Does anyone have any experience with these for household hot water?

I had a read through this and the general gist I got was to stay away from gas which I thought may be the best solution.

I'm looking to replace a 300L hotwater cylinder in our bach, its now 20years old and by the sounds its probably very near or past its end of life. We often rent the bach out and sometimes the cylinder gets depleted and there's no hotwater until it reheats (which can be up to 24 hours) I thought continuous gas would be a good solution but it would need to be a bottled solution as there's no mains gas available.

A plumber has recommended we install another 300L cylinder in-line with the other ($3500) but that then leaves me with the possibility of having to replace the other cylinder shortly as well, so the costs are getting up there. As the bach is only infrequently occupied, the cylinder sits idle for a lot of the time, so I'm guessing there's going to be some energy wastage there also.

The convenience of a continuous electric system just sounds like a winner. Isn't using power when we're not there, hot water won't ever run out, installation in place of current cylinder should be relatively simple, remove the possibility of a leaking or blown older cylinder.

The current cylinder is in a large cupboard under the stairs, so I'm hoping that an electric continuous system can be installed there in place of the hotwater cylinder but I just don't know enough about them.

• Do they need to be vented?
• Is a single phase unit sufficient for a large household?
• If 3 phase required, is there much involved in getting that installed

Any/all views/recommendations would be appreicated.