The highest quality and most stable Power Supply Units available. Protected by industrial grade protection circuitry and can output wattage at a rated maximum temperature of 50°C or below. These units are also Haswell certified, meaning they can cope with the C6 and C7 sleep states of 0.05Amps without triggering the under-current protection switch. May even go over labelled wattage and still work until it safely shuts down.
Tier Two
As above, just slightly lower quality. Still safe to use and are all stable for any PC of today's standards, including Haswell C Stepping certification. Recommended for those on a fairly limited budget.
Tier 3
Some Haswell compatible, some not (maybe unconfirmed). Still safe to use and stable, just lower quality components. Not really ideal in serious overclocking or super-high load situations, such as a Bitcoin mining rig or a high end gaming system.
Tier 4
Built down to a low price. Not exactly the most stable units ever created. Very basic safety circuitry or even thin gauge wiring used. Not for gaming rigs or overclocking systems of any kind. Avoid unless your budget dictates your choice.
Tier 5
Avoid IMMEDIATELY. These units are highly unsafe to use. No such protections added, very thin gauge wiring used, false advertising and too much to list. Reference to a higher tiered unit for a better, money saving and a much safer unit. For your safety's sake, please don't order or pick one up for use in your system. These units are a potential fire hazard and could even kill you, let alone your system.