Worked example: a 4,000 m² warehouse from fluorescent to LED high bay

Starting points of the example

This is a fictional worked example. The hall does not exist and the figures were chosen to show the calculation steps. Use them as a template for your own project, not as a result.

• Hall: 80 x 50 metres, floor area 4,000 m², ceiling height 10 metres, mounting height of the luminaires about 9 metres.
• Required level: 200 lux average on the floor, maintained value, so at the end of the maintenance interval.
• Operating hours: 3,000 hours per year (day shift, 12 hours a day, 250 days).
• Existing installation: 200 fluorescent luminaires 2x58 W with magnetic ballasts, about 140 W system power per luminaire.
• New luminaire: sensor-ready 150 W UFO high bay from the ATRIA series, calculated in this example with 22,500 lm (150 lm/W), 4000K and a 90° optic.
• Energy price used: 0.25 euro per kWh excl. VAT. Fill in your own tariff here.

Step 1: number of high bays using the lumen method

For a first estimate you use the lumen method: the number of luminaires is the required lux level times the floor area, divided by the luminous flux per luminaire times the utilisation factor times the maintenance factor.

• Luminous flux required on the floor: 200 lux x 4,000 m² = 800,000 lumens.
• Utilisation factor: 0.65. That is the share of the light that reaches the floor in a high hall with light-coloured walls and a 90° optic.
• Maintenance factor: 0.8 for soiling and lumen depreciation over the years.
• Effective flux per luminaire: 22,500 x 0.65 x 0.8 = 11,700 lumens.
• Quantity: 800,000 / 11,700 = 68.4, rounded up to a grid of 70 luminaires.

Seventy units fit a grid of 10 rows along the length and 7 across the width: 8 metres by 7.1 metres between centres. At 9 metres mounting height the spacing-to-height ratio is well below 1, which gives an even result with a 90° optic. The calculated average comes to about 205 lux maintained.

Step 2: connected load and kWh per year

The connected load drops from 200 x 140 W = 28 kW to 70 x 150 W = 10.5 kW. At 3,000 operating hours that becomes:

• Fluorescent, existing: 28 kW x 3,000 hours = 84,000 kWh per year.
• LED high bay, without control: 10.5 kW x 3,000 hours = 31,500 kWh per year.
• Saving: 52,500 kWh per year, or 62.5 per cent.
• In euros: 52,500 x 0.25 = 13,125 euro per year.

On top of that, an older fluorescent installation often no longer reaches the design level of 200 lux: because of tube ageing and dirty reflectors, measured levels in practice are regularly lower, for instance 120 to 150 lux. The LED installation does reach the design level, and with the 5-year warranty on the ATRIA luminaires it still will in five years' time.

Step 3: adding sensors

The 150 W UFOs are sensor-ready: a plug-in sensor clicks into the luminaire without any wiring. In a warehouse with order picking, an aisle is only occupied for part of the day. A microwave sensor with daylight sensor dims the luminaire to standby when nobody is present and, when enough daylight comes in through the rooflights, down to the set level. If you choose sensors with a Zhaga socket and grouping function, the whole aisle switches on together and a forklift never drives towards a dark stretch.

In this example we count conservatively on a third less consumption from presence and daylight control:

• LED with sensors: 31,500 x 2/3 = 21,000 kWh per year.
• Saving compared with fluorescent: 63,000 kWh per year, 75 per cent.
• In euros: 63,000 x 0.25 = 15,750 euro per year.

For 70 luminaires you need 70 plug-in sensors. Order them together with the luminaires: clicking them in later is possible too, but then the access platform turns up twice.

Step 4: payback period, indicative

The payback period is the total investment (luminaires, sensors, installation, any changes to the cabling) divided by the annual saving. Because the unit price differs per project, we give the result per 10,000 euro invested:

• Without sensors: 10,000 / 13,125 = 0.76 years, just over 9 months per 10,000 euro.
• With sensors: 10,000 / 15,750 = 0.63 years, about 8 months per 10,000 euro.

Not included: maintenance. In 200 fluorescent luminaires sit 400 tubes that are replaced in groups every few years, each time with an access platform and two people. That disappears. The energy price is also an assumption: at 0.30 euro per kWh the saving with sensors becomes 18,900 euro per year. As long as the total investment in this example stays below about 78,000 euro (five times the annual saving with sensors), the payback period falls within the 5-year warranty period.

What the lumen method does not show

The lumen method assumes an empty hall. A warehouse is not empty: racking 8 metres high turns the floor into a series of narrow aisles and catches a large part of the light on the top of the racks. There are three ways to deal with that:

• Hang the UFOs above the aisles instead of in a fixed grid, so that every aisle gets its own row.
• Linear high bays with a 30x90° optic from the same ATRIA series, which put the light along the length of the aisle and not on the racking.
• A lighting calculation in simulation software with the racking drawn in. Lichtplein produces one for 350 euro excl. VAT, including luminaire positions and the lux distribution per aisle.

Glare has not been assessed in this example either. Where staff work under the light for long periods, for instance at a packing bench, look at the flat high bays with UGR<21 from ATRIA for that zone.

Frequently asked questions