Power Dissipation Calculator

Electrical power dissipation calculations appear across electronics design, industrial electrical engineering, HVAC sizing, EV charging and PCB layout. This calculator covers all four practical scenarios in a single tool: DC and resistive circuits, AC single-phase loads (as found in household and light-commercial installations), balanced three-phase systems (motors, distribution panels, industrial drives) and thermal/heatsink derating for semiconductor components.

How it works

DC and resistive circuits

For a purely resistive load — a heater element, a shunt resistor, a cable run — all three forms of Joule’s law give the same answer:

P = V × I (voltage times current) P = I² × R (current squared times resistance) P = V² / R (voltage squared divided by resistance)

Choose which quantity to solve for. The tool cross-checks the result against the other two formulae whenever resistance is provided, so you can instantly spot a data-entry error.

AC single-phase power triangle

For AC circuits with reactive loads (motors, transformers, fluorescent ballasts) the picture is more complex. The apparent power S (in volt-amperes, VA) is the product of RMS voltage and RMS current. Only the fraction that is in phase with the voltage does real work; that fraction is the real power P (in watts), related to S by the power factor: P = V · I · pf. The remaining component is the reactive power Q (in volt-amperes reactive, VAR):

Q = √(S² − P²)

The phase angle φ satisfies pf = cos(φ). A power factor near 1.0 means an efficient, mostly resistive load; a low power factor means the grid and wiring must carry more current than the load actually needs, raising losses and conductor sizes.

Balanced three-phase

In a balanced three-phase system the real power is:

P = √3 × V_LL × I_L × pf

where V_LL is the line-to-line voltage and I_L is the line current. Equivalently, using the line-to-neutral (phase) voltage V_LN = V_LL / √3:

P = 3 × V_LN × I_L × pf

The factor √3 ≈ 1.732 arises because the three phase voltages are 120° apart and do not simply add; their vector sum gives a line voltage √3 times larger than the phase voltage.

Thermal / heatsink derating

Once you know the dissipated power P (from any of the formulae above), the junction temperature model gives the operating temperature of a semiconductor die:

T_j = T_a + P × Rθ_JA

Rθ_JA is the junction-to-ambient thermal resistance in °C/W, found on the device datasheet. The calculator highlights junctions above 85 °C (amber warning) and above 125 °C (red — exceeds the typical silicon limit).

Worked example

A 230 V single-phase induction motor draws 8 A at a power factor of 0.82:

• Apparent power: S = 230 × 8 = 1,840 VA
• Real power: P = 1,840 × 0.82 = 1,509 W ≈ 1.51 kW
• Reactive power: Q = √(1,840² − 1,509²) = 1,054 VAR
• Phase angle: φ = arccos(0.82) ≈ 34.9°

The wiring and switchgear must be rated for 8 A (the apparent current), even though only 1.51 kW of real work is delivered. This is why industrial sites install power factor correction capacitors — the capacitive VAR cancels inductive VAR, reducing the line current and hence cable losses.

Formula reference