Power: units and measurement guide

What is being measured?

Power is the rate of energy transfer. Power and energy differ by a time interval; watts and watt-hours cannot be exchanged by a unit factor alone.

Horsepower is mechanical unless marked metric. PS equals metric horsepower. Refrigeration tons use the US 12,000 BTU IT/h convention.

Choose a unit definition

Symbols, prefixes and conventions matter. Each row links to a definition, a worked example and a comparison table. Units of different quantities are kept separate even when symbols or dimensions look similar.

Choose the conversion direction

Start with the unit attached to the original measurement. Select the unit needed by the receiving report, specification or formula. Conversion tools preserve the measured quantity; calculating another property can require additional measured inputs and assumptions.

Precision and common mistakes

• Keep the source and target quantity consistent. A matching symbol is not enough to establish a compatible definition.
• Record the original measurement precision. Showing twelve significant digits in a table does not make a rounded input more precise.
• Check symbol capitalization, prefixes and any regional convention in the source. Use the displayed unit definition rather than a similarly named unit.
• Factors involving repeating fractions or π use finite numerical precision. Round the final answer to the precision justified by your input.

Use a working converter

• Power converter
• Watts to Kilowatts converter
• Watts to Megawatts converter
• Watts to metric horsepower converter
• Watts to btu per hour converter
• Watts to tons of refrigeration converter

Source and verification scope

Examples and tables are computed from the versioned unit definitions used by the linked converters. The source access dates appear below. Computandum is independent of the cited organizations; a citation does not mean that a publisher endorses this page or an actual project.

Read the calculation and sourcing methodology →