In common with so many questions the answer starts with..."it depends". It depends mostly upon where you live because the first of two key issues is the ambient temperature of your garage.

If your home is in the one of the warmer parts of the world you have more options to choose from but if there are months of the year when your garage may be 60°F/16°C or lower for weeks or months at a time you will need a dehumidifier capable of effective low temperature operation.

A simple room dehumidifier will be sufficient but, if it is a refrigerant type it must have an active defrost feature. An active defrost system, often referred to as "hot gas defrost" stops the compressor and redirects warm air across the cold coil to melt the accumulated ice. This quickly removes the problem, the compressor restarts and your dehumidifier is back in business.

Few room dehumidifiers have this feature. In its place they have a passive defrost feature usually referred to as "auto-defrost". What happens when ice forms is that the compressor stops, the fan keeps running and the ice is melted by the incoming air.

The lower the ambient temperature the longer it takes for the ice to melt. In low temperatures the dehumidifier will be defrosting for more time than it is dehumidifying.

Dehumidifiers with "auto-defrost" are often rated for operation in temperatures as low as 41°F/5°C but all this really means is that the dehumidifier will not be damaged at such low temperatures. The sad truth is they are scarcely effective when it gets so cold.

The second key issue is the level of relative humidity you need to maintain. To protect your car, your tools, equipment and anything else made with steel from corrosion it is important to keep relative humidity no higher than 55%, preferably 50%.

Refrigerant dehumidifiers begin to struggle when relative humidity falls below 50%. A room dehumidifier of reasonable quality will maintain 50% RH but it will do so more reliably if it has a little more power than the area of your garage might suggest is needed. It will cost more to buy a slightly larger dehumidifier but it will remove more moisture per Dollar, or Pound, of energy consumed, and should run for fewer hours per day, so may be less expensive in the longer term.

The ideal solution is a desiccant dehumidifier. What we are talking about here is a unit that looks like a refrigerant dehumidifier, sounds like a refrigerant dehumidifier and has a fan, but removes moisture by capturing individual water molecules on a desiccant (water adsorbing) wheel or rotor. That "d" in adsorbing is not a typo. The word describes a chemical process, distinct from absorption which is a physical action typified by a sponge soaking up water. This type of dehumidifier is not to be confused with containers of silica gel, without moving parts, which remove small amounts of water and are best suited to closets and drawers.

Because refrigeration is not involved in the process this type of dehumidifier will operate quite happily down to 33°F/1°C and will comfortably reduce relative humidity below 50%.

In the UK both types of dehumidifier are readily available. The Mitsubishi MJE16VX is an excellent refrigerant dehumidifier with hot gas defrost and the ideal refrigerant dehumidifier for a garage. If a desiccant dehumidifier is preferred there are several to choose from, including the highly regarded Meaco DD122FW-MK4.

In the USA the situation is not so good. Desiccant units are very hard to find and are not available from the major retailers. Refrigerant dehumidifiers with hot gas defrost are almost equally rare although one or two suppliers are listing the Mitsubishi unit, but at the ridiculous price of nearly $800, more than twice the price of those on sale in the UK.

For $800 or a little less a small industrial dehumidifier, such as the Ebac CD30, may be purchased. A unit such as this provides hot gas defrost, is rated down to 33°F/1°C and offers the greater reliability of a unit built for demanding industrial applications. Small desiccant dehumidifiers built for industrial use are also available but are much more expensive.

This kind of expense may not even be considered by most people but if you have a classic car in storage in a garage you may feel it is worth the investment.

One final point; if the contents of your garage are sufficiently precious to justify the expense of a dehumidifier it may be advisable to invest a little more to purchase a good quality hygrometer, a device for measuring relative humidity. The humidistats on dehumidifiers, more particularly room dehumidifiers, are not always accurately calibrated. Better to be safe than sorry.