Choosing a Power Supply:
How Many Watts Are Needed?

Wattage calculation: what is it based on?

What determines the total capacity is what the system at the same time is the highest power that can be drawn. In practice this figure is dominated by two components: the graphics card and the processor. The total of the other components (drive, memory, fans) remains small alongside these two.

The correct approach is as follows: what the graphics card manufacturer states recommended system power value as your basis. This value is already stated taking a typical processor and the surrounding components into account. Leaving some margin on top of it is necessary for two reasons.

Why is headroom left?

• Sudden spikes in consumption: Modern graphics cards produce very brief but very high consumption peaks. Even though the average consumption may look reasonable, if these sudden spikes cannot be covered the system shuts down instantly. This is one of the frequent causes of the “it shuts down during games” complaint in our service centre.
• Ageing: Capacitors weaken over time. A power supply chosen at the limit becomes unable to supply the same system after a few years.

These two reasons make it sensible to choose one step up rather than “just enough”. In addition, if you are considering a graphics card upgrade later on, buying the power supply accordingly from the outset is more economical overall.

What do the 80 PLUS certifications tell you?

These certificates measure efficiency: they show how much of the electricity drawn from the socket is passed on to the computer and how much is lost as heat.

The last point is particularly important. If there is a considerable price difference between two power supplies with the same wattage rating, the difference usually lies precisely here: a cheap product can deliver the power stated on the label only under ideal conditions and for a short time.

The real cost of a cheap power supply

The power supply is the only component in the system that, when it fails, can damage other partsIn a power supply with inadequate protection circuits, voltage fluctuations are passed directly on to the motherboard and the other components.

The protections to look for are as follows: over-voltage, under-voltage, over-current, over-power, short-circuit and over-temperature protection. The fact that these protections are listed in the product description is a concrete indication of the manufacturer's seriousness.

The symptoms produced by a poor-quality power supply are familiar: random restarts, shutdowns under load and unexplained blue screens. In systems that come to us with these complaints we always include the power supply in the scope of testing; the details in our guide on computers shutting down by themselves.

Cable design: modular or fixed?

• Fixed cabling: All cables are attached. This is the most economical option; unused cables take up space inside the case and can obstruct the airflow.
• Semi-modular: The essential cables are fixed and the rest can be plugged in. A common choice in terms of the balance between price and use.
• Fully modular: All cables are detachable. This is the best option for tidiness inside the case and for the airflow.

Important warning: Never swap modular cables between power supplies of different brands or models. Even if the connectors look the same, the pin layouts may differ, and this leads to damage to the connected hardware.

Physical fit and connectors

The physical fit must be checked just as much as the capacity:

• Form factor: ATX is used in standard desktop cases; small cases may require SFX. The supported type is stated in the case manual.
• Length: High-capacity power supplies are longer and can make cable management difficult in narrow cases.
• Graphics card connectors: Make sure that the power inputs your card requires are present on the power supply. Instead of solving it with converter adapters, choose a power supply that has enough connectors directly.

If you would like to check the compatibility of all the components with one another one by one our component compatibility guide provides a comprehensive checklist.

When should it be replaced?

The following symptoms place the power supply on the list of suspects: sudden shutdowns under load, crackling or electrical noise coming from the case, a smell of burning, a fan that does not turn at all, and unexplained instabilities that appear after years. If there is a smell of burning, stop operating the device immediately.

At our service centre we test power supplies under load and measure voltage stability. In this way the uncertainty of “could it be this?” is removed and a sound part is not replaced unnecessarily. To determine the capacity suitable for your system, it is enough to send us your current component list.