Perhaps an explanation will help.
You need about 24 Volts to run your valves and the electronics. This is produced by a transformer. The transformer does two main things.
1). It reduces (steps down) the 120 Volt AC mains power to a safer 24 Volt AC.
2). It is a transformer and is made by two separate windings of copper wire around an iron core. Which is why irrigation power supplies are fairly heavy when you pick one up. The two windings are known a primary, which connects to the mains and the secondary which produces the 24 Volts. There is no electrical connection between the primary and secondary windings. Coupling is entirely magnetic. This means that the secondary winding that you will use to power all of your irrigation system is a). at a lower and safe voltage and b). completely isolated and disconnected from the house mains supply.
The amount of power (current) any given transformer or power supply gives is measured in Amps or, milliAmps. 1 Amp = 1000 milliAmps. Thus your 1500mA transformer provides 1.5 Amps. This is more than enough to run a couple of solenoids (Main and one zone) and the OSPi electronics. Power is really measured in Watts. Watts is a number that is derived by multiplying Voltage by Amps. Therefore your 24 Volt, 1500mA transformer may also be called a 36 Watt, 24 Volt transformer (24 * 1.5 = 36). Thus the more Amps you have for a given voltage, the more power in Watts you have available and, the heavier and more costly the transformer.
I have deliberately not explored AC vs. DC voltages because the OP was asking about the power of a transformer was AC voltage exclusively. In any event the wattage calculation is exactly the same.
I hope this explanation is clear. If you need more information, please post here.