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CISCEISC (Science) ISC Class 12
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अवधारणा स्पष्टीकरण और वीडियो८५६
समय सारणी१६
पाठ्यक्रम

Commercial Units of Electricity Consumption

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Estimated time: 10 minutes
CISCE: Class 12

Need of a Special Unit

The joule (J) — the SI unit of energy — is far too small for everyday electricity bills. If your electricity bill were calculated in joules, the numbers would run into millions, making it impractical. So electricity companies use a bigger, more convenient unit: the kilowatt-hour (kWh), commonly just called a "unit" on your electricity bill.

1 kilowatt-hour (kWh) = 1 kilowatt × 1 hour
CISCE: Class 12

Derivation

  1. Convert kilowatt to watt: 1 kW = 1000 W
  2. Convert hours to seconds: 1 hour = 3600 s
  3. Multiply: 1 kWh = 1000 W × 3600 s = 3.6 × 106 W-s
  4. Since 1 watt-second = 1 joule: 1 kWh = 3.6 × 106 J
CISCE: Class 12

Watt vs Kilowatt vs Kilowatt-hour

Term What it measures Unit type Example
Watt (W) Rate of energy use (power) Power A 100 W bulb
Kilowatt (kW) Power, in bigger units Power 1000 W AC unit = 1 kW
Kilowatt-hour (kWh) Total energy consumed over time Energy 1 kW device run for 1 hour = 1 kWh

This distinction trips up many students — remember, kW is power (how fast energy is used), while kWh is energy (total amount used over time).

CISCE: Class 12

Formula: Cost Calculation

If a device draws current I at voltage V for t hours:

Energy consumed (units) = \[\frac {V×I×t}{1000}\] = \[\frac {Power (watts)×time (hours)}{1000}\]
CISCE: Class 12

Bulb Brightness Rules

  • Series circuit: The bulb with lower wattage (higher resistance) glows brighter, since current is the same everywhere and heat produced (I2R) is higher for higher resistance.
  • Parallel circuit (as in houses): The bulb with higher wattage (lower resistance) glows brighter, since voltage is the same everywhere and heat produced (V2/R) is higher for lower resistance.
  • If a 60W-240V bulb is connected to a 300V supply, more current flows than its safe rating, causing it to fuse — unless a suitable resistor is added in series to limit current.
CISCE: Class 12

Example

A house has 20 bulbs (100 W each), 10 fans (0.5 A each at 220 V), and a 100 Ω kettle. Supply voltage = 220 V, rate = ₹0.50/unit. Appliances run 6 hours/day for 30 days.

 
Appliance Calculation Power (W)
20 bulbs 20 × 100 W 2000
10 fans 10 × 220 V × 0.5 A 1100
Kettle V2/R = 2202/100 484
Total power 2000 + 1100 + 484 3584 W

Energy consumed = \[\frac {3584×(6×30)}{1000}\] = 645.12 units

Total cost = 645.12 × ₹0.50 = ₹322.56

CISCE: Class 12

Real-Life Analogy

Think of a kilowatt-hour like a "bucket of energy." A 1 kW appliance fills one bucket every hour; a 2 kW appliance fills the same bucket in just 30 minutes. Your electricity meter simply counts how many buckets you've used in a month — and that's your bill.

CISCE: Class 12

Key Points: Commercial Units of Electricity Consumption

  • The commercial unit of electrical energy is the kilowatt-hour (kWh), also called a "unit."
  • 1 kWh = 3.6 × 106 J.
  • Cost = Number of units × Rate per unit, where units = (Total power in watts × time in hours) / 1000.
  • In series, lower-wattage bulbs glow brighter; in parallel, higher-wattage bulbs glow brighter.
  • A bulb's resistance is fixed by its rated power and voltage, and doesn't change with the supply voltage — but the current through it does.
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