Wattage Rating
Power is the rate at which things generate or use energy.
- measured in watts (W)
- calculated for electrical components as
- calculated for electrical components as
- A PSU must be able to meet the combined power requirements of the PC’s components
- PSU’s output capability is measured as its wattage rating
- PSU for a standard desktop PC is typically rated at around 200–300 W
- Enterprise workstation PCs and servers often have units rated over 300 W to meet the demands of multiple CPUs, additional memory modules, disk drives, and tape units
- Gaming PCs might require 500 W or better power supplies to cope with the high specification CPU and graphics card(s)
Info
- The power requirement of different components varies widely.
- E.g., CPUs can range from 17 W to over 100 W, depending on the model
- If you are building or upgrading a system, the simplest way to work out the power requirement is to use an online calculator
Tip
The power output is not the same as the power the PSU draws from grid power. If a PSU works at around 75% efficiency, a 300 W supply would draw 400 W from the outlet.
The extra energy is lost mainly as heat. As energy becomes more expensive both in terms of cost and in terms of the climate, power efficiency is an important criterion to use when selecting a PSU. An ENERGY STAR 80 PLUS compliant PSU must be 80% efficient at 20–100% of load.
When specifying a PSU for a system with high power requirements, it is also important to assess the power distribution for its output voltages (3.3 VDC, 5 VDC, and 12 VDC).
- Distribution refers to how much power is supplied over each rail
- A rail is a wire providing current at a particular voltage
The following table shows an example of how power distribution for a PSU might be configured:
| Output Rail (VDC) | Maximum Load (A) | Maximum Output (W) |
|---|---|---|
| +3.3 | 20 | 130 |
| +5 | 20 | 130 |
| +12 | 33 | 396 |
| -12 | 0.8 | 9.6 |
| +5 (standby) | 2.5 | 12.5 |
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Note that the output of +3.3 V and +5 V has a combined limit. For a modern computer, the output rating of the +12 VDC rail (or rails) is the most important factor, as +12 VDC is the most heavily used.