PSU Sizing Sheet
Size for spikes, and centre your TYPICAL load near half the supply's rating.
BFCBrilliance · bfcbrilliance.com/tools/psu-wattage-calculator
The arithmetic
- PEAK DRAW
- CPU + GPU + about 100 W for the rest
- RECOMMENDED
- peak x 1.3, rounded up to a real size
- Efficiency target
- TYPICAL load near 50% of the rating
- Typical load
- roughly 70% of peak while gaming
- 80 PLUS tests at
- 20%, 50% and 100% load
- Gold is specified
- 87% / 90% / 87% across those three
- ⚠ TDP is not peak
- cards spike far above it for milliseconds
- ⚠ A tripped PSU looks like
- a random reboot under load
- ⚠ Watts are not quality
- and 80 PLUS rates efficiency only
Builds sized
| Build | CPU W | GPU W | Peak | PSU chosen | Typical load % |
|---|---|---|---|---|---|
This build
- Build
- Date
- CPU and TDP
- GPU and board power
- Everything else
- PEAK DRAW
- Headroom %
- RECOMMENDED PSU
- Typical load %
- SIZE FOR 50% TYPICAL
- Unit chosen
- 80 PLUS rating
- PCIe connectors needed
- Connectors on the unit
Before buying a power supply
- CPU and GPU figures taken from the manufacturer's spec page
- Headroom added for transient spikes, not just the sum
- Typical load checked against the 50% efficiency band
- PCIe connector count and type checked against the graphics card
- 12VHPWR cable confirmed if the card needs one
- Physical length checked against the case
- Reviews consulted for QUALITY — wattage does not describe it
- Future upgrades considered while the machine is open anyway
Size for the spike, not the sum
A graphics card rated at 250 W can pull far more for a few milliseconds during a load change. A supply that is adequate on paper can trip its own protection on those transients and shut the machine down — which presents as a random reboot under load, not as an obvious power fault. That is what the headroom is really for, and what ATX 3.0 was written to address.
Two arguments, one answer
30% headroom over peak gives 618 W on the defaults. Centring typical load at 50% — where 80 PLUS units are most efficient — gives 665 W. Both round to a 650 W unit. When a rule of thumb and an efficiency argument agree, the answer is probably right; if they diverge sharply on your build, question the typical-load estimate rather than picking a favourite.