Watts vs lumens - what's the difference?

8th July 2013

At Brightgreen, we understand how confusing the lighting market can be. Over the next few weeks we hope to explain some key factors to look for when choosing lighting solutions, starting with the difference between watts and lumens.

It’s a common misconception that wattage is the deciding factor when choosing lights. In fact, a bulb’s brightness has nothing to do with its wattage – instead, brightness is measured as lumen output.

Where wattage measures electrical power, lumen output measures how much light a bulb emits. A bulb with a low lumen output will seem dull and will only illuminate a small space, whereas a bulb with a high lumen output will appear bright and illuminate a large space.

The easiest way to understand this is by imagining two equally sized birthday cakes, one with a single candle and one with 100 candles. The cake with only one candle will not emit much light, but the cake with 100 candles will emit a lot of light.

Lumens output comparison for halogen and LED bulbs

Wattage and lumen output can be combined to find out a light’s efficacy. Efficacy describes how efficient a lightbulb is at turning electrical energy into light. It is calculated by dividing the lumen output of the light by its wattage to give a lumens per watts value. The lumens per watt values of different lights can be compared to find out which product is the most energy efficient.

Premium 50W halogen light bulbs have an output of roughly 900 lumens, which equates to 18 lumens per watt. In contrast, Brightgreen’s 15.7W D900 Classic LED downlight offers between 906 and 1060 lumens (depending on the light temperature you choose), giving an average efficacy value of 63 lumens per watt – a much better rating that will result in long-term savings.

Lumen and wattage comparison for major LED bulb manufacturers

Efficacy comparison using lumens per watts measurements

When choosing lighting, lumen output should be your top priority, closely followed by the lumens per watt rating. Some lighting packaging now shows the lumen output value – we include it on all of our packaging – but not all, so be wary of brands that don’t tell you their products’ lumen outputs. 

Is there another area of lighting that you would like to learn more about? Our Lighting Glossary contains more useful information on lighting terminology, or feel free to get in touch to recommend a topic for next week's article.