Power is how fast a device spends energy, measured in watts (W) — one watt is one joule spent per second. The main road: $P = V\,I$ — read aloud: P equals V I — the push times the flow. A 120 V outlet driving 2 A is spending $240\ \mathrm{W}$, right there on the spot.

Sometimes the problem hides the voltage and hands you a resistance instead. There is a detour built for exactly that: $P = I^2 R$ — P equals I-squared R — power straight from current and resistance. Choosing between the two IS the skill: read what the story gives you, and take the road built from those parts.