Watt's the Cost

Why it matters

Your geyser gulps more electricity in 30 minutes than an LED bulb sips in an entire month. The bill knows it — do you?

The idea in plain words

The meter on your wall runs on one line of physics. Tap it apart.

E = P × t · 1 kWh = 1000 W × 1 h

A 2000 W geyser runs 30 minutes daily. Tariff ₹8 per unit. Monthly cost?

₹240 a month, just for hot water

• kWh (the 'unit' on the bill): E — electrical energy consumed — what you pay for
• watt (W): P — power — how fast the appliance eats energy (P = V × I)
• hour (h): t — time it stays on
• E = 2 kW × 0.5 h = 1 kWh per day
• Month: 1 × 30 = 30 kWh → 30 × ₹8

Predict first

Which adds the most to the monthly bill?

Cost follows ENERGY = power × time. Geyser: 2 kWh/day. LED all day: 0.22 kWh. Fan: 0.6 kWh. Big watts beat long hours.

• Geyser (2000 W) for 1 hour a day — correct
• LED bulb (9 W) left on 24 hours a day
• Ceiling fan (75 W) for 8 hours a day

What you do

Run the home: the fridge, the fan and the LED must work every day. Keep the month's bill under ₹1500 — the geyser and the AC will test you.

Check yourself

The kilowatt-hour is a unit of…

kWh = power × time = energy. Power is the RATE (kW); the bill charges for the total energy.

A device draws 0.5 A from 220 V mains. Its power is…

P = V × I = 220 × 0.5 = 110 W.

1 kWh equals how many joules?

1000 W × 3600 s = 3,600,000 J. That is why the grid bills in kWh, not joules.

• energy — correct
• power
• current
• 110 W — correct
• 440 W
• 219.5 W
• 3.6 × 10⁶ J — correct
• 1000 J
• 3600 J