LED Efficiency: How to Calculate It

This guide shows you how to calculate the efficiency of an LED, which tells you how much visible light you get for each watt of electrical power it consumes. You’ll need the LED’s luminous flux (in lumens) and its power draw (in watts) — both are usually printed on the packaging or datasheet. With those two numbers, you can compute efficacy in lumens per watt (lm/W), the standard measure of LED efficiency.

Formula

LED efficiency is properly called luminous efficacy. The formula is:

Efficacy (lm/W) = Luminous flux (lm) ÷ Power (W)

Where:

• Luminous flux (lm) — the total amount of visible light emitted by the LED, measured in lumens.
• Power (W) — the electrical power the LED consumes, measured in watts.
• Efficacy (lm/W) — the light output per watt of power; higher is better.

For example, a typical 9-watt LED bulb that produces 800 lumens has an efficacy of 800 ÷ 9 ≈ 88.9 lm/W.

Worked Example

Let’s use the default values from the calculator above:

1. Luminous flux = 800 lm
2. Power = 9 W
3. Divide flux by power: 800 ÷ 9 = 88.888…
4. Round to one decimal place: 88.9 lm/W

That matches the calculator’s output. So this LED produces about 89 lumens per watt.

When to Use This Calculation

Calculate LED efficacy whenever you want to compare the energy efficiency of different bulbs or LEDs. It’s especially useful when:

• Choosing a replacement bulb — a higher lm/W means more light for the same electricity.
• Estimating operating costs — if you know the wattage and hours of use, efficacy helps you compare.
• Designing lighting layouts — you need to know how many lumens each fixture provides per watt.

The result is a simple ratio. A typical modern LED bulb achieves 80–100 lm/W, while some high-end LEDs exceed 150 lm/W. Older incandescent bulbs manage only about 15 lm/W, so the difference is dramatic.

How to Read the Result

The efficacy number tells you how efficiently the LED converts electricity into visible light. A higher number means more light per watt, which generally means lower electricity bills for the same brightness. However, efficacy alone doesn’t tell you about light quality (color rendering, color temperature) or lifespan — it’s just one metric.

Common Mistakes

• Using input power instead of actual power: Some LED bulbs list an “equivalent wattage” (e.g., “60W equivalent”) but the actual power draw is much lower (e.g., 9W). Always use the real wattage, not the equivalent.
• Mixing up lumens and lux: Lumens measure total light output; lux measures illuminance on a surface. Efficacy uses lumens, not lux.
• Forgetting to convert units: If your power is in milliwatts, convert to watts first (divide by 1000).
• Using the wrong flux value: Some datasheets list radiant flux (in watts) instead of luminous flux (in lumens). Radiant flux measures all electromagnetic radiation, not just visible light. Use luminous flux for efficacy.
• Rounding too early: If you round the flux or power before dividing, you can get a slightly off result. Keep full precision until the final step.

Tips

• Check the LED’s datasheet or packaging for the exact luminous flux and power. If only the wattage is given, you can estimate flux by multiplying wattage by a typical efficacy (e.g., 90 lm/W for a good LED).
• For a more accurate measure, use a integrating sphere or a lux meter with a known distance — but for most practical purposes, the manufacturer’s numbers are fine.
• Remember that efficacy changes with drive current: running an LED at lower current often increases efficacy (in lm/W) but reduces total light output.
• When comparing bulbs, also consider the beam angle and color temperature — a narrow beam might appear brighter even if the total lumens are lower.

Troubleshooting

My result seems too high or too low. Double-check that you’re using the actual wattage, not equivalent wattage. Also verify the luminous flux — some bulbs list “initial lumens” which can be higher than the average over the bulb’s life.

The calculator says “Cannot compute” — this happens if you enter a power of 0. Enter a positive wattage (even 0.1 W works).

I don’t know the luminous flux. If you only have the wattage, you can estimate flux using a typical efficacy. For example, a 9W LED with 90 lm/W efficacy would produce about 810 lumens. But for precise comparisons, get the actual flux from the manufacturer.

My LED is dimmable — does that change efficacy? Yes, dimming reduces power and light output, but the efficacy may change slightly. The calculation above uses the rated values at full power.

Frequently Asked Questions

What is a good LED efficacy? Modern LEDs typically range from 80 to 150 lm/W. Above 100 lm/W is considered very good; above 150 lm/W is excellent and usually found in high-end or specialty LEDs.

Why is LED efficiency called “efficacy” not “efficiency”? Efficiency is a unitless ratio (output/input). Efficacy has units (lumens per watt) because it measures light output per power input, not a pure percentage. It’s the standard term in lighting.

Can I calculate efficacy if I only know the wattage? No, you need the luminous flux. But you can estimate flux if you assume a typical efficacy — for example, 90 lm/W for a good LED. That gives you flux = wattage × 90.

Does color temperature affect efficacy? Generally, cooler (higher Kelvin) LEDs tend to have slightly higher efficacy than warm ones because they produce less red light, which is less efficient. But the difference is small.

How does LED efficacy compare to other bulbs? Incandescent bulbs are about 15 lm/W, halogen about 20 lm/W, compact fluorescent (CFL) about 60 lm/W, and LEDs typically 80–150 lm/W. LEDs are the most efficient common option.

Does the driver or power supply affect efficiency? Yes, the LED driver converts AC to DC and can waste some power. The efficacy calculation above uses the LED’s own power draw, not the whole fixture. To include driver losses, divide the fixture’s total power (including driver) by the light output.

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in lm | Luminous flux | lm | 800 | 0 | 100000
in w | Power draw | W | 9 | 0 | 1000
out eff | Efficacy | lm/W | lm / w | 1