How to size a PC power supply
A power supply unit (PSU) should cover the system’s peak load with comfortable headroom. Running a PSU at about 50 to 70% of its rating keeps it in its most efficient range, quieter and cooler, and leaves room for brief power spikes and future upgrades.
Estimating the load
The calculator adds:
- CPU power: the TDP or, better, the package power limit (PPT or PL2) when boosting.
- Graphics card: its total board power (TBP), the largest item in most gaming PCs.
- Motherboard and chipset: about 50 W.
- Memory: about 3 W per stick; SSDs about 5 W; 3.5-inch hard drives about 8 W; fans and pumps about 3 W each.
- USB devices, lighting and other: whatever you add.
The recommended rating is the load divided by 0.65, rounded up to the next 50 W.
Example
A 125 W CPU and a 300 W graphics card with two memory sticks, two SSDs, five fans and 20 W of extras total about 526 W. The calculator suggests an 850 W unit, which runs at about 62% at peak.
Power spikes
Modern graphics cards can draw twice their rated power for a few milliseconds. The ATX 3.0 and 3.1 standards require PSUs to tolerate these excursions, and the 12V-2x6 connector feeds high-end cards. With an older PSU, add more headroom.
Efficiency ratings
| 80 Plus level | Efficiency at 50% load (115 V) |
|---|---|
| Bronze | 85% |
| Gold | 90% |
| Platinum | 92% |
| Titanium | 94% |
Higher efficiency means less heat and slightly lower bills, but the quality of components and protections matters more than the badge.
Running cost
While gaming, a PC typically draws about 70% of its estimated peak. With 3 hours a day at $0.17 per kWh, the example PC uses about 448 kWh and costs about $76 a year. Idle and desktop use draw much less, often 50 to 100 W.
Tips
- Do not buy a 1,200 W unit “just in case”: oversized PSUs are less efficient at light loads.
- Check the PSU has enough PCIe power connectors for your card.
- Choose a reputable brand with full protections (OCP, OVP, OTP, SCP).