To size a power supply you must match the supply voltage to the voltage of your outputs, then choose a wattage that is big enough to power all the outputs that will be on at any given time. See below for an example.

Voltage
Determine the voltage of your output devices, generally these are solenoids or relays. The voltage must be 12 or 24 volts DC to be used with our solid-state output products. The voltage of your power supply MUST match the voltage of your output devices!

Choosing a Voltage
If you are going to use the solid-state outputs on our products we highly recommend you use 24 volt solenoids and relays. The reason for this is the current capacity of solid-state outputs. Each solid-state output has a current limit of 0.5 amps, which is 6 watts at 12 volts, or 12 watts at 24 volts. There is also a 1.0 amp fuse inside each solid-state product that protects the driver chip from overheating. This prevents the total current draw at any one time from exceeding 12 watts at 12 volts, or 24 watts at 24 volts. Therefore when you use 24 volt solenoids you can get away with using twice as much wattage without exceeding the current limitations. This is a common limitation for solid-state controllers from almost every manufacturer.

Wattage
The wattage of your supply must meet or exceed the total wattage of all your outputs plus 3 watts for the controller. To do this, add up the wattage of all your valves, this should be clearly marked on the label of each valve, 5 to 6 watts is common. You may not have all the outputs turned on at any one time, so you can cheat a bit here. Add up the wattages of the most outputs that will be turned on at any given time.

Example
Let's say you have six 24VDC 6 watt valves connected to a controller, but a maximum of three of them will be turned on at any given time.

3 x 6 watts = 18 watts
Controller = 3 watts
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Total = 21 watts

Common Solenoid Wattages
MAC Valve 3-Way 1/8" Port: 12v = 5.4 watts / 24v = 5.4 watts
MAC Valve 3-Way 1/4" Port: 12v = 8.5 watts / 24v = 6 watts
MAC Valve 3-Way 3/8" Port: 12v = 6 watts / 24v = 6 watts
MAC Valve 3-Way 1/2" Port: 12v = 6 watts / 24v = 6 watts