Fluorescent to LED Replacement: Match Lumens, Not Watts
Why the “half the watts” rule loses 40% of your light, real lumen output of T8, T5 and PL lamps, a lumen-based replacement table, ballast losses and what lifetime figures actually measure.
How many watts of LED replaces a 36 W fluorescent tube?
The honest answer is that the question is asked in the wrong unit. Watts measure consumption, not light. A 36 W T8 fluorescent lamp produces roughly 3,300 lumens; to keep the same light you need an LED tube that produces 3,300 lumens, which is about 32 W in an economy LED, 21 W in an efficient one and 16 W in a premium one.
The widespread “half the watts” rule — 36 W fluorescent to 18 W LED — delivers around 1,800 lumens and therefore loses close to 40% of the light. It is the single most common and most expensive mistake in a fluorescent-to-LED conversion, because the lighting level is only noticed after the whole building has been relamped.
Why watt-for-watt matching fails
Follow the arithmetic through one fitting and the loss becomes visible:
| Step | 36 W T8 fluorescent | Economy 18 W LED tube |
|---|---|---|
| Lamp lumens | 3,300 lm | 1,800–1,900 lm |
| Ratio | reference | 55–58% |
| Luminaire efficiency (same fitting, example values) | ≈ 75% | ≈ 81% |
| Light leaving the fitting | ≈ 2,475 lm | ≈ 1,539 lm |
| Result | — | ≈ 38% less light |
The LED is more efficient per lumen and still loses, because it was never producing the same number of lumens to begin with.
Real lumen output of fluorescent lamps
Match against these figures rather than against the wattage printed on the old tube. Values are for 840 colour at the reference temperature each lamp family is rated at.
| Nominal power | Lamp lumens | lm/W | Length (mm) | Rated life |
|---|---|---|---|---|
| 18 W | 1,300 lm | 72 | 604 | 20,000 h |
| 30 W | 2,400 lm | 80 | 909 | 20,000 h |
| 36 W | 3,250–3,350 lm | 90–93 | 1,214 | 15,000–20,000 h |
| 58 W | 5,200–5,240 lm | 90 | 1,514 | 20,000 h |
| 70 W | 6,200 lm | 89 | 1,778 | 20,000 h |
| Type / power | Lamp lumens | lm/W | Length (mm) | Rated life |
|---|---|---|---|---|
| HE 14 W | 1,200 lm (1,350 lm at 35 °C) | 86 | 549 | 24,000 h |
| HE 21 W | 1,900 lm | 90 | 849 | 24,000 h |
| HE 28 W | 2,600 lm | 93 | 1,149 | 24,000 h |
| HE 35 W | 3,320 lm | 95 | 1,449 | 24,000 h |
| HO 24 W | 1,750 lm | 73 | 549 | 24,000 h |
| HO 39 W | 3,100 lm | 79 | 849 | 24,000 h |
| HO 49 W | 4,310 lm | 88 | 1,449 | 24,000 h |
| HO 54 W | 4,450–4,500 lm | 82–83 | 1,149 | 24,000–30,000 h |
| HO 80 W | 6,150 lm | 77 | 1,449 | 24,000 h |
T5 lamps are optimised for 35 °C: datasheets give the rated flux at 25 °C and a higher output at 35 °C, roughly 10–14% more depending on type (HE 14 W: 1,200 lm at 25 °C, 1,350 lm at 35 °C). This table uses the 25 °C rated values; say which temperature you are comparing at. Lengths: T8 figures include the pins, T5 figures exclude them (add about 14 mm). Lumen figures differ slightly between manufacturers and datasheet editions; check the one for the lamp you actually have.
| Type / power | Lamp lumens | Cap |
|---|---|---|
| PL-C 18 W | 1,200 lm | G24q-2 |
| PL-C 26 W | 1,800 lm | G24q-3 |
| PL-L 18 W | 1,200 lm | 2G11 |
| PL-L 24 W | 1,800 lm | 2G11 |
| PL-L 36 W | 2,900 lm | 2G11 |
| PL-L 55 W | 4,800 lm | 2G11 |
Replacement table, based on lumens
Read across from the lamp you have to the LED wattage that preserves the same light. Which column applies depends on the efficacy of the LED you buy — and that varies by a factor of two across the market.
