The Honolulu Board of Water Supply on Monday unanimously gave a final OK to a five-year rate hike schedule that increases water bills for most of its residential users while keeping rates about the same for those who use the bare minimum.

The first round of increases won’t take effect until July 1, and three subsequent adjustments would kick in on July 1 in 2020, 2021 and 2022.

BWS Manager Ernest Lau said he was grateful for the board’s support. “The new rates will allow us to move forward with implementing our increased pipe replacement initiative as well as provide subsidies for affordable and homeless housing,” he said in a statement.

Pinpointing how much more the average customer will pay will vary depending on the customer class and how much water is used.

The monthly bill, beginning July 1, will consist of two parts: a new monthly customer charge based on the size of the customer’s meter which will replace the current, flat-rate billing charge; and a revamped schedule of tiers and rates for the variable-quantity charge, based on how many thousands of gallons of water is used from month to month.

Additionally, those customers that receive private fire service will need to pay a new monthly fire meter standby charge, which will be based on the size of a fire meter.

Generally speaking, the average single-family residential household uses 9,000 gallons of water a month. That family now pays $49.04 a month (including a billing charge) and would see its bill go up about 21.5 percent over the five years to $59.56 (including a customer charge) by July 1, 2022, under the new schedule.

Water officials believe a gradual increase will make it easier for families to absorb than a one-time spike.

But not all water users would be paying much more at the end of the schedule period. For instance, some multifamily residential customers using less than 2,000 gallons a month would pay roughly the same as they do now at the end of the five years.

BWS officials said they wanted to ensure that all rate increases were spread across all rate classes fairly and equitably. A recent study showed that single-family residences currently pay less than it costs to serve them, while multiunit family residences and nonresidential customers pay more than the actual costs to serve them.

The schedule is based on a long-range financial plan the board adopted in March which points out that it will need to increase revenue to keep up with its fixed costs as well as its aim to increase the miles of pipes it replaces islandwide annually to 21 from the current six. There are an estimated 2,100 miles of pipe in all.

The semi-autonomous board estimates it needs to invest in more than 300 projects at a cost of $5.3 billion over the next 30 years. The rate increases are expected to generate about $60 million more over the five years.

Sewer rates are approved by the City Council and will not be affected by the new water rate schedule.