Power & Energy Calculator
Calculate electrical power, energy consumption, and electricity cost from any two values.
Component Values
Energy & Cost
Results
P = V × I = V²/R = I²R
Electrical Power and Energy
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.
Power (P)
P = V×I = V²/R = I²REnergy (Wh)
E = P × tKey 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
Practical Examples
230 V AC motor drawing 10 A with a power factor of 0.85. Find real power (W) and apparent power (VA) for cable sizing.
P = 230 × 10 × 0.85 = 1955 W · S = 230 × 10 = 2300 VA · Cable must handle 10 A
5 V RGB LED strip, 60 LEDs/m × 5 m, each LED draws 60 mA at full white. Size the PSU.
I = 300 × 0.06 = 18 A · P = 5 × 18 = 90 W · Use 100 W (20% headroom) 5 V PSU
Did you know? James Watt did not invent the steam engine, but his improvements in the 1760s made it practical. The watt unit was named in his honor in 1882. A single AA battery stores about 3–4 Wh — enough to power a 1 W LED for 3–4 hours.