A cylinder should hold enough water so that it rarely runs out under normal use. A family of four will probably need about 300 litres capacity. Cylinders for solar and wetback systems Some cylinders are specially designed to work with solar systems and wetbacks.
Some older cylinders don’t work well in these situations. Ask your installer if yours is suitable. Wetbacks must be vented, so cannot be connected directly to a high-pressure supply. They can be connected directly to a vented, low pressure cylinder, or be connected to a vented heat exchanger that heats a high pressure supply.
For solar hot water, it’s better to have a larger cylinder as you will be less likely to run out of water.