5W vs 100W Bulb Electricity Bill: Cost, Power Consumption & Savings

1. Basic Electrical Relationship

For a simple resistive electrical load, such as an ideal resistor or approximately a traditional incandescent filament:

P = V × I = V / R Therefore: P = V² / R

Where:

Suppose two simple resistive bulbs operate from the same 230 V supply.

R = V² / P R = 230² / 5 R = 10,580 Ω I = P / V I = 5 / 230 I ≈ 0.0217 A

Therefore, the 5 W load has an effective operating resistance of approximately 10.6 kΩ and draws about 21.7 mA.

R = V² / P R = 230² / 100 R = 529 Ω I = P / V I = 100 / 230 I ≈ 0.435 A

Therefore, the 100 W load has an effective operating resistance of approximately 529 Ω and draws about 435 mA.

Bulb	Voltage	Power	Effective Resistance	Current
5 W	230 V	5 W	≈ 10,580 Ω	≈ 0.0217 A
100 W	230 V	100 W	≈ 529 Ω	≈ 0.435 A

Electricity cost depends on the amount of electrical energy consumed.

Energy (kWh) = Power (kW) × Time (hours) Cost = Energy (kWh) × Electricity Rate

Assume the electricity rate is ₹8/kWh and both bulbs operate for 10 hours.

5 W = 0.005 kW Energy = 0.005 × 10 = 0.05 kWh Cost = 0.05 × ₹8 = ₹0.40

5 W bulb: approximately ₹0.40 for 10 hours.

100 W = 0.1 kW Energy = 0.1 × 10 = 1 kWh Cost = 1 × ₹8 = ₹8.00

100 W bulb: approximately ₹8.00 for 10 hours.

Conclusion: At the same voltage and operating time, a 100 W load consumes 20 times as much energy as a 5 W load.