Temperature
Every extra degree requires higher compressor pressure and sometimes backup heater assistance. The right setting depends on the cylinder and the manufacturer's hygiene requirements, so change it only following manufacturer or installer instructions.
Legionella cycle
Legionella protection is necessary for hygiene; its temperature and frequency come from the manufacturer's manual. On its own it does not guarantee the whole system is safe (pipework and standing water matter too). Never switch it off.
Timing
Producing hot water during warmer daytime hours or a cheaper tariff period reduces cost. Avoid a cycle right before peak demand when the tank is already full.