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.