Electric power

P = VI. With Ohm's law it can also be written P = I²R or P = V²/R. The SI unit is the watt, W. One watt is used when 1 A flows under 1 V, so 1 W = 1 V × 1 A. A kilowatt, kW, is 1000 W.

Energy is power multiplied by time. The electricity bill counts energy in kilowatt hours, kW h, called "units". One kW h is 3.6 × 10⁶ J.

A small example

An electric kettle is rated 1100 W, 220 V. It runs for 2 hours.

• Current: I = P/V = 1100 W / 220 V = 5 A.
• Resistance: R = V/I = 220 V / 5 A = 44 Ω.
• Energy: 1.1 kW × 2 h = 2.2 kW h, or 2.2 units.

Where marks go

Question 31 of CBSE's 2026-27 Science sample paper is a 1-mark choice question. An appliance rated 1000 W runs at 250 V. The 2026-27 Science marking scheme gives the answer 4.0 A, from I = P/V = 1000 W / 250 V.

In a written numerical, the formula, the working and the answer with its unit can each carry marks. Our article on where the marks go in CBSE Science shows a 2-mark power question from the 2026 board exam, split into four half marks.

About the author

Deepti Sardana

Founder Member & Academic Lead

An IIT alumna (M.Sc., M.Phil.) with over 25 years in teaching, child psychology and learning assessment. As Academic Lead at EduBrahma, she leads the method TRUtesting uses to find where and why a child loses marks.

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