To compute the power supply required you need to add up the power used by each connected component, and use a power supply that is adequate to run your peripherals.
Assuming (At 5V)
500mA for the RPI
500mA for the display
1A for the HDD,
1A for any additional USB
This puts you in the
3A range (
With a 25% buffer -
20W) seems like a reasonable choice.
Power Supply Choices
The simplest, is to continue to use a 5V supply with any external connector you wish.
You can Split the power internally using a simple wiring harness. That is, from the connector, Individual wires go to the Raspberry PI, USB Hub, Display, HDD, etc.
In a real pinch you could use the 5V OUT pin on RPI, which is the same as powering directly off of USB port. This is limited to 2A (10W) by a fuse regardless of supply, and will likely be starved. This can impact the processor if the voltage droops or spikes as the HDD spins up/turns on.
The better way is to use a power regulator board that has a higher voltage input (
12-24V common) with internal regulators for separate power rails.
This lets you use a smaller power supply (fewer Amps), and the impact of the supply wire resistance is minimized. It also reduces interference between different devices in the system.
simulate this circuit – Schematic created using CircuitLab
Separating the loads means less impact if one device dies/shorts or goes berserk. It is also the most obvious place to add a battery, supply voltage monitoring, soft-shutdown, etc. However this adds complexity to the design.
Note, you will probably not do better than the off-the-shelf supply manufacturers in designing a fresh power supply in cost, efficiency, reliability.
It's much easier to use a beefy linear regulator or an off-the-shelf buck/boost and not spend time developing your own power supply circuit beyond the most trivial power supply board .
Its much easier to buy a wall-wart with an extra amp than to design a switching supply from scratch.