which ends up as about 1,789 BTU/h of heat in the room.
Sources: NVIDIA: Compare GeForce graphics cards (Total Graphics Power, Required System Power) · AMD Radeon RX 9070 XT specifications · AMD Radeon RX 9070 specifications · 80 Plus efficiency levels (Wikipedia) · GameMax: How to calculate the required power supply for your build · Computer Hope: How much power do computer components use?
Frequently asked questions
How many watts do I need for an RTX 5070 or RTX 5080?
NVIDIA lists 250 W of graphics power and a 650 W minimum PSU for the RTX 5070, 300 W and 750 W for the RTX 5070 Ti, 360 W and 850 W for the RTX 5080, and 575 W and 1,000 W for the RTX 5090. Pick the card in the calculator to check the sum of your own parts against those minimums.
Does a bigger PSU waste electricity?
No. A supply only delivers what the parts draw, so a 1,000 W unit powering a 400 W PC still delivers 400 W. The wall draw depends on efficiency at that load, and a quality supply is close to its best between about 20% and 80% load, so moderate oversizing costs money up front, not running power.
Why add headroom instead of matching the exact draw?
CPUs and GPUs draw millisecond spikes well above their rating, and a supply sized to the average can trip its protection and shut the PC down. Headroom also covers ageing and future upgrades, and keeps the fan slower. 20–30% is the usual margin; 40–50% if you plan a bigger card later.
Should I use TDP or maximum power for the CPU?
Use maximum package power. AMD Ryzen chips run up to their PPT, about 1.35 × TDP (88 W for a 65 W part, 142 W for 105 W, 230 W for 170 W), and Intel’s unlocked desktop chips run up to PL2, 253 W on a Core i9-14900K.
What 80 Plus rating should I buy?
Bronze is a sound baseline and Gold is the common choice for gaming PCs. At 115 V, Bronze must reach 85% efficiency at half load, Gold 90%, Platinum 92% and Titanium 94%. Higher ratings mean less heat and a slightly lower power bill, which matters most for PCs that run under load for many hours.
Do I need an ATX 3.1 power supply?
For recent high-power cards it is the safest choice. ATX 3.x supplies are designed for the brief power excursions of modern GPUs and come with a native 12V-2x6 cable for cards that use the 16-pin connector, so you avoid multi-plug adapters.
Related calculators
Estimates for planning. Check your parts’ spec sheets and buy a quality supply from a reputable maker; a cheap unit may not deliver its label rating.