Anyone got thoughts as to which is the best way to go for new house build?

A bit of info. 3 adults and 2 kids for showers etc and don't really need hot water in the kitchen, laundry or bathroom sinks.

We are in Phuket so no cold weather anticipated.

If your hot water demand is limited to showers, I would stick with the single point of use 'instantaneous' model. 3.5KW should be good for Phuket.

Advantages and disadvantages to both.

If your points of use are fairly close together a central system will place less peak demand upon your power supply (important if you're out of town), but of course there's the danger of running out of hot water.

Instantaneous point of use heaters provide a constant flow of hot water, but for those who require high flows of blistering water they put a significant strain on the electrical supply.

We have two 7kW Redring multipoint instant heaters (variable 3.5, 5 and 7kW settings), in northern BKK they spend summer on 3.5kW (or even off) and winter on 5kW with occasional blips of 7kW if the weather is really cold.

Don't discount solar, either as a primary heat source or as a pre-heat for a conventional heater, lots of potential savings.

Actually water heater for me is likely in use for about two or three minutes for a shower so even if at full power solar would never pay back (have solar in form of SS water tanks above ground so not much heating required most days). Those that insist on rain showers or continuous flow when soaping and cleaning might have higher demands but doubt they are that much for most. As for bathroom sinks if you locate right the shower hose can serve the sink for hot shaving water without extra plumbing. We get by in Bangkok with 3.5kw heaters although may have to reduce flow a bit on coldest days (perhaps a handful per year). But admit do not like steam room temperature.

Don't discount solar, either as a primary heat source or as a pre-heat for a conventional heater, lots of potential savings.

Have you ever done the math on solar hot water payback time vs a same priced grid-tie PV system?

Of course, YMMV, but for me it worked out to something like 6+ years payback. Grid tied solar PV is < 3 years. No brainer for me....

I've not done the sums recently, but it does make sense not to waste all that lovely sunshine, solar water heating can be very low cost to implement.

If you can get grid-tie PV down to < 3 years without an advantageous feed-in tariff I need to know your supplier, looking to purchase some time soon (after the wet now I expect).

Don't discount solar, either as a primary heat source or as a pre-heat for a conventional heater, lots of potential savings.

Have you ever done the math on solar hot water payback time vs a same priced grid-tie PV system?

Of course, YMMV, but for me it worked out to something like 6+ years payback. Grid tied solar PV is < 3 years. No brainer for me....

Some passive solar hot water systems can be fabricated by the DIY homeowner for a low cost.