Flood Light Wattage and High Bay Lumens: How Much Light a Space Needs

The short answer

• Lumens, not watts. Watts are what a fitting draws; lumens are the light it gives. Good LED high bays give about 130–160 lm/W and floods about 100–150 lm/W.
• Target lux – about 100–200 for storage, 300 for packing, welding and machining, 500 for fine work and 20–50 for outdoor yards (EN 12464-1:2021 and EN 12464-2:2014; check the current editions).
• Lumens needed = lux × area ÷ (UF × MF). UF × MF is often about 0.5, so you need roughly 2 × lux × m² lumens; more in tall, narrow or dirty spaces.
• Spacing – up to about 1 to 1.5 times the height above the working plane, or the datasheet limit if lower.
• Site ratings – IP65 or more, a temperature rating that covers a hot roof, a stated surge rating, and MCBs chosen for driver inrush.

Watts, lumens and target lux

Luminous efficacy links watts and lumens: lumens = watts × lm/W. A 150 W fitting at 140 lm/W gives 21,000 lm; at 100 lm/W only 15,000 lm, so you need 40 percent more fittings. Compare the rated lumens of the complete fitting, not the LED chip figure.

Standards give maintained illuminance: the average must not fall below it, even just before cleaning. They are recommendations; local codes or client specifications may differ.

Indoor values are the “required” (minimum) column of EN 12464-1:2021; its higher “modified” column (for example 500 lux for welding, rough machining and packing) applies when the task is critical, errors are costly, the work is long or eyesight is below normal. Outdoor values come from the 2014 tables of EN 12464-2, revised in 2024, so check the current edition. In workshops the darkest point should be at least 0.6 of the average (0.4 in storage areas).

How many fittings: the lumen method

N = E × A ÷ (Φ × UF × MF), where N is the number of fittings, E the target lux, A the area in m² and Φ the rated lumens of one fitting.

• Utilisation factor (UF) – the share of lumens reaching the working plane, read from the maker’s table for the surface reflectances and the room index k = L × W ÷ (Hm × (L + W)), with Hm the height of the fittings above the working plane. For one wide-beam high bay and medium to dark surfaces it is about 0.5 at k = 1, 0.7 at k = 2 and 0.85 at k = 3.
• Maintenance factor (MF) – light lost to ageing and dirt. For sealed LED fittings use about 0.8 in clean areas, 0.7 in a typical workshop or warehouse and 0.6 or less with heavy dust or rare cleaning. If conditions are unknown, one major maker uses 0.57 for normal industrial rooms.

Worked example: a 600 m² workshop

A 30 m × 20 m workshop needs 300 lux for welding and machining. The fittings hang at 7 m above 0.85 m benches, so Hm = 6.15 m. Sealed IP65 high bays will be cleaned once a year.

1. k = 600 ÷ (6.15 × 50) = 1.95, so UF = 0.7 from a typical table; take MF = 0.7.
2. A 150 W high bay at 140 lm/W gives 21,000 lm.
3. N = 300 × 600 ÷ (21,000 × 0.7 × 0.7) = 17.5, so 18 fittings: 2.7 kW, or 4.5 W/m².
4. Six rows of three give 5.0 m × 6.7 m spacing, within 1.5 × 6.15 = 9.2 m, with 2.5 m and 3.3 m to the walls. Result: 309 lux maintained, about 440 lux when new.

This gives only the average; for evenness and glare, ask for a point-by-point layout from the maker’s photometric file.

Worked example: an outdoor loading yard

A 30 m × 20 m yard where lorries are loaded every night needs 50 lux. With no walls or ceiling to reflect light, and some spill past the edges, assume UF 0.4, and MF 0.7 for sealed IP66 floods.

1. Lumens needed: 50 × 600 ÷ (0.4 × 0.7) = 107,000 lm.
2. A 150 W flood at 130 lm/W gives 19,500 lm: 107,000 ÷ 19,500 = 5.5, so six floods (900 W), for example three on each long side at 8 m or more, aimed so the beams overlap without dazzling drivers. Confirm this first estimate with a layout.

Mounting height, spacing and beam angle

Start with wide-beam fittings no further apart than about 1 to 1.5 times their height above the working plane, never beyond the maximum spacing-to-height ratio (SHR max) on the datasheet, and about half that spacing from the walls. Narrow beams need closer spacing but reach down tall aisles; wide beams suit low roofs. Indoors, avoid tilted floods, which cause glare.

IP, IK, heat and lifetime ratings

Heat. Datasheets give ta, the highest ambient for safe operation (25 °C if unmarked), and tq, the ambient at which lumen and life figures apply. Lahore reaches about 45 °C in the shade in heatwaves, and air under a metal roof is hotter, so choose a ta of at least 50 °C.

Lumen maintenance. L80 at 50,000 h means at least 80 percent of the initial lumens remain after 50,000 hours, usually as a median (B50) value; L70 allows more loss. Good high bays quote L80 or better at 50,000 h at a tq of 25–35 °C. These are projections, not measurements.

Colour temperature and colour rendering

4000 K and 5000 K both suit factories: 5000 K looks crisper, 4000 K softer next to offices. 6500 K looks bluish, can feel harsh at night and often adds little or no efficacy. Use Ra 80 or more where people work, as EN 12464-1:2021 asks for most industrial tasks, and Ra 90 at 4000–6500 K for colour inspection. Ra 70 suits circulation routes, where it asks only Ra 40; loading areas where people work ask Ra 80.

Drivers, circuits and switching

• Surge and supply. The basic immunity test for lighting (IEC 61547) is only 1 kV line to line and 2 kV line to earth. Better drivers state higher ratings, often 4 kV on high bays and 6–10 kV on outdoor floods (state line to line or line to earth when you compare), plus a surge protective device (SPD) in the board feeding outdoor circuits, a power factor of 0.9 or more and a wide input range, such as 90–305 V AC.
• Flicker. Poor drivers ripple at 100 Hz, which can make rotating machines look slower or stopped. Ask for flicker data. As a benchmark, EU ecodesign rules for mains LED lamps set PstLM 1.0 and SVM 0.4 (industrial and outdoor products are exempt from SVM). Spread neighbouring fittings over the three phases.
• Inrush sets the MCB count. A typical 150 W driver draws a 50 to 65 A pulse at switch-on, and its maker may allow only about 4 per B16 or 6 to 8 per C16 MCB, not the 20 its running current suggests. Follow the datasheet table, and never raise the MCB rating to fit more drivers: the cable, derated for 45 °C, must still be protected (see MCB vs MCCB vs RCCB). With such drivers the workshop needs three C16 circuits, one per phase.
• Contactors and RCDs. Size a lighting contactor from the maker’s lamp or LED-driver rating, not its AC-1 (heater) rating, since inrush can weld contacts, and let the timer or photocell drive its coil (see relay vs contactor vs SSR). Driver earth leakage (under 0.75 mA each on that driver) adds up, so limit the fittings per RCD.

Buying checklist

1. Area, mounting height, task, target lux and evenness.
2. Lumens needed = lux × area ÷ (UF × MF), with UF from the maker’s table.
3. Datasheet lumens, number of fittings and a layout within the spacing limit.
4. Beam to suit the height; asymmetric optics for yards.
5. IP65 or more (IP66 on exposed sites), IK08 where fittings can be hit, and a ta for summer heat.
6. Ra 80 or more and one colour temperature, usually 4000 K or 5000 K.
7. Driver surge rating, power factor, input range, flicker, and L80 at a stated tq.
8. Drivers per MCB from inrush data, contactor lamp rating, RCD leakage, SPD and balanced phases.

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