A/B Testing PC Power Supply Calculator Formula
Learn how the calculator estimates reserved PC power demand, PSU load, spare capacity, and the PSU rating needed for two builds.
This calculator adds the estimated high-load CPU, GPU, and other-component power for each PC build. It adds a selected reserve for short power spikes, then sizes a power supply so the more demanding build stays at or below your chosen target load.
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Recommended PSU Rating for Either Build
Where:
Calculate each build's component load, increase it by the transient reserve, divide by the target PSU loading percentage, round each result up to the next 10 W, and use the larger recommendation.
Variables Explained
| Variable | What It Means | Unit |
|---|---|---|
| cpuWattsA - Build A CPU power | Estimated high-load CPU package power for Build A. | watt |
| gpuWattsA - Build A GPU power | Estimated demanding-workload graphics card board power for Build A. | watt |
| otherWattsA - Build A other components | Estimated power for the motherboard, memory, storage, fans, pumps, USB devices, and accessories in Build A. | watt |
| cpuWattsB - Build B CPU power | Estimated high-load CPU package power for Build B. | watt |
| gpuWattsB - Build B GPU power | Estimated graphics card board power for Build B. | watt |
| otherWattsB - Build B other components | Estimated non-CPU and non-GPU component power for Build B. | watt |
| transientReservePercent - Transient power reserve | Planning allowance added to each build load for short power spikes. | percent |
| targetLoadPercent - Target maximum PSU load | Maximum continuous percentage of the PSU rating used after the reserve is included. | percent |
| psuRatedWatts - Current PSU rated power | The power supply's stated continuous output rating. | watt |
Step-by-Step Calculation
Add Build A component demand
This creates Build A's estimated simultaneous high-load component draw.
configurationATotalWatts = cpuWattsA + gpuWattsA + otherWattsA
Add Build B component demand
This creates Build B's estimated simultaneous high-load component draw.
configurationBTotalWatts = cpuWattsB + gpuWattsB + otherWattsB
Apply the transient reserve
The same reserve is applied to each build to create a reserved peak planning load.
configurationAPeakWatts = configurationATotalWatts * (1 + transientReservePercent / 100)
Calculate the PSU recommendation
Dividing by the target load converts reserved demand into a PSU rating, then rounding up avoids understating the result.
recommendedPsuForA = ceil((configurationAPeakWatts / (targetLoadPercent / 100)) / 10) * 10
Select the larger build requirement
One PSU intended to support either configuration must meet the higher of the two estimates.
recommendedPsuWatts = max(recommendedPsuForA, recommendedPsuForB)
Check current PSU utilization
This shows how much of the current PSU rating each build would use after reserve.
configurationAPeakLoadPercent = configurationAPeakWatts / psuRatedWatts * 100
Compare a 320 W GPU build with a 450 W GPU build
Build A base load
125 + 320 + 80
525 W
Build B base load
125 + 450 + 85
660 W
Build A reserved peak
525 * 1.20
630 W
Build B reserved peak
660 * 1.20
792 W
Build B PSU recommendation
ceil((792 / 0.80) / 10) * 10
990 W
Build B load on current PSU
792 / 750 * 100
105.6%
Final Result
The estimated PSU rating for either build is 990 W. Build A uses an estimated 84.0% of the current 750 W PSU after reserve, while Build B uses 105.6%.
Assumptions
- ✓The wattage inputs represent realistic simultaneous high-load DC component demand rather than idle power.
- ✓The PSU's stated wattage is treated as its available continuous output capacity.
- ✓The transient reserve is a planning allowance, not a measurement of a specific component's exact peak.
- ✓The selected target load reflects a user preference for headroom rather than a universal requirement.
Limitations
- !Actual power draw varies by workload, game, application, component sample, settings, and connected devices.
- !The calculation does not evaluate PSU build quality, condition, protection features, voltage behavior, rail limits, or cable ratings.
- !It does not confirm that the PSU has the required motherboard, CPU, and GPU connectors.
- !PSU efficiency affects power drawn from the wall and is not included in this DC output-capacity calculation.
Common Mistakes to Avoid
Using a CPU's base power figure instead of an expected high-load package power estimate.
Leaving out motherboard, memory, storage, cooling pumps, fans, USB devices, and lighting from other-component power.
Comparing the unreserved component total with the PSU rating while ignoring the selected transient reserve.
Entering the PSU's wall-power consumption instead of its continuous output rating.
Treating a rounded calculator result as proof of connector compatibility or PSU quality.
Related Formulas
Frequently Asked Questions
How is recommended PSU wattage calculated for two PC builds?
The calculator sizes each build after adding its component load and transient reserve, then returns the larger rounded-up PSU requirement.
Why is the recommended PSU higher than the component wattage total?
The result includes the selected reserve and target maximum PSU load. For example, a target below 100% requires a larger rating than the reserved load alone.
What does a PSU load result above 100% mean?
It means the build's reserved estimate is greater than the entered PSU's rated continuous output. Recheck inputs and evaluate a higher-capacity suitable unit.
Does the formula include PSU efficiency?
No. The formula compares estimated DC component demand with the PSU's rated DC output. Efficiency is relevant to wall draw, not this capacity comparison.
Ready to calculate your result?
Use the calculator to get instant results with your own inputs.