Large Power Supplies

Whether you're building a new computer or repairing an existing one, I think there is a lot of mystery about what size power supply to use. More often than not people buy too large of a power supply for their needs. Take for example the following specs - a brand new Core i5 CPU, 4 GB of RAM, 750 GB hard drive, DVD burner, gigabit network adapter, Intel graphics card, and onboard high-definition audio. How large of power supply should be paired with this hardware? 400W? 500W? 600W? Me personally, I would use something around 250W for such a system. You could probably safely go as low as 200W, and I wouldn't go any larger than 300W.

The confusion about power supply size is understandable. After all, you can't exactly find a chart of telling you what size to get. And manufactures and distributors don't make it any easier. For example, here is a graph showing the distribution of power supplies at newegg.com:



By this graph you can clearly see they expect most people to buy power supplies in the 500W - 700W range. Unless you're building a super hot system (pun intended) with multiple high-end graphics cards, multiple hard drives with RAID, etc. you should never need more than 300W.

You could make the argument that it doesn't really hurt to buy more than you'll need, just the additional cost of the larger supply. But there is a hidden cost associated with a larger power supply. Most power supplies are more efficient only when delivering close to their maximum amount. So a power supply which is 80% efficient at 500W is probably only 50% efficient at 300W, and maybe only 40% efficient at 100W. So in the long run you'll spend more on electricity.

Speaking of efficiency, there is a standard called "80-plus" for power supplies which meet or exceed 80% efficiency at their rated wattage. These power supplies cost more but I highly recommend buying them. This falls under my "buy nice or buy twice" philosophy. To achieve these higher efficiency levels they have to use higher quality components which, in addition to being more efficient, last longer. I've had a lot of power supplies die over the years, and every one that has died has been a cheap power supply. Never once has an 80-plus supply died on me. I've also noticed that an 80-plus power supply will outperform a higher wattage regular power supply. So an 80-plus 300W will provide better power than a 400W regular supply - through the entire range, even into the 300+ range.

So the next time you're in the market for a power supply, unless you're using multiple video cards or a large number of hard drives, I would suggest no more than 300W for the power supply. The money you save buying a smaller power supply, put that money into upgrading to an 80-plus model. In the long run you'll be glad you did. Oh, me personally I love the brand SeaSonic. They make very high quality power supplies without any of the gimmics (like embedded blue LEDs). They just make really great power supplies.