TENS machine purchase: What do I need to know to buy a TENS unit?

A lot of what you need to know about buying a TENS unit is information you already possess. You do need to do some TENS machine purchase research as well, though. First things first, you need to ask yourself why you need a TENS device, and what you are going to use it for, and for how long, and where, and you need to answer those questions truthfully. You also need to find out what these machines do, and how they do it; that is, how they work and how you use them.   
Why. More likely than not, you are experiencing some sort of pain, and that is what has led you to consider the idea of buying a TENS unit. However, first you need to understand that pain is not always a condition in and of itself, but is often a symptom of a condition.  

As such, you have to determine the source and type of that pain; in other words whether it is acute or chronic, or whether it is caused by an injury or a by a condition like arthritis or sciatica. The best, if not only way to achieve this is to consult a doctor who can diagnose the underlying condition and prescribe a treatment; in this case TENS therapy. Here, for the sake of brevity, we’ll summarize it this way; a TENS unit has been developed to relieve musculoskeletal pain (joints and muscles).

What. Now that you know what a TENS device is for, you can freely decide whether you need one or not. Most units available for TENS machine purchase are portable and lightweight, and designed to be use for several lengths of time, including long periods. They can be stored and used at home conveniently. Moreover, smaller and lighter handheld units can be easily carried around. The size and weight you choose thus depends on where you are planning use it, and whether you plan to take it with you someplace in particular. 
How. TENS is a non-invasive, drug-free pain management therapy. However, it does involve electrical current flowing through the skin and into the nerves. Though when done correctly it is painless and has virtually no side effects, you still should know the basics of its mechanism of action, for you not only need to know how a TENS unit works, but how to safely operate one. The general principle of electrotherapy is the same for all models, but the particulars may vary. Most modern devices are digital, but there are still some with analog interfaces. Some have three modes, others five, yet others ten, and so on. Electrodes, batteries, manual, etc., also need to be considered; in fact, make sure if possible to not leave any loose ends whatsoever. 

Related Read: Which TENS Unit is the Best?