PC Power Supply Annual Cost Formula
Learn how annual PC power supply energy use, electricity cost, and savings are calculated from load, efficiency, usage, and electricity price.
This calculation estimates how much electricity two power supplies draw from the wall to deliver the same PC component load. It converts those losses into annual kWh and estimated running costs, making it easier to compare PSU efficiency at your typical usage level.
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Annual Cost Saving With Power Supply B
Where:
Estimate each PSU's wall power by dividing PC component load by efficiency. Find the difference, multiply by annual operating hours, convert watt-hours to kWh, then multiply by the electricity price.
Variables Explained
| Variable | What It Means | Unit |
|---|---|---|
| L - PC component load | Average power delivered to the PC's internal components, before power supply losses. | watt |
| ηA - Power supply A efficiency | Estimated efficiency of power supply A at the stated load. | percent |
| ηB - Power supply B efficiency | Estimated efficiency of power supply B at the same load. | percent |
| H - Hours per day | Average number of hours the PC runs daily at the stated load. | hours |
| D - Days per year | Number of days the PC is used during the year. | days |
| R - Electricity rate | Price paid for one kilowatt-hour of electricity. | currency per kWh |
Step-by-Step Calculation
Convert efficiency to a decimal
An 80% efficiency value becomes 0.80, while 90% becomes 0.90.
efficiencyDecimal = efficiencyPercent / 100
Calculate wall power for PSU A
This includes the input power needed to cover PSU A's conversion losses.
wallPowerA = pcLoadWatts / (psuAEfficiency / 100)
Calculate wall power for PSU B
The same component load is divided by PSU B's efficiency.
wallPowerB = pcLoadWatts / (psuBEfficiency / 100)
Calculate annual energy for each PSU
This converts annual watt-hours to kilowatt-hours.
annualEnergy = wallPower * hoursPerDay * daysPerYear / 1000
Calculate annual electricity cost
Multiply annual kWh by the entered price per kWh.
annualCost = annualEnergy * electricityRate
Calculate the savings from PSU B
A positive result means PSU B is estimated to cost less to run.
annualCostSavings = annualCostA - annualCostB
400 W PC load: 80% PSU A versus 90% PSU B
PSU A wall power
400 / 0.80
500 W
PSU B wall power
400 / 0.90
444.44 W
PSU A annual energy
500 × 5 × 365 / 1000
912.50 kWh
PSU B annual energy
444.44 × 5 × 365 / 1000
811.11 kWh
Annual cost difference
(912.50 − 811.11) × 0.16
16.22
Final Result
PSU B is estimated to use 101.39 fewer kWh and save 16.22 per year, or about 11.1% of PSU A's estimated running cost.
Assumptions
- ✓Both PSUs provide the same PC component load for the same operating hours.
- ✓Each entered efficiency figure is representative of the PSU at the typical load.
- ✓The electricity rate stays constant over the estimated year.
- ✓The PC load is treated as an average rather than a changing real-world load profile.
Limitations
- !Actual PSU efficiency varies with load, input voltage, temperature, component age, and model design.
- !The estimate excludes monitor, speakers, networking equipment, standby power, and other peripherals.
- !A PSU's rated wattage does not by itself indicate its efficiency or annual power use.
- !Electricity tariffs can include time-based pricing, fixed charges, or taxes not represented by a single kWh rate.
Common Mistakes to Avoid
Entering total wall power instead of component load, which double-counts PSU losses.
Using a peak gaming or benchmark wattage as the average load for every hour.
Treating a PSU's maximum watt rating as its normal power consumption.
Using an efficiency figure measured at a very different load level.
Comparing systems with different component loads instead of isolating the PSU change.
Related Formulas
Frequently Asked Questions
What is the formula for PC PSU wall power?
Wall power equals PC component load divided by PSU efficiency expressed as a decimal. For example, 400 W divided by 0.80 equals 500 W.
How is annual PSU electricity cost calculated?
Multiply wall power by daily hours and yearly days, divide by 1000 to get kWh, then multiply by the electricity price per kWh.
Why does a more efficient PSU reduce electricity use?
It wastes less energy while converting wall electricity into the DC power used by PC components.
What does a positive annual saving mean?
It means power supply B is estimated to cost less annually than power supply A under the entered assumptions.
Ready to calculate your result?
Use the calculator to get instant results with your own inputs.