| Existing lamp | Target lumens | Economy LED (≈105 lm/W) | Efficient LED (≈160 lm/W) | Premium LED (≈210 lm/W) |
|---|---|---|---|---|
| T8 18 W | 1,300 lm | ≈ 13 W | ≈ 8 W | ≈ 6 W |
| T8 36 W | 3,300 lm | ≈ 32 W | ≈ 21 W | ≈ 16 W |
| T8 58 W | 5,200 lm | ≈ 50 W | ≈ 33 W | ≈ 25 W |
| T5 HE 14 W | 1,200 lm | ≈ 11 W | ≈ 8 W | ≈ 6 W |
| T5 HE 28 W | 2,600 lm | ≈ 25 W | ≈ 16 W | ≈ 12 W |
| T5 HE 35 W | 3,320 lm | ≈ 32 W | ≈ 21 W | ≈ 16 W |
| T5 HO 54 W | 4,450 lm | ≈ 42 W | ≈ 28 W | ≈ 21 W |
| T5 HO 80 W | 6,150 lm | ≈ 59 W | ≈ 38 W | ≈ 29 W |
| PL-L 36 W | 2,900 lm | ≈ 28 W | ≈ 18 W | ≈ 14 W |
| PL-L 55 W | 4,800 lm | ≈ 46 W | ≈ 30 W | ≈ 23 W |
Note what the columns imply commercially: the same light can be bought at 32 W or at 16 W. Over a 4,000-hour year that difference is real money, and it never appears in a price-per-tube comparison.
Ballast loss: the meter reads more than the label
A fluorescent installation consumes more than the sum of its lamp wattages, because the ballast consumes too. When you calculate the saving from a conversion, compare against the system figure, not the lamp figure — otherwise the saving is understated.
| Lamp | Electronic (HF) | Low-loss magnetic | Standard magnetic |
|---|---|---|---|
| T8 18 W | 19–21 W | 25 W | 25–28 W |
| T8 36 W | 35–38 W | 43 W | 45 W |
| T8 58 W | 59 W | 67 W | 70 W |
An old installation on standard magnetic ballasts is drawing about 25% more than its lamp labels suggest. That is the number the conversion actually replaces.
Lamp lumens and luminaire lumens are different numbers
A lamp datasheet gives the light leaving the lamp. A luminaire datasheet gives the light leaving the fitting, after losses in the reflector and diffuser. A luminaire’s light output ratio is typically 60–90%: the fitting loses 10–40% of the lamp’s light, so a lamp figure reads roughly 11–67% higher than what the luminaire actually delivers — a common way to specify a conversion that ends up too dim.
If you are replacing whole luminaires rather than tubes, ask for the luminaire lumen output and the efficacy in lm/W at that level. If you are retubing existing fittings, lamp lumens are the right comparison — but remember the old reflector was designed around a different lamp geometry.
Lifetime figures are not measured the same way
“50,000 hours” and “20,000 hours” are not the same claim measured on a longer scale — they are different definitions:
| Product | Typical figure | Definition |
|---|---|---|
| T8 fluorescent | 20,000 h | The point at which 50% of lamps have failed |
| T5 fluorescent | 24,000 h | Same — 50% failure |
| Compact fluorescent | per datasheet | Check the definition — most state B50 (50% failures), some a survival rate |
| LED tube | 30,000 h | L70 — light has fallen to 70% of initial; the lamp is still lit |
| LED luminaire | 50,000 h | L70, or L80 on panels |
| Premium LED tube | up to 100,000 h | L70 |
A fluorescent figure describes when lamps stop working. An LED figure describes when the light has dimmed by a stated amount. Comparing them directly flatters the LED — which does not mean the LED is a bad investment, only that this particular comparison is not the argument.
What to ask before you order
- Lumens, not watts. Ask for the lumen output and the efficacy in lm/W. Two tubes at the same wattage can differ by 100% in light.
