FAQ: ATX power supplies for desktop PCs
When building or upgrading a desktop PC, you must pay attention to the power supply unit: It should be powerful enough, but also efficient, quiet and affordable.
A power supply unit for a self-built or upgraded desktop PC must meet various criteria – and still remain affordable. We have answered frequently asked questions for you.
How many watts does my PC power supply need?
How do I estimate how much nominal power an ATX power supply should have for my individual desktop PC configuration?
In principle, it's simple: add up the maximum power consumption of all PC components and add a reserve of 50 to 150 watts. In practice, however, it is complicated, as it is not easy to find out how much power most components need at their peak.
However, there are some simplifications. The largest consumers are typically the main processor (CPU) and – if available – the graphics card (GPU). In comparison, the power consumption of all other components such as RAM, mass storage (SSD/hard disk) and mainboard chipset is hardly significant. If you install one or two SSDs and a maximum of 64 GB RAM, you can assume a flat rate of 20 watts.
Things get complicated with the CPU, because the supposed rated power – called Thermal Design Power (TDP) or Processor Base Power (PBP) – is no good for estimating. Modern processors from AMD and Intel are allowed to consume considerably more power for a short time. For Intel processors in the 65-watt class, this is over 200 watts for 28 seconds. For the 125-watt types, the maximum turbo power is even over 250 watts. You should therefore assume a power supply capacity of around 300 watts for the CPU alone. Although these extreme values can be throttled in the BIOS setup of many motherboards, not all of them (keyword: Configurable TDP). This also slightly reduces the computing power. It is therefore easier to use a thicker power supply, especially as there are hardly any with less than 350 watts.
It is easier with graphics cards because many manufacturers give recommendations for their respective products. However, they include plenty of reserves. Several manufacturers of ATX power supply units provide dimensioning calculators on their websites (PSU Calculator, see box). You can use this as a guide.
We used to recommend that ATX power supply units should not be too generously dimensioned in order to save energy. This is because most PCs spend the majority of their operating time in idle mode and weaker power supply units tend to be more economical under low load. However, this only applies with restrictions. After all, how efficiently an ATX power supply works at a low secondary load of 10 to 20 watts depends less on its rated power than on its individual design.
Why are there no more 250-watt power supply units?
For my economical office PC, a 200-watt power supply unit would actually suffice. But I can only find ATX power supplies with at least 350 watts in online shops – why?
The desktop PC market is shrinking because notebooks are dominating. Mini PCs with mobile processors are also popular. If you build a classic desktop PC in ATX design yourself, you usually also plug in a graphics card and need a powerful power supply unit. And 500-watt power supply units are available from around 25 euros. Because most buyers are primarily concerned with the price, the demand for weaker power supply units is falling.
The importance of idling
I keep reading the energy-saving tip in c't that the most important thing for desktop PCs is the power consumption in idle mode. But I also want to use my PC – doesn't that matter?
Modern PC processors fall asleep within fractions of a second if they have nothing to do. If no other application is running apart from word processing, the CPU cores even clock down between two keystrokes, no matter how fast you type. The average power consumption value for typical PC use is therefore closer to the idle value than to the maximum.
If you switch on your computer 230 days a year for 10 hours each –, i.e. 365 days minus weekends and 30 days of vacation –, it will run for 2300 hours. At 15 watts in idle mode, this adds up to at least 34.5 kilowatt hours (kWh), at 30 watts it would be 69. At 30 cents per kWh, the difference adds up to more than 10 euros per year, i.e. around 50 euros over five years of use. Counterexample: A gaming PC that consumes 200 watts under load. If you play with it for 10 hours every week and switch it off otherwise, it will consume a total of 520 kWh for 156 euros over five years. Although the gaming PC consumes much more power, this does not have a proportional effect on electricity costs due to the shorter power-on time.
12VHPWR and ATX 3.0
Do I need an ATX 3.0 specification power supply with a 12VHPWR cable for my gaming PC?
That depends on the graphics card you want. Cards priced from around 550 euros – roughly from Nvidia GeForce RTX 4070 – have a socket for 12VHPWR plugs. However, adapters for the established PCIe power connectors with six or eight poles are usually also included.
If the power supply is powerful enough, the graphics cards will run stably even with adapters. However, a wire jam can occur: Some high-end cards, which consume over 400 watts, come with adapters for four eight-pin PCIe cables. These are rather wobbly constructions, in which case we recommend buying a power supply with a 12VHPWR output and ATX 3.0. The latter are also designed for particularly high pulse loads.
Length of the power supply cable
Do I have to pay attention to the length of the low-voltage cables when buying an ATX power supply?
Yes, at least if you are using a relatively large ATX case. Because if the power supply unit is in an unfavorable position or the main power ATX connector of the mainboard is in an unfavorable position, the cable length is not sufficient for some power supply units. If in doubt, install the mainboard on a trial basis and measure the required cable length before purchasing a power supply unit.
Power supply units with DC-DC technology
What are the advantages of so-called DC-DC technology for ATX power supply units and what is it anyway?
In the past, transformers with several taps for the different voltages of 12 volts, 5 volts and 3.3 volts on the secondary side were common in ATX power supply units. With this circuit technology, individual voltage rails can come close to their tolerance limits of plus or minus 5 or 10 percent in unfavorable load situations.
ATX power supply units with DC-DC technology can in principle regulate more precisely: They contain a particularly powerful converter for 12 volts and also generate 5 and 3.3 volts from this direct voltage (direct current, DC) via downstream converters. In practice, ATX power supplies are very rarely so heavily loaded that the theoretical advantages of DC-DC technology actually have an effect.
ATX12VO technology
For particularly economical desktop PCs, there is the ATX12VO specification – but I can't find any components for it. Do they still exist?
With "ATX 12 Volts Only" (ATX12VO), the power supply unit only supplies 12 volts and no other voltages such as 5 and 3.3 volts. You therefore need both special ATX12VO power supplies and compatible mainboards. However, these have not yet become established.
There are devices from larger PC brands with proprietary 12-volt technology, which are often particularly economical. However, with a few exceptions, there are no power supply units and mainboards with ATX12VO on the market. However, you can buy adapter cables for some normal ATX power supplies – but then the actual ATX12VO advantage is lost, namely the absence of the converter stages for 5 and 3.3 volts.
If you are looking for a particularly economical desktop PC, then mini PCs with mobile processors are hard to beat. The market niche for ATX12VO is therefore small.
(ciw)
