Water for Schools
Menu

Lesson plan Grade 9 Science

Tiered Water Rates in Braemoor: A Grade 9 Conservation Lesson

A 50-minute grade 9 lesson where students compute household bills under an increasing-block water rate and analyze how tiered pricing rewards conservation.

Objectives

  1. Students will compute a household water bill under an increasing-block rate structure.
  2. Students will calculate the savings a household earns by reducing its water use.
  3. Students will explain why a tiered rate encourages conservation more than a flat rate.

Vocabulary

Fixed charge
The part of a water bill that stays the same every month no matter how much water a family uses. It helps pay for the pipes, meters, and workers the utility needs even when no water is flowing.

Also: increasing-block rate, tier, conservation, marginal price

Teacher overview

Many utilities charge for water in tiers: the first block of water is cheap, and each higher block costs more per unit, so households that use a great deal pay a rising price on their heaviest use. This lesson uses the fictional town of Braemoor to show how an increasing-block rate turns a water bill into a conservation signal. A common misconception is that a household's average price per unit stays fixed; under a tiered rate the average price climbs once use reaches the higher tiers, because more of the water is billed at the higher prices. Students compute several bills, measure a household's savings from cutting back, and explain how the structure rewards conservation.

Procedure

Hook (5 min): Ask students to predict, with a show of hands, whether a household that uses three times as much water as its neighbor pays exactly three times as much on the water part of its bill. Record the split without revealing the answer, and tell them the lesson will settle it.

Direct instruction and model (12 min): Introduce Braemoor's increasing-block rate. The fictional town of Braemoor charges a fixed monthly charge of ten dollars, then bills water in three tiers: the first 4 hundred-cubic-foot units (CCF) cost three dollars each, the next 6 units (CCF 5 through 10) cost five dollars each, and every unit above 10 CCF costs eight dollars each. Model one bill on the board for a household that uses 10 CCF: the fixed charge of ten dollars, plus 4 units at three dollars (twelve dollars), plus 6 units at five dollars (thirty dollars), for a total of fifty-two dollars. Emphasize splitting the usage into the correct blocks before multiplying.

Guided practice (10 min): In pairs, students complete handout questions 1 through 3, computing bills for households that use 4, 8, and 13 CCF. Circulate and check that each pair splits the usage into tiers correctly before adding.

Independent practice (15 min): Students complete handout questions 4 through 8 on their own, finding the share of a bill that comes from the top tier, the savings from a cutback, the average price per unit at two usage levels, and the marginal price of a single added unit.

Formative check and exit ticket (8 min): On an index card, each student answers: "In one or two sentences, explain why Household C (13 CCF) pays a higher average price per CCF than Household A (4 CCF) under Braemoor's rate." Collect the cards; if fewer than four of five students connect higher use to more water billed at higher tiers, reteach the tier-splitting step before the next class.

Tiered Water Rates in Braemoor: A Grade 9 Conservation Lesson — Grade 9 Science
Name: ____________________________ Date: ______________

Student Handout

Braemoor (a fictional town) charges a fixed monthly charge of ten dollars, plus water billed in three tiers:

  • Tier 1: the first 4 CCF at three dollars per CCF.
  • Tier 2: the next 6 CCF (CCF 5 through 10) at five dollars per CCF.
  • Tier 3: every CCF above 10 at eight dollars per CCF.
  1. Household A uses 4 CCF in a month. Calculate its total bill.
  2. Household B uses 8 CCF. Calculate its total bill.
  3. Household C uses 13 CCF. Calculate its total bill.
  4. Of Household C's bill, how many dollars come from Tier 3 water alone?
  5. Household B cuts its use from 8 CCF to 6 CCF. Calculate its new bill, then find how many dollars it saves compared with the 8 CCF bill from question 2.
  6. Find the average price per CCF for Household A (question 1) and for Household C (question 3). Divide each total bill by the number of CCF used, and round to the nearest cent.
  7. What is the price of the 13th CCF a household uses, and what is the price of the 4th CCF? Explain which tier each falls in.
  8. Explain why Braemoor's tiered rate encourages households to conserve more than a flat rate of one fixed price per CCF would.

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key

  1. Answer: $22.
    Fixed charge ten dollars plus 4 CCF at three dollars (twelve dollars) = 10 + 12 = 22 dollars.

  2. Answer: $42.
    Fixed ten dollars, plus Tier 1 (4 at three = twelve dollars), plus Tier 2 (4 units, CCF 5 through 8, at five = twenty dollars) = 10 + 12 + 20 = 42 dollars.

  3. Answer: $76.
    Fixed ten dollars, plus Tier 1 (4 at three = twelve dollars), plus Tier 2 (6 units, CCF 5 through 10, at five = thirty dollars), plus Tier 3 (3 units, CCF 11 through 13, at eight = twenty-four dollars) = 10 + 12 + 30 + 24 = 76 dollars.

  4. Answer: $24.
    Household C uses 3 CCF in Tier 3 (CCF 11, 12, and 13) at eight dollars each: 3 x 8 = 24 dollars.

  5. Answer: $10 saved.
    New bill at 6 CCF = 10 + 12 (Tier 1) + 10 (2 units of Tier 2 at five) = 32 dollars. Savings = 42 minus 32 = 10 dollars.

  6. Answer: $5.50 per CCF and $5.85 per CCF.
    Household A: 22 divided by 4 = 5.50 dollars per CCF. Household C: 76 divided by 13 = 5.846, which rounds to 5.85 dollars per CCF. Household C's average is higher because 3 of its units are billed at eight dollars.

  7. Answer: $8 for the 13th CCF and $3 for the 4th CCF.
    The 13th CCF falls in Tier 3 (above 10 CCF), priced at eight dollars. The 4th CCF falls in Tier 1 (the first 4 CCF), priced at three dollars.

  8. Answer: Each extra unit of heavy use costs more, so cutting back saves more.
    Under the tiered rate, a household's heaviest use is billed at the highest tier, so every unit a heavy user saves comes off the eight-dollar tier rather than a low flat price. A flat rate charges the same price for the first unit and the hundredth, giving less reason to cut back. Accept any answer that connects the rising tier price to a stronger reason to conserve.

Differentiation

Support

Provide a partially completed bill template that lists the three tiers as labeled rows, so students fill in units and multiply rather than building the structure from scratch. Allow a calculator for every question and pair struggling students with a peer for the guided-practice set.

Extension

Ask students to design their own three-tier rate for Braemoor that would raise the same total from Households A, B, and C combined while charging the lowest tier even less, then explain the trade-off their design makes.

ELL supports

Pre-teach "tier," "fixed charge," and "conserve" with a labeled diagram of a bill split into blocks of rising price. Provide a sentence frame for the exit ticket: "Household C pays a higher average price than Household A because ___."