This will mostly depend on several factors such as your energy usage, available roof space, budget, and energy goals. A size between 4kW and 8kW solar panel system is common for the average single family residential home. Typically, a larger system will generate more energy and allow you to save more money in the long run but are more expensive upfront. A solar expert should be able to tell you what would be the best size based on the information that you give them. They will ask you about what your energy usage is, the total free space you’ll have on your roof, and a few additional questions to figure out which size best fits your unique criteria.