This pressure tank is 19 litres and available for just a few dollars
The weight is 4kg
This one is a different brand and 100 litres
The weight of the 100 litre tank is only 15.4kg. I believe both use a similar membrane, just obviously the 100 litre is larger. However the price of the larger one is 20x more expensive, whereas the capacity is only slightly over 5x higher.
According to the use, which is to stop a submersible well pump of 16l/minute flow from running continuously, in conjunction with a digital pressure gauge/switch (e.g. run the pump from 2 bar up to 4 bar), I believe that around 95-100 litres is an appropriate size. The end user (in Indonesia), who is not me, has a limited budget, hence considering using 5 tanks instead of 1. I can see some advantages in that the larger tank does not appear to be very high quality (higher quality tanks are available but much more expensive again), whereas the smaller tanks are cheap enough to be disposable. But maybe there is some problem with 5 tanks in series that I have overlooked which would make this a bad idea?