- Lamp or luminaire figure? Make the supplier state which one the number refers to.
- Colour temperature and CRI. Matching 3,300 lumens at the wrong colour still changes how the space looks.
- Ballast compatibility. A retrofit tube either works with the existing ballast, requires it to be bypassed, or needs it removed — and the labour difference between those three is larger than the price of the tube.
- Warranty terms in hours as well as years. A five-year warranty means something different at 2,000 h/year than at 6,000 h/year.
⚠ Availability: in the EU the RoHS mercury exemptions ended on 24 February 2023 for compact fluorescent lamps without an integrated ballast and on 24 August 2023 for linear T5 and T8 tubes; lamps already installed may still be used, but replacements are increasingly limited to remaining stock. The Turkish market’s dates and legal references are listed in our Turkish guide. Practically, a fluorescent installation should be planned for conversion rather than for continued relamping.
Frequently Asked Questions
How many watts of LED replaces a 36 W fluorescent tube?
Between roughly 16 W and 32 W, depending on the efficacy of the LED you buy. The target is not a wattage but 3,300 lumens — the output of the 36 W tube. An economy LED at about 105 lm/W needs 32 W to produce that; an efficient one at 160 lm/W needs 21 W; a premium one at 210 lm/W needs 16 W.
Is an 18 W LED tube a direct replacement for a 36 W fluorescent?
No, and this is the most common error in a conversion. An 18 W LED tube typically produces 1,800–1,900 lumens against the fluorescent’s 3,300 — around 40% less light. It is only a valid swap where the original installation was over-lit or where a lower level is acceptable, and that should be a deliberate decision rather than an accident.
Why does my old fluorescent installation use more power than the lamp labels?
Because the ballast consumes power too. A 36 W T8 on a standard magnetic ballast draws about 45 W at the system level, and on an electronic ballast about 35–38 W. When calculating conversion savings, compare against the system figure — using the lamp label understates the saving by roughly a quarter on older installations.
Do LED tubes work with existing fluorescent ballasts?
It depends on the tube. Some are designed to run on an existing electronic ballast, some require the ballast to be bypassed and the fitting rewired, and some need it removed entirely. The three options have very different labour costs, so this should be settled before ordering rather than discovered on site.
Is a 50,000-hour LED really five times better than a 20,000-hour fluorescent?
Not in the way the numbers suggest, because they measure different things. The fluorescent figure is the point at which half the lamps have failed. The LED figure is usually L70 — the point at which output has dropped to 70% of initial, with the lamp still working. Both are useful; they are simply not the same measurement.
Can I compare a lamp lumen figure with a luminaire lumen figure?
No. A luminaire figure is already reduced by losses in the reflector and diffuser: the light output ratio is typically 60–90%, so the fitting loses 10–40% of the lamp’s light and a lamp figure reads roughly 11–67% higher than the luminaire delivers. Comparing the two is a reliable way to end up with an installation that is too dim.
What colour temperature should an office use?
Neutral white — around 4000 K, the 840 designation — is the usual choice for offices and is what most fluorescent installations already use, so it keeps the space looking the same after conversion. What matters more is consistency: mixing colour temperatures in one room is far more noticeable than the choice itself.
Sources
- Signify — EU regulations: end of RoHS exemptions for fluorescent lamps (CFLni 24 February 2023; T5 and T8 24 August 2023)
- Commission Delegated Directive (EU) 2022/284 — RoHS Annex III exemption for mercury in double-capped linear fluorescent lamps
- Commission Delegated Directive (EU) 2022/276 — RoHS Annex III exemption for mercury in single-capped (compact) fluorescent lamps
- Lamp lumen, length and rated-life figures: Osram/Ledvance and Philips/Signify lamp datasheets, 840 colour
- LED efficacy and L70 life for the premium column: Philips MASTER LEDtube UE product datasheet
How we choose sources, verify figures and date our pages is set out in how we source and verify what we publish. Spotted an error? Write to info@aksco.com.tr — verified errors are corrected.
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