What the meter counts
Show on a big screenA 1000 W heater runs for two hours. What does the home electricity meter add?
- The meter reads 1250 units.
- A 1000 W heater runs for 2 hours.
- The meter now reads 1252 units — 2 more, or 2 kWh.
Now build a bill. Drag the lamp and the heater onto it, or tap them. Set the hours and read the and the cost.
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The meter adds 2 kilowatt-hours (kWh), also called 2 .
New words
A kilowatt (kW) is 1000 watts. A watt-hour (Wh) is 1 W used for 1 hour.
Unit — the meter's own name for one kilowatt-hour (kWh). 1 kWh = 3.6 × 10⁶ J.
The rule
The meter counts energy: power times time. One is the energy a thousand watts deliver in one hour. The bill grows with the watts and with the hours.
Energy = P × t
Here P is power in watts and t is time in hours.
Energy = 1000 W × 2 h = 2000 Wh. Now 1000 Wh = 1 kWh, so 2000 Wh = 2 kWh. That is 2 .
In kW it is shorter: Energy (kWh) = P (kW) × t (h). A 1000 W heater is 1 kW. So 1 kW × 2 h = 2 kWh.
Cost = × rate. At ₹6 a unit, 2 units cost 2 × 6 = ₹12.
In joules, 2 kWh = 2 × 3.6 × 10⁶ = 7.2 × 10⁶ J.
A 100 W lamp is used for 20 hours. It uses 100 × 20 = 2000 Wh. That is also 2 . It takes longer to reach them.
Try the lab to finish this lesson.