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Energy and Cost
Electrical energy depends on power and time; household bills use kilowatt-hours.
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Energy and Cost
Recommended for Class 10
Energy and Cost
Electrical energy used by a device depends on power and time.
**E = Pt**
If power is measured in watts and time in seconds, energy is obtained in joules.
Kilowatt-hour
For household electricity, energy is commonly measured in kilowatt-hours (kWh).
**1 kWh = 3.6 × 10⁶ J**
A kilowatt-hour is a unit of energy, not power.
Cost
Electricity cost can be calculated using:
Cost = energy used in kWh × price per kWh
Worked Example
A 2 kW heater operates for 3 h.
Energy:
**E = Pt**
`=2 kW × 3 h`
**=6 kWh**
If electricity costs 50 currency units per kWh:
**Cost = 6 × 50 = 300**
Energy and Cost — Advanced
Common Mistakes
Common Mistakes
- Mistake
Do not treat kWh as kilowatts per hour.
- Mistake
Convert watts to kilowatts when calculating household kWh.
Revision Zone
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Quick cues from this topic — use them for last-minute revision.
- Definition
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Practice
PQ
Q1. State `E=Pt`.
PQ
Q2. Define kWh.
PQ
Q3. Convert 2 kWh.
PQ
Q4. 1.5 kW for 4 h.
PQ
Q5. At 30/unit, cost.
Exam Practice
EQ
Q1. Define kWh.
EQ
Q2. Explain difference power/energy.
EQ
Q3. Convert kWh to joules.
EQ
Q4. Appliance energy numerical.
EQ
Q5. Cost numerical.
School-standard facts for Matric and Intermediate. Original explanations — not copied from textbooks.