Rectifier Efficiency for Cathodic Protection

Quick Definition

Rectifier efficiency is calculated as:

Efficiency is often shown as a percentage:

Where:

• output power is the DC output power from the rectifier;
• input power is the electrical input power to the rectifier;
• efficiency compares output power to input power.

When Rectifier Efficiency Is Used in CP Work

Rectifier efficiency may be used when reviewing:

• approximate DC output power;
• approximate input power;
• power loss between input and output;
• rectifier operating records;
• energy-use discussions;
• basic equipment review.

Rectifier efficiency is not the same as CP effectiveness.

A rectifier can be efficient and still not provide adequate current distribution to all protected areas. A rectifier can also be inefficient while the structure measurements still require separate evaluation.

Protection must be evaluated with proper CP field measurements and criteria.

Formula Reference

Variable and Unit Table

Common power relationship:

Use matching power units:

• if output power is in watts, input power should also be in watts;
• efficiency as a decimal is unitless;
• efficiency as a percent is decimal efficiency multiplied by 100.

How to Use the Formula

First, identify output power and input power.

For a basic learner calculation, output power may be calculated from DC rectifier output:

Then calculate efficiency:

To express it as a percent:

Do not use efficiency alone to decide whether the CP system is protecting the structure.

Do not mix AC input and DC output values without understanding what each value represents.

Worked Example 1 — Efficiency as a Percentage

A rectifier has an output power of 800 W and an input power of 1,000 W. What is the efficiency?

Known values:

• output power = 800 W
• input power = 1,000 W
• efficiency = ?

Use:

Substitute:

Calculate:

Answer: The rectifier efficiency is 80%.

Field meaning: This compares output power to input power. It does not prove CP protection.

Worked Example 2 — Calculate Output Power First

A rectifier has a DC output of 40 V and 10 A. The input power is 600 W. What is the efficiency?

Known values:

• DC output voltage = 40 V
• DC output current = 10 A
• input power = 600 W

Calculate output power:

Use:

Substitute:

Calculate:

Answer: The rectifier efficiency is approximately 66.7%.

Field meaning: This is a power comparison. It does not identify the cause of losses or prove whether the structure is protected.

Worked Example 3 — Output Power from Efficiency

A rectifier input power is 500 W. The decimal efficiency is 0.80. What output power does that represent?

Known values:

• input power = 500 W
• efficiency = 0.80
• output power = ?

Use:

Substitute:

Calculate:

Answer: The output power is 400 W.

Field meaning: Efficiency as a decimal can be used to estimate output power from input power in a basic calculation.

Worked Example 4 — Input Power from Output Power

A rectifier output power is 360 W. The decimal efficiency is 0.75. What input power is required in this basic calculation?

Known values:

• output power = 360 W
• efficiency = 0.75
• input power = ?

Use:

Substitute:

Calculate:

Answer: The input power is 480 W.

Field meaning: This is a basic efficiency relationship. Actual rectifier evaluation may require more information than this formula alone provides.

Common Mistakes

1. Confusing efficiency with CP effectiveness. Efficiency compares power input and output. It does not prove structure protection.
2. Mixing input power and output power. Keep AC input power and DC output power clearly separated.
3. Forgetting to multiply by 100 for percent. A decimal efficiency of 0.80 is 80%.
4. Using percent as a decimal without converting. 80% should be used as 0.80 when multiplying.
5. Assuming an efficiency value identifies the exact problem. Efficiency is a clue, not a complete troubleshooting result.
6. Using volts and amps from different operating conditions. Use values from the same operating condition when calculating power.
7. Treating efficiency as an acceptance criterion. This page does not define acceptance limits.
8. Ignoring structure measurements. CP protection still requires proper field measurements and criteria evaluation.

Practice Problems

1. A rectifier output power is 600 W and input power is 750 W. What is the efficiency percentage?
2. A rectifier output power is 300 W and input power is 500 W. What is the efficiency percentage?
3. A rectifier output is 24 V and 5 A. What is the DC output power?
4. A rectifier output is 24 V and 5 A. Input power is 200 W. What is the efficiency percentage?
5. A decimal efficiency is 0.85. What percentage is that?
6. An efficiency is 70%. What decimal efficiency is that?
7. Output power is 450 W and decimal efficiency is 0.90. What input power does that represent?
8. Does rectifier efficiency by itself prove that a structure satisfies CP protection criteria?

Practice Problem Answer Key

Related Learning

Use these pages to continue studying related concepts:

• Rectifier Output
• Ohm’s Law
• Voltage Drop
• Current Requirement
• Current Interruption
• Pipe-to-Soil Potential Survey

Related formulas:

• Rectifier Output
• Ohm’s Law
• Voltage Drop
• Current Requirement