Household arithmetic: the electricity bill
Read the meter, find the units used this month, and work out the bill tier by tier. Change the units from 15 to 200 and see why using more electricity costs more for each unit.
Core concept
Units used = new reading − last reading; the tariff charges each tier of units at its own rate, plus a service charge, so a big user pays more per unit
Topic: Household arithmetic
An electricity meter counts units (kilowatt-hours) like a car's odometer counts kilometres. The units used in a month are the new reading minus last month's reading. The bill is worked out in tiers: in this lab the first 20 units cost Rs 3 each, units 21 to 50 cost Rs 7, units 51 to 150 cost Rs 9 and every unit above 150 costs Rs 11. A service charge of Rs 50 is added. Cheap first units help small homes; big users pay more for each extra unit, which encourages saving electricity. Drinking water and telephone bills are worked out in the same way, from a meter and a tariff.
What you will learn
- Find the units used from two meter readings.
- Calculate a bill on a tiered tariff.
- Find the average cost of a unit and explain why it rises.
Before you start
- Know how to multiply and add money.
Materials
- Your family's last electricity bill, if you have one
Equipment
- None
Procedure
- Record a month of 45 units.
- Record a month of 15 units.
- Record a month of 200 units.
- Record a month of 80 units.
How this simulation is simplified
- The tariff is simplified, in the style of NEA domestic tiers; real tariffs and service charges are different and change.
- Last month's reading was 02450.