I watched my water bills closely, and found the following:
On average, every person in my household uses approximately 1 ton of water per week, regardless of the season.
On average, the lawn sprinkler system uses up to 8 tons per week in summer, for a lawn that is about 8,000 square feet. I do not over water either.
So, you can figure out how much the water usage would be for the tenants and for the lawn, respectively. For example, if you have 36 tenants in the 24-plex, they would use about 36 tons of water per week. If the lawn is 18,000 square feet, the lawn sprinklers can use something like 18 tons per week, which is about 50% over the tenant water usage. Also, if your sprinkler settings are higher, they can use more water even for a smaller lawn. Go figure!
