Hi all,
I have 4 bathrooms and looking at the recommended chart by Tsunami water pump, I need at least 0.5kW model..

But since my bathroom not all the time all 4 also using at the same time, would buying a 0.35kW pump be sufficient?

Water pump causes havoc to the internal piping system in the long term. Speaking from experience, water starts to leak everywhere once the pipe aged. I'm referral to the internal pipe the developers use. I wonder why they must use the ciput blue colour pipe which according to my plumber it's low quality. Until today, no end in sight as to the solution.

The blue color is ABS which was probably popular 10+ years ago. In 15 years it become brittle and easy to crumble unlike what manufacturer stated. Screw thread also after once or twice screw unscrew, thread gone.

I have cut and joined with PVC type near tank there. Not sure when they will crumble

If you need to install one, go with inverter type. Water jetting can cause pipe burst. Inverter type is gentler on pipe and produce constant pressure unlike pressure switched with tank.

Is the water too slow? Actually slow water flow also good. Lower water bill significantly

I think can go for 3 bathroom usage at one time if cost matter.

Back to TS question, how to calculate the pressure required for a house? Unnecessary pressure for sure will cause the pipe to degrade faster.

Depend on how much the pipe can stand is the max. I cannot remember but for in house there is recommended range. The pump stop at preset pressure. Once more and more pipe opened, pressure drops.

That is why use inverter. You set the pressure in pipe normally around 1.5 bar(?) and the pump will make the pressure constant at that lever no matter how many taps opened until max the pump can provide.