PC Power Supply Monthly Cost Formula
Learn how monthly PC electricity use and cost are estimated from component load, PSU efficiency, daily hours, and electricity price.
This calculation converts the DC power used by PC components into estimated wall power, then estimates monthly energy use and electricity cost. It is useful when comparing two power supply efficiencies, PC loads, or daily usage patterns.
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Monthly PC Electricity Cost
Where:
Divide the PC's component load by the PSU efficiency to estimate wall power. Convert watts to kilowatts, multiply by usage time for monthly kWh, then multiply by the price per kWh.
Variables Explained
| Variable | What It Means | Unit |
|---|---|---|
| componentLoad - Average PC component load | Typical DC power required by the PC components during active use. | watt |
| psuEfficiency - PSU efficiency | Estimated power supply efficiency at the typical operating load. | percent |
| hoursPerDay - Daily use time | Average number of hours the PC runs per day. | hours |
| daysPerMonth - Days per month | Number of days included in the monthly estimate. | days |
| electricityRate - Electricity price | Cost charged for each kilowatt-hour of electricity. | currency |
Step-by-Step Calculation
Convert efficiency to a decimal
An efficiency of 85% becomes 0.85.
efficiencyFactor = psuEfficiency / 100
Estimate wall power
The wall must supply more power than the components use because some energy is lost in the PSU.
wallPower = componentLoad / efficiencyFactor
Calculate monthly energy
This converts watts to kilowatts and multiplies by total monthly operating hours.
monthlyEnergy = wallPower / 1000 * hoursPerDay * daysPerMonth
Calculate monthly electricity cost
Monthly kWh is multiplied by the local electricity price.
monthlyCost = monthlyEnergy * electricityRate
Compare the two scenarios
The cost gap is the absolute difference between the two estimated monthly costs.
monthlySavings = abs(monthlyCostA - monthlyCostB)
Comparing 85% and 92% efficient PSUs
Scenario A wall power
350 / 0.85
411.76 W
Scenario A monthly energy
411.76 / 1000 * 6 * 30
74.12 kWh
Scenario A monthly cost
74.12 * 0.18
$13.34
Scenario B wall power
350 / 0.92
380.43 W
Scenario B monthly cost
(380.43 / 1000 * 6 * 30) * 0.18
$12.33
Monthly cost gap
13.34 - 12.33
$1.01
Final Result
Scenario B is estimated to use about 68.48 kWh and cost $12.33 per month, compared with $13.34 for Scenario A.
Assumptions
- ✓The entered component load represents typical DC demand while the PC is in use.
- ✓PSU efficiency is treated as constant at the entered load.
- ✓The electricity price remains constant during the selected month.
- ✓Monitor, speakers, networking equipment, standby power, and other devices are excluded unless included in the load.
Limitations
- !Actual PC demand changes between idle work, gaming, rendering, and other tasks.
- !PSU efficiency can change with load, temperature, input voltage, and unit design.
- !Electricity bills may include fixed charges, tiered prices, taxes, or time-of-use pricing not represented by one rate.
- !A component-load estimate may be less accurate than a properly measured wall-power reading.
Common Mistakes to Avoid
Entering a PSU wattage rating instead of the PC's actual component load.
Using wall-meter power as component load and also applying PSU efficiency, which counts conversion losses twice.
Entering 0.85 instead of 85 for an 85% efficiency input.
Forgetting to include a monitor or peripherals when estimating the whole desk setup.
Using a daytime electricity rate when the PC normally runs under a different tariff period.
Related Formulas
Frequently Asked Questions
How is PC monthly electricity cost calculated?
Estimated wall power is converted to kWh using daily hours and days per month, then multiplied by the electricity price per kWh.
Why divide PC load by PSU efficiency?
Component load is DC demand. Dividing by efficiency estimates the higher AC power drawn from the wall to provide that DC power.
Does a larger PSU use more electricity?
Not necessarily. The wattage rating is a capacity limit; actual consumption depends mainly on load and efficiency.
What efficiency should I enter for my PSU?
Use an estimated efficiency at your typical load level, preferably from reliable product test data when available.
Can I use a plug-in power meter reading?
Yes. If the reading is already wall power, use 100% efficiency so conversion losses are not applied again.
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