50L is usually RM750-850 if direct install plus some piping for bypass and overflow.

The tank is like a hot water flask. It boils water till 70 degree and reboil when drop below 60. If not been using, the tank is good to hold the temperature for 1 reboil within 12 hours. I personally find it slightly cheaper than instant heater. But at the same time we are taking bath twice as long, so maybe even cheaper if you keep your habit.