Lumens Per Watt. Why Does It Matter?

The higher the wattage of a bulb, the brighter the light. The statement holds true for the almost extinct incandescent bulbs, however, it’s not relevant in the case of LED lighting. The efficiency of an LED bulb is measured in ‘Lumens per watt’ which shows how much light or lumens a bulb is capable of producing using one watt of electricity. To better understand the concept of Lumens per watt, we first need to understand the difference between a lumen and a watt.

Lumens per watt (lm/W), also called luminous efficacy, is the light a lamp produces for each watt it uses. Higher is better: an incandescent bulb gives about 16 lm/W, a CFL about 53 and a good LED about 100 or more. To compare bulbs, divide the lumens on the pack by the watts.

If you have had incandescent bulbs in your home earlier, you might remember the times when you chose a 100-watt bulb over a 60-watt bulb for a space that needed more lighting. The more the wattage of an incandescent bulb, the more the brightness and thus more the power consumption.

However, in the case of LED bulbs, wattage is no more a norm, as an LED bulb can produce more lumens using fewer watts depending on its type and design. This means that one must check for lumens per watt instead of just the wattage when buying LED bulbs. Let us take a deep dive into the concept of lumens per watt and to paint a better picture, we have shown the three generations of bulbs in a comparative manner as per the average industry standards.

Say a standard 100-watt incandescent bulb uses 100 watts of electricity to deliver 1600 lumens. Its efficiency comes out to be 16 Lumens per watt. First, let us replace it with 30-watt CFL bulb equivalent and if you calculate the efficiency now, it increases to 53.33 Lumens per watt.

Now if we replace it with a 15-watt LED bulb, the energy efficiency increases drastically to about 106.66 Lumens per watt. This clearly shows that a 15-watt LED bulb can produce as much brightness as a 100-watt incandescent bulb while consuming very less energy, making it almost 7 times more efficient than the 100-watt incandescent bulb it is replacing.

How do I calculate lumens per watt?

Lumens per watt = lumens ÷ watts. Using the figures in this post, a bulb giving 1,600 lumens works out as:

The 15W LED gives the same light as the 100W bulb for 15% of the power.

How many lumens does a room need?

A common rule of thumb is 10–20 lumens per square foot for a living room or bedroom. Multiply the room's area in square feet by the figure.

Example: a 12 ft × 14 ft living room is 168 sq ft, so it needs about 1,680–3,360 lumens. At 100 lm/W that is roughly 17–34W of LED lighting in total.

Lumens, watts and lux: what is the difference?

Is a higher lumens-per-watt figure always better?

A higher lm/W means lower running cost, but it is not the only quality measure. Check colour rendering (CRI), colour temperature and glare as well. A very efficient lamp with poor CRI can make colours look dull.

Switch to Smart. Switch to LEDs.

With LEDs offering a range of benefits including energy efficiency, longevity and greater design flexibility, lighting is now no longer a necessity but a lifestyle requirement. The LED lightbulbs generally have a lifespan of 25,000 hours or more, last longer than conventional lighting and offer 80% energy savings, which makes them a preferred choice of lighting. Decreasing prices, increasing efficiency, and government policies and initiatives have further given a boost to LED adoption throughout the country in the last couple of years.

However, it is important for the consumer to choose quality LED bulbs to get true value for their money. Earlier, there were no standardised norms for LED bulbs and thus people had to buy anything they were offered. Understanding the quality of the LED bulbs was a challenge. Last year, BEE, under its LED labelling programme, started rating bulbs on a five-star scale for energy efficiency on parameters that included ‘Lumens per watt’ and other safety requirements like photo-biological safety. As a leading brand in the industry, Orient Electric became the first Indian lighting brand to receive a BEE 3-Star rating for its 5W, 7W & 9W LED bulbs, which ensures that these bulbs deliver more than 100 Lumens per watt as compared to other bulbs in same wattage that produce 70-85 Lumens per watt.

Switching from conventional lighting to LED at your home might seem like a very small step towards energy efficiency but when every household in India makes the same switch, the impact will be huge. Replacing just one inefficient bulb (average usage of 5.5 hours daily) with an LED bulb can give an annual cost saving of around Rs. 520 while saving 130 kWh of annual energy and reducing annual CO2 emissions by 0.11 tonnes. If you already know, there are approximately 24.8 crore households in India. Think of the big picture now!!

FAQs

Q: What is lumens per watt?

A: Lumens per watt is the amount of visible light a lamp produces for every watt of electricity it uses. It is also called luminous efficacy. A higher number means a more efficient lamp: the same brightness for less power, and a lower electricity bill.

Q: How many lumens per watt is good for an LED?

A: A good LED bulb gives around 100 lumens per watt or more, compared with about 16 for an incandescent bulb and about 53 for a CFL. The BEE star table sets the thresholds for rated lamps. 

Q: How do I convert watts to lumens?

A: There is no fixed conversion, because it depends on the lamp's efficiency. Divide the lamp's lumens by its watts to get its lm/W, then multiply lm/W by watts. As a guide, a 60W old bulb gives about 800 lumens, which a good LED delivers at about 8W.

Q: How many lumens is a 9W LED bulb?

A: At about 100 lumens per watt, a 9W LED bulb gives roughly 900 lumens. The exact number depends on the model, so read the lumens printed on the pack. 

Q: Are lumens or watts more important when buying an LED bulb?

A: Lumens matter more for brightness, and watts matter for running cost. Choose the lumens you need first, then pick the lamp with the lowest watts for that output. That is the same as choosing the highest lumens per watt.

Q: What is the difference between lumens and lux?

A: Lumens measure the total light a lamp emits. Lux measures how much of that light lands on a surface: one lux is one lumen per square metre. A lamp's lumens stay the same, but the lux on your desk falls as the light moves farther away.