A Watt is a unit of power, not of light output. But we’ve been conditioned to think of lighting in terms of Watts (ie power consumption) because that’s what was stamped on the top of the bulbs we bought. From experience, we know that a 60W light bulb provides a decent amount of light for an overhead fixture for example. However, now with LED lighting & LED bulbs available everywhere, this reference of light output no longer makes sense. LED bulbs are up to 10x more efficient at producing light.

So, now we need to use a proper measure of light output in order to be able to compare one light source to another. That measure is “Lumens”. Lumens refers to the volume of photons produced by a light source. Note that this is not the same as “Lux”, which measures light hitting a specific surface area – I’ll address Lux in a future post).

Here’s a quick reference…

100 watt (W) incandescent bulb = approx 1600 lumens
75W bulb = approx 1100 lumens
60W bulb = approx 800 lumens
40W bulb = approx 450 lumens

This will help you choose an LED bulb or light fixture for indoor and general purpose overhead room lighting.
For example, we can now expect about 800 lumens from a 10W LED light source. More light from less power!

For outdoor flood lights, we are focusing the light somewhat, so it gets a little more difficult to compare. Instead of spreading 11100 Lumens in all directions, we will be throwing it all in one direction down a driveway or garden path. And perhaps we use two lamps to provide better coverage. So, now how do we know how much light is enough?
This should help…

Small backyard deck : 500-700 lumens
Driveway : 1,000-2,000 lumens
Small Garden (50m²) : 1,500-2,000 lumens
Medium Garden (150m²) : 2,000-3,000 lumens
Parking Lot : 8,000-20,000 lumens

It is usually better to divide the desired lumens among several light sources in order to provide a more uniform, and softer illumination of an area. For example, use three or four 500-700 lumen lamps to illuminate a driveway from several directions and heights. Much better than trying to light-up the entire space with one big lamp from a single direction.

The installation location, beam angle, and height of lights greatly affects their performance. Placing two stronger lamps higher up a tree or pole will often provide better results than weaker floodlights placed at awkward angles down low. No one, especially your neighbours, appreciate staring into bright lights! Take the time to do a little geometry, and you will be happier with the end results.

A Note Regarding Light Colour
Incandescent light bulbs, and most halogens, produce a light that is on the warm side of white. Slightly yellow. This is understandable, since at the centre of the bulb is a glowing filament of metal. This slightly warm light source is pleasing, and is actually somewhat relaxing.
Compare this to the cold, blue, and even green of those terrible fluorescent tube lights that you find in many stores & office buildings. We’re also all familiar with the freaky blue-green light that some out of that cheap LED flashlight you got at the dollar store.

So, how do we choose a light that is either neutral daylight white, or on the warm side of the colour spectrum? Check the temperature scale on the side of the box, or on the spec sheet.

3000-4000K range is warm (yellow/white)
5000-6000K range is daylight (white/white)
7000-8000K range is cold white (blue/white)

As an easy rule-of-thumb, 4000K is pleasant for indoor residential lighting, while 6000K is a good choice for outdoor flood lights.

Cheers!