Spot welding machines vary from the small hand held spot welders through to large and very powerful floor mounted hand fed spot welders.
There are many spot welding machines used in the automotive industry the majority are used with robotic systems with a relatively powerful output so they are able to provided 100% duty cycle.
The power required for the spot welding machine that you need depends on the thickness of the material you wish to join, what size spot welds you have to achieve and of course the time taken to complete the spot weld and be ready for the next weld.
People will frequently purchase a spot welding machine based on a "price level" rather than the job they need to complete, this generally results in frustration and disappointment because although a spot welding machine may well have the ability to weld the required material thickness the lower power / lower cost machine does not have the necessary duty cycle to be able to do the job.
Sizing a spot welding machine correctly is important, probably more important than most other welding machines because with other welding methods there are ways around achieving the weld results, with spot welding you really only get one shot to get the weld right and each spot weld needs to be right.