> If a place gets less than 40cm of rain per year, adding water collection for the entire roof of a 100 sq meters building will only result in 40 cubic meters o

> If a place gets less than 40cm of rain per year, adding water collection for the entire roof of a 100 sq meters building will only result in 40 cubic meters of water, or 40000 liters. At 100 liters per day usage (lower than a typical European), collecting rainwater gives you 40 days of usage, 40000/100 = 400 days of usage.