Almost. I assume you know, but P=IV is only an approximation when reactive loads are close to zero. It is not the "DC formula". S is "complex power", meaning i

Almost. I assume you know, but P=IV is only an approximation when reactive loads are close to zero. It is not the "DC formula". S is "complex power", meaning it includes both the real (resistive) and imaginary (reactive) parts. It is measured in volt-ampere, and is calculated as S=I_z x V, where I_z is the impedance current ("complex current"). P is "real power" (resistive) measured in watts, and is calculated as P=I_r x V or P=S x cos(φ), where I_r is the resistive current ("real current"), S is the complex power, and φ is the phase angle or "power factor"—the delay between voltage and current as an angle. In a pure DC system (think "incandescent bulb on a battery"), the phase angle φ is 0 making P equal S, as cos(0) equals 1. However, in real life, it is only a vague approximation. Electronics switch currents and have reactive components, giving them a non-zero phase angle. They're more complicated to calculate on than an pure sine-wave AC system, not less. (I apologize for any hiccups above. I stopped being an electrician a looooong time ago.)