No home should be without at least one fire extinguisher. Yet thousands are. Part of the problem is that buying an extinguisher can be confusing. More than once I've watched a potential customer look over a rack of them in my local hardware store, then turn away in bewilderment - but it doesn't have to be that way.

There are three types of fires: Class A, fueled by ordinary combustible materials such as wood, paper or cloth; Class B, involving flammable liquids such as gasoline or kitchen fats; and Class C, involving electrical equipment.

Underwriters Laboratories assigns numerical ratings to extinguishers: The higher the rating, the larger the fire it can handle. A small, multipurpose extinguisher - one designed to fi* ght all three classes of fire - might be rated 1-A: 10-B C. This means it has a rating of 1 for Class A fires and 10 for Class B fires. The C rating means that the fire-dousing agent in the extinguisher is non-conducting, suitable for electrical fires.

Most extinguishers sold today are the multipurpose type, more versatile than the old-fashioned water types that can handle only Class A fires or dry-powder bicarbonate-based types designed only for B and C fires. Multipurpose extinguishers contain ammonium phosphate powder and nitrogen gas under pressure that shoots the powder out the nozzle.

Most experts recommend that you have at least one multipurpose extinguisher, and that it be rated no smaller than 2-A: 10-B: C. An extinguisher with this rating will probably be at least a foot-high and four inches or more in diameter. But check the label - the rating may be in small print, but it will be there. Don't buy an extinguisher without a rating, and don't buy one without the UL seal.

What about the little beer-can-size extinguishers? There are lots of these around, most rated 1-A: 10-B: C. They may be okay for small fires, but most discharge in about 10 seconds, and that's just not enough time to fight larger fires.

The ammonium phosphate extinguisher has one weakness - kitchen fires involve hot grease. The multipurpose will put one out, but if the grease is still hot enough, it will flare right up again.

The answer to this problem is to dry-powder bicarbonate extinguisher. It's rated only for B and C fires, but it's especially good on grease fires.

How much will fire extinguishers cost? You can get a good multipurpose unit rated 2-A: 10-B: C for about $30. A bicarbonate type for the kitchen, rated at 5-B: C, is only $10 to $15.