The power supply is arguably one of the more important parts of our systems, it is what powers our whole system and a faulty power supply unit could ruin your system just as easily as tossing the whole build into a tub of water.
When building a computer we often spend most of our money on the latest and greatest processors and the best graphics card we can afford. Next we buy RAM, then the motherboard, leaving the power supply as one of the most over looked parts, usually people go for the first one they find with enough wattage to ensure their maxed out CPU and graphics card have enough power.
So what should you look for in a power supply? How many watts do you need? Should you buy modular, semi-modular or a non-modular power supply? What is the difference between an 80+ certified gold/silver/bronze and a power supply that is not 80+ certified at all?
When considering a power supply for your build the first thing you need to decide is how many watts you need. Generally the more watts a power supply unit supplies, the more expensive. This leads many people to buy a power supply unit that is the equivalent or only 50w more than they need. To figure out how much wattage you need take the max power draw of all your components (pcpartpicker is helpful here) and then add about another 100-150 watts. The extra 100-150 watts is a like a buffer, ensuring that if you add an extra drive or two you'll still be getting all the power your system needs. While some buy the bare minimum, Others may think the more watts the better, this is also not true. If you plan to upgrade your system's CPU or graphics card at some point in the near future you may consider adding an extra 500 or so watts just to be sure it's completely future proof and that's fine, however if you have a system that uses a max of 300 watts a 1050 watt power supply is going to be a waste as those extra watts will never be utilized and the power supply will be wasting power as they are usually most efficient at about 80-90% the max draw of the system.
This is also where the 80+ certification comes in, the more efficient the supply, the better rating; with gold being the best. I highly recommend you choose a silver or better power supply, that way you can be sure your supply is not only efficient, but it's actually giving you the amount of watts you paid for.