When you consider the problem of choosing a sprinkler timer for a landscape you first look to divide the landscape into comparable zones.  Sunny lawn areas all grouped together, drought tolerant shrubs in another zone, shaded areas or pots in the next.  Each of these sections of the irrigated landscape will have different needs of irrigation frequency as well as the length of time the sprinkler timer will be set for each section or zone.   Each type of landscape will require a separate program on the timer.  Most sprinkler timers come with 2-4 programs.  

The biggest limiting factor is to consider how much water can be supplied to the landscape through the meter, measured in gallons per minute.  Sprinkler heads also rated in terms of gallons per minute at a given pressure.    In areas that are going to be irrigated with spray heads I mark out on a plan how many heads will fit around the perimeter of the space to be irrigated.  This can be done on the ground if the landscape is already in place.  The heads should be spaced so that one sprays onto the next and that all heads in each area are of the same type.  If heads of different types are mixed the rate of precipitation will not be consistent, leaving you with wet and dry spots.  Once the sprinkler heads have been located I mark on the plan the amount that each sprinkler head emits under ideal conditions.  I then divide the area into equal parts each requiring significantly less water than the meter can provide.  

 Planning a sprinkler system from scratch is often easier than fixing an existing one.  

When I choose the sprinkler timer I start by calculating how many stations, or valves will be in a system.  Then I look to see who will be using the timer.  If it is the gardeners and they are comfortable with timers, using a good quality timer will save time and money in the end.  If some of the people likely to use the timer are computer illiterate or if the electricity is not dependable a mechanical timer becomes more appropriate.  If money is the key aspect of your planning, go for a home center special.  I find these sprinkler timers frustrating to figure out but they work, and they are cheap.  Each one seems to have a different programming logic.  So I often have to read the manual repeatedly to finally figure each one out.  A battery powered sprinkler timer is a choice of last resort, they rarely work for long and are fragile if the pressure is high, but if the cost of running power out to a valve is too extreme then this may be the best answer available.  Recently it has become common for water companies to provide partial rebates for water saving smart timers.  

Typical Program: I water lawns two to three times a week, sometimes with a double cycle on the days that they run in order to get better penetration.  I run potted plants as often as once a day.  Many shrubs and fruit trees perform best on a weekly watering or even less frequently.  A new lawn will be watered three times daily.  With each of these different conditions I need another program for the sprinkler timer.

Design Criteria: I design sprinkler systems so that they do not lose pressure if someone flushes a toilet. I look at the meter size and check the pressure. I cross reference those two pieces of information. And figure out the gallons per minute that the meter can supply. I then check to see that the pipes are working reasonably by running a hose at full force to see that the pipes are not blocked by rust. An old pipe may only have a space the size of a pencil for the water to pass through. I use no more that 75% of the water that I estimate is available. Each head is rated to emit a certain amount of water. I add up the ratings for each head that I use on a system until I reach that 75% of the available water. Once I have enough heads on a valve to use up all of that water I put in a new valve. I try to keep all of the sprinklers on a valve watering similar types of plants: i.e. lawns, shrubs, groundcover, vegetable garden . . . etc. That way I won't have to keep that fuchsia happy when the rest of the more drought tolerant plants don't need it. I like using a timer because I often get distracted and forget to turn off the water in a timely manner. I also like to run the water at night when the evaporation of that water will be at the lowest rate possible.

How much Power? The more complicated problem is to decide how many features are necessary.  Sometimes the answer is none, because it is just too much of a headache.  With the Nelson Traveling Sprinkler you set the hose to the path that you want the sprinkler to follow and turn it on.

Battery Timer: Battery Powered single station sprinkler timers can fulfill a need for awhile. I find that they rarely last more than a single season. And sometimes not even that.  Every one that I have put in eventually came to a bad end.  The weather is hard on them. The minerals in the water harder. The pressure of the waterline is the final killer.  There are lots of single station sprinkler timers out there, none that I recommend without some caution.  Disconnecting these irrigation timers during the rainy season is essential.  Ensuring that the pressure is not over 60 psi is critical.  I think that the best way to use them is like a kitchen timer where you turn the water on and it turns the water off.  Then you go back out once you remember and turn the water off to the timer.  This is certainly cheaper than to risk allowing the water bill to mount unchecked when the valve finally breaks.