my personal experience of instataneous water heaters is crap, they do have there uses but i think a cylinder is far better.  Because you are storing an amount of water when the power goes off you still have volume of water to use, now i know infinty's can come with battery back up but that isn't included when you purchase the item off the shelf it is an EXTRA cost.  Dont get me wrong infinitys are good but they have there place, like if i had a holiday home i would have one cause i could arrive and a flick of a switch you have hot water.  If it comes down to running out of hot water then you could time showers, install a tempering valve, addjust tempering valve, turn up temperature on thermostat.  Because you are turning your hwc off the cylinder has to use power to heat the volume of water to set temp on thermostat this requires power usage, thermostats should be set at temp around 55-60 thermostat has a 7 degree + - heating range, your cylinder should heat water to 60 degrees once in a 24 hour period this is to kill bacteria.  If that doesn't solve your problem then maybe the cylinder is too small for the amount of occupants in the house a possible upgrade might be the case.  Oh and the power board switches your cylinder off for you through the ripple rely at the main feed into property this is to save on power and prevent grid over loading.