Power P = VI = $I^{2R}$ = $V^2$/R. SI unit: watt (W). Energy E = Pt = VIt = $I^{2Rt}$. SI unit: joule (J). Commercial unit: kWh (1 kWh = 3.6 x 10^6 J). In series (same I): P proportional to R — the larger resistor dissipates more power. In parallel (same V): P proportional to 1/R — the smaller resistor dissipates more power. Total power: series $P_{total}$ < individual P (because $R_{eq}$ > any $R_i$ with same I). Parallel $P_{total}$ > individual P.

Heating effect (Joule's law): H = $I^{2Rt}$. Fuse rating: a fuse melts when $I^{2R}$ exceeds its thermal capacity. Fuses are low-melting-point alloys with specific current ratings.