UNIT 1.7 — POWER & ENERGY

Power, energy, and I²R heating

CERT 1 · DC/AC Theory & Safety · ~9 min

Community content — not verified by a licensed electrician. Lessons and questions here are written for study practice and pinned to NEC 2023. Code editions and local amendments vary by jurisdiction. Always confirm against your adopted code book and your licensing board's published exam outline before test day.

Power (watts) is the *rate* of doing electrical work; energy is power sustained over time. Utilities bill energy in kilowatt-hours: a 1 500 W heater running 4 hours consumes 1.5 kW × 4 h = 6 kWh. Exams convert between W, kW, and kWh constantly.

Every real conductor has resistance, so every ampere flowing through it dissipates heat at P = I²R. Because current is squared, doubling the current quadruples the heat — this single fact drives conductor sizing, breaker ratings, and why loose (high-resistance) connections start fires.

It also explains why transmission uses high voltage: for the same delivered power, higher voltage means lower current (P = V × I), and lower current collapses the I²R losses in the line.

Exam focus: kWh cost problems ('at $0.15/kWh, what does 8 hours cost?') and conceptual I²R questions about loose connections and undersized conductors.

Energy

kWh = kW × hours

Heat in a conductor

P = I²R

Double the current

4× the heat

Knowledge check

Quiz — 3 questions

1. A 1 200 W appliance runs 5 hours. Energy consumed:

2. Current through a conductor doubles. Heat dissipated in it:

3. A loose termination is dangerous primarily because it:

← Syllabus Next: AC fundamentals: sine waves, RMS, frequency →