Designing an Avionics Power Supply
The first thing to do is to define the problem. Work done earlier in the Avionics Power section have gone a long way to satisfy this first step. The calculations suggest a worst case current of $3.152\,A$ needs to be considered. Also, using a 3S battery, pegs the power dissipation at $19.2\,W$, so some way of dissipating the heat will need to be considered. If a higher voltage battery is to be used, the power dissipation will be higher still.
This is why many avionics power (BEC) devices have large aluminum heat sinks. It is important that the heat sinks have a good amount of cooling air. They are the only thing standing between effective regulation, and a burnt out avionics power system, with the ensuing pile of broken parts which were once your aircraft.
Power is the ability to do work. There is lots of work to be done in an RC aircraft's avionics system.
One of the most obvious things that requires work to be done in an aircraft, are the control surfaces. In a remotely piloted aircraft servos are used to do this work. By far servos consume the lions share of the avionics power budget.
Modern receivers, used in RC aircraft, with no servos attached, consume very little power. They use so little power, that it is often safe to ignore them as loads to the power system.
Only after servos are connected does any significant current pass into the receiver's power connector. This servo current, for the most part, is simply passed through the receiver's bus to the receiver power connector. It is the aggregate current of the servos attached to the receiver, which dissipate the power. The receiver itself consumes very little, regardless of the number of servos attached to it.
Servos are typically the most significant loads to the avionics power system. Other systems, such as electronic retractable landing gear, can contribute significantly to the avionics power system load.
In the context of the "park flier" rules, the servos alone will be considered. However, the principle is the same. Add up the worst case load, then add a little more load for safety's sake, and design for that grand total.