I would like to heat up my small 1800 litre plunge pool to jacuzzi type temperatures with a combination of six 3 Kw geyser elements in a home made heat exchanger. I have done the maths and reckon to increase the water temp from 18°C to 38°C will take just over 2 hours. This will cost an arm and a leg in electricity I know.

Normally the pool is heated by a donkey boiler, but this requires keeping the fire stoked for a whole day, which in turn requires forward planning. Normally, pool parties are a spur of the moment thing.

My new plan is to be able to remotely activate a Smart Plug, to energize the coil on a contactor, that will close and allow power through to the elements.
I will build in some safety with a theromostat and maybe a flow control switch.

What I would like to know is
1. What size wires would I need if I wired 2 elements per phase, over a distance of about 12 metres from the main DB?
2. What size contactor would I need to handle the load of 6 elements (3 kw each)?
3. Can I trust a Smart Plug to not activate randomly, or not turn off when I instruct it to?