@Weina Shi Yes, RUBS is the way to go if you want to avoid hardware costs. Funny, Sant, the other commenter so far, and I were just having this conversation in another thread. I used to implement RUBS in a 6 unit. Here was my experience:
For the most part it worked well but the reason I was swayed away and looked into submetering was because it increased the number of calls/complaints I would get from my tenants. I would get calls and emails over the weekend from one tenant letting me know the other tenants had friends staying the weekend with them and they wanted me to know because they did not want to be assessed the additional water usage. It was basically a "point the finger at other tenants for high usage each and every month" To their benefit in many ways I agreed with them as I wouldn't want to pay for other people who weren't responsible for water usage. It didn't cost me anything but I got tired of the complaints. Clearly the ultimatum is RUBS is free to implement and only costs a monthly fee to pay a company to do it for you or your own labor to do it each month vs. pay for a larger up front cost to install meters (that is not always cheap) and a monthly monitoring cost.