The electricity bill: units and cost
Choose the bulbs, the TV time, the way you cook and whether you have a fridge, then read the meter and work out the month's units and the bill.
Core concept
Electrical energy is measured in units: 1 unit = 1 kilowatt-hour (kWh) = power in kW × hours of use; the bill counts the units, charged at a higher rate for each slab, so efficient appliances and less use save money
Topic: Electricity
The meter at your home counts the electrical energy: What is needed to make things move, heat up, light up or grow. All key words you use, in units. One unit is a kilowatt-hour (kWh): the energy a 1000 W (1 kW) appliance uses in one hour, or a 100 W TV in ten hours. To find the units, multiply each appliance's power in kW by the hours it is used. Electricity is sold in slabs: the first few units are cheap, and each slab above costs more per unit. In this lab a home has five bulbs lit for 5 hours a day, a 100 W TV, a way of cooking and maybe a fridge. The bill uses a simple slab tariff made up for this lab: Rs 4 a unit for the first 20 units, Rs 7 a unit up to 50, Rs 9 a unit up to 150 and Rs 11 a unit above that. Look at your own bill for the real tariff.
What you will learn
- Define the unit (kWh) of electrical energy.
- Calculate the units used by appliances from their power and hours of use.
- Calculate an electricity bill on a slab tariff.
- Suggest ways to save electricity at home.
Before you start
- Know that power is measured in watts (W) and 1 kW = 1000 W.
Materials
- Your home's electricity bill, the power labels on appliances
Equipment
- A calculator
Procedure
- Record the units and bill with LED bulbs, 4 hours of TV, a gas stove and no fridge.
- Record with filament bulbs.
- Record with an induction stove.
- Record with a fridge.
How this simulation is simplified
- A month is 30 days; the five bulbs are on for 5 hours a day.
- The induction stove uses 1.5 units a day, a rice cooker 0.5 and a fridge 1.
- The slab tariff is made up for this lab; real tariffs also have a fixed monthly charge.