A/B Testing PC Power Supply Formula
Learn how the calculator estimates PSU capacity, power headroom, and PSU load for two individual PC configurations.
This calculator estimates the power supply size for each PC build by adding expected CPU, GPU, and other component power, applying a chosen headroom percentage, and rounding upward to a 50 W PSU size. Comparing both results helps show whether the same PSU capacity is suitable for each individual system.
- 100% Free
- No Sign-Up Required
- Private & Secure
- Mobile Friendly
Recommended PSU Wattage
Where:
Add the estimated peak power of the CPU, GPU, and remaining components. Increase that total by the chosen headroom percentage, then round up to the next 50 W PSU capacity.
Variables Explained
| Variable | What It Means | Unit |
|---|---|---|
| cpuPower - CPU power | Expected peak CPU package power for one configuration. | watt |
| gpuPower - GPU power | Expected peak graphics card board power for one configuration. | watt |
| otherPower - Other component power | Estimated demand from the motherboard, memory, drives, cooling, USB devices, and other internal components. | watt |
| headroomPercent - Desired power headroom | Extra capacity added above estimated component demand. | percent |
| psuWattage - Selected PSU wattage | Rated continuous DC output capacity of the PSU being checked. | watt |
Step-by-Step Calculation
Estimate component demand
Combine the expected peak power of the main PC components for Configuration A or Configuration B.
estimatedPower = cpuPower + gpuPower + otherPower
Convert the headroom percentage
Turn the selected percentage into a multiplier. For example, 30% headroom becomes 1.30.
headroomMultiplier = 1 + headroomPercent / 100
Calculate required capacity before rounding
Increase estimated component demand by the selected headroom.
requiredCapacity = estimatedPower * headroomMultiplier
Round up to a PSU size
Round upward to the next 50 W increment so the suggested capacity is not below the calculated requirement.
recommendedPsu = ceil(requiredCapacity / 50) * 50
Check selected PSU load
Divide estimated component demand by the selected PSU rating to show the estimated load percentage.
psuLoadPercent = estimatedPower / psuWattage * 100
Compare the two recommendations
Find the absolute difference between the two rounded PSU recommendations.
recommendedPsuDifference = abs(recommendedPsuA - recommendedPsuB)
Comparing two gaming PC configurations
Calculate Configuration A demand
125 + 320 + 100
545 W
Add Configuration A headroom
545 × 1.30
708.5 W
Round Configuration A recommendation
ceil(708.5 / 50) × 50
750 W
Calculate Configuration B demand
65 + 220 + 90
375 W
Round Configuration B recommendation
ceil((375 × 1.30) / 50) × 50
500 W
Check selected PSU loads
545 / 750 × 100; 375 / 750 × 100
72.7%; 50.0%
Final Result
Configuration A is estimated to need a 750 W PSU, while Configuration B is estimated to need a 500 W PSU. The rounded recommendation difference is 250 W.
Assumptions
- ✓Entered component figures represent realistic peak DC demand rather than idle use or CPU base-power labels.
- ✓The chosen headroom covers uncertainty, sustained loads, normal aging, possible upgrades, and some short power spikes.
- ✓PSU recommendations are rounded upward in 50 W increments.
- ✓Each result applies to one separate PC, user, or workstation.
Limitations
- !The estimate does not verify the PSU's connector count, cable type, rail design, physical dimensions, protections, or overall build quality.
- !Actual CPU and GPU power can vary with settings, workloads, firmware, and overclocking or power-limit changes.
- !Very high-power GPUs can have transient behavior that a simple component-wattage estimate may not fully capture.
- !PSU efficiency affects power drawn from the wall but is not included in the rated DC output capacity calculation.
Common Mistakes to Avoid
Using a CPU base-power figure when its boost or sustained peak power is higher.
Leaving out fans, liquid-cooling pumps, storage, USB-powered devices, and motherboard power from other components.
Entering wall power draw instead of estimated component DC demand.
Treating a wattage result as confirmation that a PSU has the required CPU and GPU connectors.
Multiplying a per-PC PSU result by the number of users even though each separate workstation needs its own PSU.
Related Formulas
Frequently Asked Questions
How is recommended PSU wattage calculated?
The calculator adds CPU, GPU, and other component power, applies the selected headroom percentage, then rounds upward to the next 50 W size.
Why does the formula round up to 50 W?
Rounding up avoids recommending a capacity below the calculated target and reflects common PSU wattage increments.
How is PSU load percentage calculated?
Estimated component demand is divided by the PSU's rated wattage and multiplied by 100.
Does PSU efficiency change the recommended wattage?
No. Efficiency mainly affects wall power draw and heat losses; the calculation compares estimated DC component demand with rated DC output capacity.
Is a 30% headroom setting required?
No. It is a user-selected estimate. The appropriate margin can depend on uncertainty in the inputs, power limits, upgrades, and transient behavior.
Ready to calculate your result?
Use the calculator to get instant results with your own inputs.