What is the formula for Maximum Power Transfer?

The Maximum Power Transfer formula is Pmax = Vth² / (4 × Rth) (when Rload = Rth), where Pmax—Maximum power into the load (W); Vth—Thévenin (open-circuit) voltage (V); Rth—Thévenin (source) resistance (Ω).

What does Pmax mean in the Maximum Power Transfer formula?

Pmax is Maximum power into the load (W).

Maximum Power Transfer

Q: How do you calculate Maximum Power Transfer?

Use the formula Pmax = Vth² / (4 × Rth) (when Rload = Rth). The maximum power a source can deliver to a load occurs when the load resistance equals the source’s internal (Thévenin) resistance. Enter the values you know in the calculator above and it solves for the remaining variable.

Pmax = Vth² / (4 · Rth)

Calculator

Maximum Power

Thévenin Voltage

Thévenin Resistance

Result

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Formula

Pmax = Vth² / (4 × Rth) (when Rload = Rth)

Description

The maximum power a source can deliver to a load occurs when the load resistance equals the source’s internal (Thévenin) resistance. At that matched point the load receives one quarter of the open-circuit voltage squared divided by the source resistance, and exactly half the total power (the other half is lost in the source). This sets the ceiling on deliverable power and underlies impedance matching.

Variables

Pmax — Maximum power into the load (W)
Vth — Thévenin (open-circuit) voltage (V)
Rth — Thévenin (source) resistance (Ω)

Practical Notes

Matching maximizes power but caps efficiency at 50%; for power delivery (not signal) systems prefer Rload ≫ Rsource for efficiency. Matching matters most in RF and small-signal work.

Related Concepts

All Basic Electronics formulas →

Ohm's Law Electrical Power Voltage Divider Current Divider Thévenin Equivalent Norton Equivalent Temperature Coefficient of Resistance Kirchhoff's Voltage Law (KVL)

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