I have recently let a townhouse to a couple who complain of running out of hot water with approx. one dishwash and one shower. Up until this a single person resided there and never complained. The power is off-peak (coming on at 11pm & off at approx. 6am), and the cylinder is 180 litre Mains Pressure. I have tripped the relief valves and tested shower flow and all is normal. The more I think about it I'm wondering if the cylinder is large enough considering the input of HP cold as soon as say, the kitchen tap is turned on, and of course it wont matter what the thermostat does because there's no power until 11pm. Am I barking up the wrong tree? I thought there used to be a booster system here in Christchurch where you could do a daytime boost, but have not been able to verify this.
Any help would be appreciated.

Linkback: https://www.plumbers.nz/q-and-a-hot-water-cylinders/44/night-rate-power-vs-cylinder-capacity/897/

Thanks for those points Intergrated...sorry for not stating the flow from the shower but its 11 litres p/m, and I'd been told the figs. you quoted before, so figured it was within normal margins. A couple of points you raise are worth following up though - the temp. (goddam so obvious!) and the arvo boost which I will follow up with Meridian & see what they can do, or perhaps I have to get my sparky to fit something. Anyway thanks heaps!