I originally didn't want to influence the results, but it seems people are having trouble with Per's descriptions, so here, I will try to describe them in more detail impartially.
First, I will call the systems "direct debit with allocation" (2.3), "direct debit with power queue", and "power flow".
I will refer to the actions of manufacturing, researching, and building as "power actions". These are (currently) the only things that use power.
When you have more power than you need:
- The two direct debit systems work identically. When you do a power action that costs X, your power will immediately go down by X at the start.
(Some direct debit systems have an allocation phase, so the "immediately" above could be a few seconds, but this is easy to change and is purely cosmetic.)
- The power flow system will slowly decrease your power over the course of the power action, to a total of X.
There are several major differences here:
- With the direct debit systems, if you have more power than the cost of the power action, you are guaranteed to have enough power to do the power action at full speed. With the power flow system, that's not enough, since your other power actions might drain enough of your power that you no longer have enough midway through the power action.
- With the direct debit systems, your power rises constantly (and it always rises), so not only do you know how much power you have at any given point, but you also know how much power you'll have any any point in the future. With power flow, how fast your power rises/falls changes each time you start or finish a power action - hence TVR's comment about "requiring calculus".
- With the power flow system, power is drained gradually, so you could have enough power even if you currently have less than the power action's cost. However, as above, this does make it effectively impossible to estimate whether or not you have enough power to do something, since it depends on so many factors.
When you have less power than you need:
First, you might think, "Being at low power should be fairly rare". If you do, it's probably because you play on maps with way too much oil.

For a skilled player, their power should always be low - having a lot of power means you've spent a lot less power, and since the strength of your army depends on how much power you've spent on it, that means your opponent has an advantage.
This is best done with a case study. Say you have 60 power, and you manufacture five units that cost 60 power each, and take 60 seconds to manufacture. You gain power at the rate of 1 per second.
- With direct debit and power queue, you get one unit after 1 minute, another 1 minute later, another 1 minute later, etc.
- With direct debit and allocation, the allocation is spread out over the five units, you don't get any units until 5 minutes in.
- With power flow, the power spending is spread out over the five units, you don't get any units until 4 minutes in.
In other words, unless you use power queue, you have to micromanage if you want a unit ASAP - wait until the first unit finishes before starting on the second.
The purpose of a power queue is to automate the micromanagement of power allocation. You can add a power queue to power flow, too, but it kind of cancels out a lot of the advantages of power flow, so you might as well, just use a direct debit power queue if you're going to do that.
Here's another case study. Say you have a bunch of power actions actions queued or in progress, and not enough power to go through it all full speed. You suddenly have an urgent power action you need to get done fast (i.e. at maximum speed).
- With direct debit and power queue, you start the urgent power action, then double-click on the the urgent power action to place it in the front of the queue.
- With direct debit and allocation, you have to hunt through all your in-progress power actions, and cancel all the ones that haven't fully allocated, start the urgent power action (you should have enough power from your canceled power actions for it to allocate completely, so you can start the next step immediately), then start the other power actions back up.
- With power flow, you have to pause/cancel
all your in-progress power actions, start the urgent power action, leave the others cancelled and wait for the urgent power action to finish, then re-start all your other actions.
So as you can see, in low power, micromanaging is most difficult in power flow, and easiest in direct debit power queue. However, the consequences of not micromanaging are most severe in direct debit allocation, and least severe in direct debit power queue.