The power supply provides electricity to all the parts of the computer. A bad power supply can make a good computer crash often. Also wattage is an issue an under wattage power supply can make a computer crash or not boot at all. Some power supplies work properly for quite some time then suddenly display symptoms of under wattage. Getting the right power supply can save you from many headaches as well as failed parts.
Power supplies are rated a number of different ways Wattage, and MTBF (Mean Time Before Failure) are the most common. Wattage is extremely important as it dictates the maximum load all the parts of the computer can pull, and therefore the maximum number of components all together. Most off the shelf computers come with under powered power supplies ranging from 250W to 350W, this is not enough. Many modern video card require a 350W power supply from the get go, imagine what will happen if you add a DVD burner or a second hard drive, maybe not very much it just won’t start, or maybe you will burn out the power supply, or better yet the power supply will fry some of your components. These days you really should have more than 350W, 400W-500W is really where we recommend being as far as power goes. If you have a super video card or more than 4 drives or a combination you can even go a little higher for safeties sake. Just remember that the rated wattages are maximums, one spike too close to or about this maximum in you will get a crash or worse.
MTBF dictates the manufactures prediction of how long a unit will go before if fails. Generally rated in hours, this number generally dictates how long the warranty on the unit will be as well. Higher is always better here, most manufactures don’t even advertise this number the ones who do are the units you will want to have. Make sure to look for this number any power supply with more that 50,000HR MTBF will be good, but the best will claim 100,000HR MTBF and that’s about 11.5 years; longer than the life of the computer it will drive.