PC energy cost and power supply efficiency
A computer’s running cost depends on how much power its parts draw, how long it is on, and how much the power supply wastes along the way. This page shows how the PSU Calculator works it out.
What power supply efficiency means
A power supply turns the alternating current (AC) from the socket into the direct current (DC) the components run on. Part of the input is lost as heat on the way, so a computer always draws more from the wall than its parts consume.
Efficiency is the output divided by the input.
A supply that is 90% efficient and delivers 450 W to the components draws 500 W from the socket; the other 50 W warms the room.
80 PLUS efficiency ratings
The 80 PLUS programme certifies power supplies by their efficiency at 20%, 50% and 100% of rated output. Each tier sets a minimum at all three points, and the calculator uses those minimums:
| Rating | At 20% load | At 50% load | At 100% load |
|---|---|---|---|
| 80 PLUS | 80% | 80% | 80% |
| 80 PLUS Bronze | 82% | 85% | 82% |
| 80 PLUS Silver | 85% | 88% | 85% |
| 80 PLUS Gold | 87% | 90% | 87% |
| 80 PLUS Platinum | 90% | 92% | 89% |
| 80 PLUS Titanium | 92% | 94% | 90% |
These are the thresholds for 115 V mains. On 230 V mains, common in Europe and Brazil, supplies run a couple of points more efficient, so the estimates err on the cautious side. Between the certified points the calculator interpolates in a straight line; below 20% load it keeps the 20% figure.
From the components to the wall
Efficiency changes with load, so the same parts cost more to run on a less efficient supply.
At the example build’s peak load of 492 W, its 650 W supply runs at 76% of capacity. Each tier at that load:
| Rating | Efficiency | From the wall | Lost as heat |
|---|---|---|---|
| 80 PLUS | 80.0% | 615 W | 123 W |
| 80 PLUS Bronze | 83.5% | 590 W | 98 W |
| 80 PLUS Silver | 86.5% | 569 W | 77 W |
| 80 PLUS Gold | 88.5% | 556 W | 64 W |
| 80 PLUS Platinum | 90.5% | 544 W | 52 W |
| 80 PLUS Titanium | 91.9% | 535 W | 43 W |
Energy use and cost of any device
Energy is power multiplied by time. A kilowatt-hour (kWh), the unit on an electricity bill, is 1,000 watts for one hour:
kWh per day = watts × hours per day ÷ 1000
kWh per year = kWh per day × 365
cost a year = kWh per year × price per kWhEnergy use of a PC
A PC does not draw a fixed amount. A processor at its full rating while rendering may use a fraction of that at the desktop, and a graphics card idles far below its board power. The calculator therefore splits each day in two:
- Heavy hours — gaming, video editing, 3D rendering — at the build’s full estimated load.
- Light hours — the rest of the time the computer is on — at a light-use draw: a share of each part’s rating.
Each part of the day passes through the power supply at the efficiency it has at that load. Monitors are added at the wall: they plug into the socket, not into the power supply, so they count towards the bill but never towards the PSU size.
kWh per day = ( peak load ÷ efficiency at peak × heavy hours
+ light load ÷ efficiency at light × light hours
+ monitors × hours on ) ÷ 1000The share of its peak each kind of part is assumed to draw in light use:
| Part | Light use, share of peak |
|---|---|
| Processor | 20% |
| Graphics | 10% |
| Motherboard and USB | 50% |
| Memory | 50% |
| Storage | 30% |
| Fans | 50% |
| Other components | 50% |
The light-use shares are rough by nature and lean high, so the estimate is more likely to land above your bill than below it. A plug-in power meter gives the real figure.
A worked example
The calculator’s default build — Ryzen 7 9800X3D and GeForce RTX 5070, with memory, an SSD and case fans — has an estimated peak of 492 W, draws about 96 W in light use, and is recommended a 650 W supply. Used 8 hours a day, 2 of them gaming, with monitors drawing 30 W in total, at 0.20 per kWh, it uses this much a year:
| Rating | kWh a year | Cost a year | Saving vs 80 PLUS |
|---|---|---|---|
| 80 PLUS | 799 | 159.76 | — |
| 80 PLUS Bronze | 774 | 154.76 | 5.00 |
| 80 PLUS Silver | 750 | 149.97 | 9.79 |
| 80 PLUS Gold | 735 | 146.95 | 12.81 |
| 80 PLUS Platinum | 718 | 143.55 | 16.21 |
| 80 PLUS Titanium | 706 | 141.25 | 18.51 |
Costs are in the currency of the price: the arithmetic is the same in any currency.
Average efficiency
To describe a supply’s efficiency over a whole day with one number, the two efficiencies can be averaged, each weighted by the hours spent at it:
average efficiency = efficiency at peak × heavy hours ÷ hours on
+ efficiency at light × light hours ÷ hours onIn the example, on an 80 PLUS Gold supply: 88.5% for 2 hours and 87.0% for 6 hours average to 87.4%.
Is a more efficient power supply worth it?
A more efficient supply usually costs more to buy. The payback period is how long its lower running cost takes to repay the difference:
payback in years = (price of efficient unit − price of basic unit)
÷ (yearly cost with basic unit − yearly cost with efficient unit)In the example, an 80 PLUS Gold supply saves 12.81 a year over a basic 80 PLUS one. If it costs 20.00 more, it pays for itself in 1.6 years.
The longer a machine runs and the harder it works, the sooner efficiency pays: a server that is always on, or a workstation rendering all day, recovers the difference far faster than a PC used for an evening’s gaming. Efficient units also run cooler and are often quieter, which no payback period counts.
Sizing the power supply
Efficiency decides the running cost; capacity decides whether the build runs at all. The calculator adds every part up at its maximum rating, then picks the smallest common supply that keeps that peak below a set share of its rating.
For the example: 492 W ÷ 80% = 615 W, so the next common size up, 650 W.
The spare capacity absorbs what a sum of ratings misses: graphics cards briefly draw well above their rated board power, components age, and parts get added later. A supply that is not run at its limit also sits nearer its efficiency peak.
Size your own build in the PSU Calculator