The size of the system measured in kilowatts (kW) has a significant impact on the energy cost savings that the system will provide. The current homeowner may use power differently than your client for various reasons (size of family, daily habits, air-conditioning usage, etc.).
Ask the current homeowner for a copy of a recent HECO bill and consider asking the for access to their online monitoring system to see how much energy is being produced. This will help your client plan for the future by forecasting their energy production, savings and assess the potential need for adding to the existing system.