Think about how people in your house use hot water. Are they frugal? Do they use a lot? Are they wasteful? The more hot water the occupants use, the bigger your system needs to be. The figure below is a rough guide to water usage per person. Work out the average usage per person and multiply by the number of people for a total daily usage.
Then talk to your installer about the size of cylinder and number of panels you need based on this figure; or you can work out sizing yourself by using the information in our book Solar Hot Water: choosing, fitting and using a system.