In determining what type of breast pump is the right choice for a nursing mother, it is first necessary to understand the different types of breast pumps available. Each type of pump has different strengths and weaknesses relative to the specific needs of each mother. Below are details on manual and battery/mini-electric pumps. For more types of pumps, see my article "Types of Breast Pumps - Electric and Hospital Grade" in related links below.
Manual breast pumps do not require electricity or batteries, but are generally operated by some sort of hand trigger or slide that creates suction to draw out the milk. Most manual breast pumps express milk from only one side at a time, and so to pump both breasts will take a longer time and require switching sides than a double pump (a few can be rigged up to double pump, but this requires some pretty fancy balancing of pump mechanism and breast cones to actually do the pumping).
Manual pumps are generally used for infrequent pumping to feed, to express small amounts of milk for mixing with food or to relieve breast pressure, and can also be helpful to help resolve plugged ducts. They are generally most appropriate to express milk from a mother with a well-established milk supply, and not to help build or establish supply. Some mothers who are especially "good at pumping" do use a manual pump as an everyday, frequent use pump and report high levels of satisfaction, but this is not terribly common.
Manual pumps generally cost less than $50, and are considered single-user items. Popular examples include the Medela Harmony and the Avent Isis. I believe they are a good investment and will be useful at one time or another for most nursing mothers. If desiring a "just in case" breast pump in the house, a manual is a good choice.
Battery/Mini Electric Pumps
Battery/small electric pumps tend to be a more uncommonly chosen pump. They are usually similar to manual pumps, but replace the hand trigger or mechanism with a battery-powered motor. Some come with a wall plug option, some work with a rechargeable battery and some require standard disposable batteries. Some pump one side at a time and a few can double pump.
I am sure that there are some circumstances and women for whom these pumps are just perfect and effective. But in general, I believe that either a manual pump would be just as sufficient for much less money (itís really not so much of a workout to operate a manual pump that a motor assist is truly necessary!) or that a double electric pump is more appropriate to sustain breastfeeding with frequent pumping.
The biggest trouble with battery pumps is that as the battery loses power they can become less efficient and effective. Examples of pumps in this class are the Medela Single Deluxe, Medela Swing and Avent iQ (uno or duo). Advantages include the smaller size and generally cost of these pumps compared to a standard double electric. Price generally ranges from about $50-$150.
Manual and battery pumps are most commonly effective for infrequent pumping with well-established supply. For details on frequent-use pumps that can also assist in establishing or building milk supply, see my related article on Electric and Hospital Grade Breast Pumps.