Watts, Lumens, and Colour Temperature: Reading a Light Bulb Label Correctly

Why the Old Watt Shortcut No Longer Works

For decades, shoppers chose light bulbs by a single number: watts. A 60-watt bulb went in the living room lamp; a 100-watt bulb lit the garage. That shortcut worked because all incandescent bulbs converted energy to light at roughly the same efficiency, so wattage reliably predicted brightness.

LEDs changed that relationship permanently. A modern LED can produce the same 800 lumens of light as a 60-watt incandescent while drawing only 8 to 10 watts — roughly one-seventh the energy. If you shop by watts alone today, you risk buying a bulb that is far dimmer, or far brighter, than you intended.

The shift is similar to learning that a food's calorie count doesn't tell you much about its nutritional value — both require reading the full label rather than relying on a single proxy figure. Just as reading a nutrition label correctly reveals what a food actually contains, understanding a bulb label reveals what a bulb actually does.

Lumens: The Real Measure of Brightness

Lumens quantify the total amount of visible light a bulb emits. The higher the lumen count, the brighter the light. Here are common reference points that make the number meaningful:

• 450 lumens — roughly equivalent to a 40-watt incandescent; good for accent or decorative fixtures
• 800 lumens — equivalent to a 60-watt incandescent; standard for table lamps and most ceiling fixtures
• 1,100 lumens — equivalent to a 75-watt incandescent; useful for reading areas and task lighting
• 1,600 lumens — equivalent to a 100-watt incandescent; appropriate for kitchens, workshops, and high-traffic rooms

When planning a room, consider lumens per square foot. A general guideline for ambient (overall) lighting is around 10–20 lumens per square foot, though task-heavy spaces like kitchens may benefit from higher levels. Always treat these as starting points — personal preference and ceiling height affect perception significantly.

Colour Temperature: Setting the Mood with Kelvin

Colour temperature is measured in Kelvin (K) and describes the hue of the light a bulb emits — not its heat. The scale runs from warm (yellowish) to cool (bluish), and matching it to a room's function makes a noticeable difference in comfort and usability.

Common Colour Temperature Ranges

2700K–3000K — Warm White

Produces the familiar golden glow of traditional incandescents. Well-suited to bedrooms, living rooms, and dining areas where a relaxed, welcoming atmosphere is the goal.

3500K–4100K — Neutral or Cool White

A crisp, neutral light that feels neither warm nor cold. Often chosen for kitchens, bathrooms, and home offices where clarity matters without the harshness of daylight bulbs.

5000K–6500K — Daylight

A bright, blue-tinged light that mimics natural daylight. Useful in garages, workshops, craft rooms, or anywhere accurate colour rendering is a priority. Can feel overly clinical in living spaces.

One practical tip: keep colour temperature consistent within a single open-plan space. Mixing a 2700K bulb in a floor lamp with a 5000K bulb in a ceiling fixture creates an uneven, visually distracting effect.

Other Numbers Worth Noticing

Beyond lumens, watts, and Kelvin, the Lighting Facts label typically includes a few more data points that can guide smarter choices.

Color Rendering Index (CRI)

CRI is a 0–100 scale measuring how faithfully a bulb reveals the true colors of surfaces and objects under its light, compared to natural sunlight. A CRI of 80 is generally considered acceptable for most household uses. A CRI of 90 or higher is preferable for spaces where colour accuracy matters — kitchens, dressing areas, or art studios. Most standard LEDs fall in the 80–90 range.

Estimated Yearly Energy Cost

This figure, printed on U.S. Lighting Facts labels, uses a standardized calculation based on a national average electricity rate and three hours of daily use. It allows apples-to-apples cost comparisons between bulb types. Your actual cost will vary with local electricity rates and actual usage hours, but the relative difference between two bulbs stays directionally accurate.

Bulb Life (Hours)

LED bulbs are commonly rated for 15,000 to 25,000 hours — compared to roughly 1,000 hours for a standard incandescent. That said, rated life is a laboratory estimate under controlled conditions; real-world results can vary based on heat, vibration, and how often a bulb is switched on and off.

If energy efficiency labeling interests you more broadly, the Energy Star certification guide explains how that program evaluates appliances using similar efficiency principles.