I know enough about a lot of things to get myself into trouble. And that is exactly where I find myself, not knowing enough.
I am looking into doing a municipal “backup” system. Before water “sharing” starts and water supply in jhb is not exactly improving.
And this is where I get myself into trouble.
I do not intend on using the system as a backup only, as that would require that I spend more time than id like ensuring that the water is cycled and does not stay stagnant for too long.
The tank would be on the same level as the house, so it is my understanding that pump head therefore does not make a huge difference, I am only lifting the water roughly to ceiling height.
And that is where my questions start, why does it seem like the going standard is to go for a 750W pump? DAB make a 375w pump and the specs look decent to my untrained eye. Ive seen a post from @plonkster stating that he uses one, and that flow is sufficient for his needs.
I now adding a pressure vessel is good for extending the life of the pump as it should not start as often, but would a pressure vessel not also aid in keeping the water pressure and flow rate higher?
Any advice and guidance would be appreciated.
I have a 375W pump happily feeding a double-story house. The pressure isn’t great but definitely acceptable, certainly not much worse than my municipal pressure. I’m not even using a pressure vessel. The pipes of the house already does a bit of buffering for me 
And the specs for the pump are what is important.
Pumps work on curves. The more head, the less volume. Aim for a nice trade-off, not the extreme of any curve.
(Both these parameters hit a maximum capability depending on the pump curve).
So what you have to do is figure out the maximum flow you will likely need. ( Remember that someone will flush while someone else is in the shower).
(There are actually, building regulations governing these things for big buildings, but at the domestic level, a bit of common sense suffices).
Also, estimate the likely head you will need. Again, regardless of the height of your shower, it will still need a bit of umphf behind it. Pressure translates to head.
Then, armed with those, you go and check out pump curves, that satisfy those needs.
After having chosen the pump required, that in turn will dictate the motor power needed to drive it. (There will be a recommendation of the motor to pair with that pump).
So you work the other way around, start with what you need, and work back to the motor.
Don’t start with a motor, and work towards trying to make it fit your needs.
Regarding pipe diameter, you want to keep water travelling below 2.5-3m/s in a pipe otherwise, the motor is working to overcome friction to an inefficient degree.
Obviously, as well, you don’t want the pump to be able to have enough pressure to burst the pipes either.
Pressure vessels in this scenario are a buffer, but I suppose they can take care of a bit of water-hammer effect as well.
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While Phil is completely correct, I do find that the 375W pump/motor pairing you can buy at most hardware/irrigation shops is pretty close to what you need anyway. For my double story house, I concede: It is actually a bit small. I pretty much used it only because I already had it (it was a pump I used to pump well water from a bucket into a washing machine during the cape town droughts).
Check the curves for the pumps you are looking at. If it can do the required flow at the amount of head, you’ll be fine. And when you buy these hardware/irrigation store jobbies, the motor is already paired properly with the pump.
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If you’re only requiring backup water during outages then why not use a larger pressure vessel?
This could be pressurized by mains water pressure via a non return valve.
Then you still have water under pressure and no power is required either… 
Not to sound like a doomsday prepper, a 100L pressure vessel wont get me through any sort of extended outage.I am a little lower than some of the surrounding subburbs, so have been fortunate with outages up to this point, but surrounding subburbs have had multiple outages recently, some as long as 2 or 3 days.
And as it stands last night was our first night of “water sharing” was supposed to be from 9PM til 4AM, some surrounding subburbs taps went dry by 11PM and are yet to be restored.