If you're thinking about buying an air compressor, there are several important factors to consider. If you want to be happy with your purchase, and get the performance you need, you should make sure you look at each one carefully. I will assume that you are buying either a gas powered compressor or one that is powered by 110/220 household electricity, not one of the 12-volt models powered by your car's cigarette lighter (they're good to air up a tire if you have plenty of time to wait).
- Gas Powered vs. Electric - A gas powered air compressor can be very handy, reliable and powerful. And, you're not tied down to an electrical outlet. The down side is that a gas compressor is going to be pretty expensive and will have to be vented in a closed area. Electric compressors are very convenient for shop use, and good ones are very reliable, but some household electrical systems will have problems handling the amperage draw of a even a large 110v compressor, and to have a 220v model you might have to do some extensive electrical work. Electric is best for your home garage unless your electrical system can't handle it, or you come across a great deal on a gas compressor.
- Oil-Free vs. Oiled - An oil-free compressor (sometimes advertised as Maintenance-Free) is designed for people who know little about tools and machinery and don't have much interest in learning more - it will last longer than an oiled model that has been neglected, but that is about it's only advantage. An oiled model is usually much quieter, more reliable and lasts longer. If you use a high grade synthetic pump oil, those advantages will be even greater, and you won't have to change the oil very often. And no compressor is maintenance free, you need to at least drain any accumulated water out of the tank occasionally. If you want the best, get an oiled model.
- How much Horsepower? - It really doesn't matter how much horsepower a compressor has as long as it meets your airflow requirements, which is a combined function of the power and efficiency of the compressor and the capacity of the air pump. About 2HP should be fine for any home compressor if it has enough airflow to fulfill your requirements.
- How much PSI? - Again, unless all you to do is air up some big tires, Air Flow is the important factor. As long as you get a compressor that can supply 125 or 150 PSI, you'll be prepared for anything that comes along.
- How much Airflow? - Here's where you really need to figure out exactly what you may possibly be using this compressor for in the future. If you are going to ever use any air tools, such as a half inch impact wrench or air ratchet, then you will need 6.0 SCFM at 90 PSI - memorize that, 6.0 SCFM at 90 PSI. Even that may not suffice for full time use of an air impact hammer. Other tools such as paint sprayers operate at 40 PSI, but the 90 PSI figure is key, it's the most important thing to consider to make sure you get a compressor that will provide the performance you need. If you just want to air up tires or other inflatables, or do things that require only intermittent use of the compressor, you don't need nearly as much SCFM.
- Tank Size? - For home auto technicians that foresee themselves using air tools, you need a big tank. 20 gallons would be a starting point, but 30 would probably be about ideal for most home mechanics. For airing up tires, one of the smaller "hot dog" or "pancake" style tanks is fine as long as the compressor provides 125 PSI.
SUMMARY - If you want a good home shop compressor and use air tools, spend about $300 on a 6.0 SCFM or higher compressor with 30 gallon tank. If you're not a mechanic and just want to air up tires, don't waste your money, get one of the smaller 2 gallon models.
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