Maximum Power Transfer Calculator (Load Matching)

What this tool does

A source with internal resistance gives its largest power to a load whose resistance equals its own — and at that point exactly half of the power is wasted inside the source, so efficiency is only 50 %. That is the right trade for a radio signal or a speaker driven from a weak source, where power at the load matters; it is the wrong one for a battery or a mains supply, where you want a load much larger than the source resistance so that the losses stay small. The chart shows how flat the peak is: the load can be off by a factor of two and still get about 89 % of the maximum.

Formula

P_load = V² R_L ÷ (R_S + R_L)² · maximum when R_L = R_S: P_max = V² ÷ 4R_S · efficiency = R_L ÷ (R_S + R_L)

Variables

Worked example

Limitations

• The formula assumes ideal conditions: no friction losses, no air resistance and no efficiency losses unless you enter them.
• Mixing units is the most common source of error. Convert every input to the units shown next to each field before calculating.