Power Dissipation Calculator
Find the electrical power dissipated as heat in a single component from any two of voltage, current and resistance, or solve a whole series or parallel resistor network for equivalent resistance, total current, the power in each resistor and the total. Add running hours and an electricity price to see the energy used and what it costs.
Formula
Worked example
12 V across a 6 Ω resistor draws I = V/R = 2 A, so P = V·I = 12 × 2 = 24 W. The same comes from P = I²R = 2² × 6 = 24 W and P = V²/R = 12²/6 = 24 W. For 4, 6 and 8 Ω in series, Req = 18 Ω, I = 12/18 = 0.667 A, and total P = 12²/18 = 8 W.
What power dissipation means
Power dissipation is the rate at which electrical energy is converted into heat inside a resistor or any component that opposes current. It is measured in watts, where one watt equals one joule of energy released every second. Whenever current flows through a resistance, collisions between charge carriers and the material warm it up, this is Joule heating. The same physics governs an electric heater, a glowing light-bulb filament, and the gentle warmth of a phone charger. The calculator also reports the heat in BTU per hour, which is handy when you need to size an enclosure fan or air conditioner.
The three equivalent formulas
Power can be written three ways: P = V·I, P = I²·R, and P = V²/R. They are algebraically identical because Ohm’s law (V = I·R) lets you substitute any one quantity for the others. Use P = V·I when you measure voltage and current directly, P = I²·R when you know the current and resistance, and P = V²/R when you know the supply voltage and resistance. The squared dependence on current and voltage explains why small wiring or load changes can cause surprisingly large swings in heat output.
Series and parallel resistor networks
Switch to network mode to solve a whole circuit. For resistors in series the same current flows through each one, so the equivalent resistance is simply the sum, Req = R1 + R2 + ... + Rn. For resistors in parallel each one sees the full supply voltage, and the equivalent resistance follows 1/Req = 1/R1 + 1/R2 + ... + 1/Rn, which is always smaller than the smallest resistor. Once the calculator has Req it uses Ohm’s law for the total current (I = V/Req) and P = V²/Req for the total power. In series the largest resistor runs hottest; in parallel the smallest one does, because it carries the most current.
Energy use, running cost and sizing parts
Turn on the energy estimate to see how much electricity the circuit uses and what it costs. Energy in kilowatt-hours is the power in kilowatts multiplied by the hours it runs, and the cost is that energy times your electricity price per kWh. The yearly figure assumes the same daily run time all year and is a planning estimate, not a bill. Every resistor and component also has a maximum power rating; exceed it and the part overheats, drifts in value, or fails. Engineers design with headroom, often choosing a part rated for at least twice the calculated dissipation, and add heatsinks or airflow for higher powers. Treat the result as an idealized value, real circuits add temperature coefficients, transient peaks, and ambient conditions that a final design must account for.
Typical power dissipation levels
| Power range | Example | Level |
|---|---|---|
| < 0.25 W | Signal resistor, LED indicator | Low |
| 0.25-5 W | Logic board, small power resistor | Normal |
| 5-50 W | Voltage regulator, motor driver | High |
| > 50 W | Heating element, power supply stage | High |
Rough guide to common component and appliance power ranges.
Frequently asked questions
Which power formula should I use?
Use whichever matches the values you already know. P = V·I needs voltage and current, P = I²·R needs current and resistance, and P = V²/R needs voltage and resistance. All three give the same answer for the same circuit because of Ohm’s law.
How does the series and parallel network mode work?
Enter the supply voltage, pick series or parallel, and list the resistor values. The calculator finds the equivalent resistance (a sum for series, the reciprocal rule for parallel), then uses I = V/Req for the total current and P = V²/Req for the total power dissipated by the whole network.
Why does current matter so much for heat?
In P = I²R the power grows with the square of the current, so doubling the current quadruples the heat dissipated for a fixed resistance. That is why high-current wiring uses thicker conductors and why even small resistances can run hot under heavy load.
How is the running cost calculated?
Energy per day in kilowatt-hours is the power in kilowatts times the hours it runs. Multiply that by your electricity price per kWh for the daily cost, and by 365 for a yearly estimate. It assumes a steady load and the same run time every day, so treat it as a planning figure.
Does this apply to AC circuits?
For purely resistive loads, yes, use RMS (root-mean-square) values for voltage and current and the same formulas give average power. With capacitors or inductors present you must also include the power factor, which this calculator does not model.