Equating lumen and watt

A 10 watt lamp is as bright as any other 10 watt lamp.

Watt describes the power consumed, lumen the luminous flux emitted. These are different quantities, and there is no normative formula converting one into the other — luminous efficacy depends on the technology.

Incandescent lamps had a similar luminous efficacy for decades, so watt became the habitual figure for brightness. With LED and fluorescent technology that habit no longer holds, because efficacy has risen sharply.

Read the lumen figure on the packaging for brightness comparisons, not the watt figure. Lux and lumen are also different: lux is lumen per square metre and describes illuminance on a surface. Nit or cd/m² describes the luminance of a surface. UnitCloud does not convert these three quantities into each other, because area or solid angle would be required.