When choosing a power supply, the first thing you need to do is make sure that it fits inside your case and that it can power all of your components without any physical modifications.
These are the power supply unit form factors available today:
1) ATX form factor
By far the most common form factor. ATX power supplies are 150mm wide, 86mm tall and, those which adhere to the ATX standard, 140mm deep. All ATX power supplies share the exact same width and height, but the depth of the units vary depending on their power output. Those who need more powerful units will also require larger / deeper cases.
2) BTX form factor
A form factor which appeared a few years ago (2004-2006) when Intel tried to push a new design into the market which offered better cooling. The better processor designs that followed combined with the unwillingness of the market to accept the change, sent the BTX form factor into extinction not long after. There are no BTX form factor cases and/or power supplies in production today, but if you do own a BTX case, you will need to find a compatible power supply or change the case itself.
3) CPX form factor
A form factor recently introduced by Antec. Only a few Antec products follow this factor, and no other company has embraced the standard yet either at this point of writing. Antec’s design gives more room to the power supply compartment, allowing the installation of larger (CPX form) power supplies. Larger power supply enclosures are easier to cool, as well as easier and cheaper to design and manufacture. CPX cases are backwards compatible with ATX power supplies (with an adaptor), but CPX power supplies will not fit in ATX cases.
4) miniATX, microATX, FlexATX form factors
When demand for smaller, more elegant cases increased, these form factors began to appear. The miniATX power supply is merely 3cm narrower than a standard ATX unit. The microATX is a smaller three-screw design, and the FlexATX is even smaller than that. All of these form factors are common among very small cases and ATX power supplies are incompatible with them. Very few manufacturers offer high quality power supplies based on any of these factors though.
5) SFX/LFX/CFX/TFX form factors
All of the above form factors are pretty rare and were introduced as designs for special (i.e. very small or thin) systems. Practically no manufacturer offers retail high quality / performance solutions based on these factors nowadays.
6) Non-Standard form factors
End users often believe that when they have a large case from a well-known manufacturer, any good power supply will fit inside it. Unfortunately, that is not true. Several computer manufacturers (e.g. Dell, HP, etc.) often use different, non-standard power supplies in their systems. Even if these power supplies are physically identical to ATX or microATX power supplies, their wiring differs. If you bought a complete system from a well-known manufacturer, chances are that you cannot replace its power supply with an aftermarket solution.
Number of connectors
The number of the connectors available on today’s power supplies is usually sufficient for most systems, but we still recommend that you always check the product’s specifications to make sure that there are enough connectors for all your devices.