How can I determine what kind of power supply or battery I need for my four NEMA 23 stepper motors?
2.8A is the maximum current, but actually nobody uses motors on maximal values. All stepper controllers have the possibility to set up maximum current on each motor depends on your requirements. In most cases, 50% is more than enough. For example, 3D-printers usually working on 1-1.5A current.
Additionally (for 3D-printing) Z-axis moves when other motors in idle state, so even with the maximum current you can save 25% of the expected power.