According to the American Water Works Association the average American home uses 45-65 gallons of total water (hot and cold) per day, per person. A little less than half of this would be for hot water uses.
Therefore, as a general guide, each person in your home would use 20-30 gallons of hot water per day. If your home is water efficient then you would be toward the lower end of this scale.
Your renewable energy Professional will make a more accurate assessment based on the number of people as well as the number of bedrooms and bathrooms.
Other helpful information is available at:
Go to www.solar-estimate.org