Power & Energy Calculator

Power measures how fast energy is transferred. In DC circuits, P = V × I gives you watts directly. When you know voltage and resistance but not current, use P = V²/R. When you know current and resistance, use P = I²R. All three formulas are equivalent — they come from substituting Ohm's law into P = V × I.

Energy is power multiplied by time: E = P × t. The utility company bills in kilowatt-hours (kWh): 1 kWh = 1000 watts for one hour = 3.6 million joules. A 100W light bulb running for 10 hours uses 1 kWh. At $0.15/kWh, that costs 15 cents.

For electronics designers, power dissipation determines component ratings and thermal design. A voltage regulator dropping 7V at 1A dissipates 7W as heat — enough to require a heatsink. Always check that every component in your circuit can handle its power dissipation with margin to spare.

Key Points

• P = V×I = V²/R = I²R — three equivalent formulas
• 1 kWh = 1000W × 1h = 3.6 MJ
• Power dissipation determines heatsink and component ratings
• Reactive power (AC) vs real power — this calculator covers DC/real power

Applications

• Component power rating selection
• Electricity cost estimation
• Battery runtime calculation
• Heatsink and thermal design