Retrofit & economics
High bay: why "400 W metal halide equals 150 W LED" is not a specification
What wattage-equivalence claims leave out, how depreciated HID output makes the comparison unfair in both directions, why vertical illuminance on rack faces matters more than floor lux, and the operational gains that usually exceed the energy saving.
Every high bay quotation contains an equivalence claim. It is always expressed in watts, which is the one quantity that has nothing to do with how well the space is lit. The comparison that matters is delivered illuminance where the work happens, and running it changes the answer surprisingly often — sometimes in the supplier's favour, sometimes badly against.
Why the comparison is unfair in both directions
Comparing a new LED fitting against an HID fitting's initial rated lamp lumens understates the LED. Comparing it against what the HID installation is actually delivering today understates it further. Four effects stack up.
| Effect | What happens |
|---|---|
| Lamp lumen depreciation | Metal halide in particular loses a substantial fraction of its output over life. An installation part-way through its relamp cycle is not producing rated lumens and has not been for years. |
| Ballast losses | Magnetic ballast draws meaningfully more at the circuit than the lamp rating. The existing installation's true consumption is higher than the lamp labels suggest. |
| Fixture efficiency and dirt | An HID high bay throws light into a reflector; some never leaves. Add years of dust on a reflector nobody can reach without a lift. |
| Group relamping practice | Where lamps are replaced on failure rather than on a cycle, output across the installation is wildly inconsistent. |
Before specifying anything, take a lux survey of the existing installation on a grid at working height, and note the age of the lamps. It costs an hour. It tells you what the space actually has, which is almost never what the original design intended, and it turns "equivalent to 400 W" into a comparison against a real number.
Specify delivered illuminance, not equivalence
The correct basis is the same as any other installation: maintained illuminance on the relevant plane, with uniformity, at a stated mounting height, with a maintenance factor whose terms are stated. Run it through the lumen method calculator for a first count, then require a proper photometric calculation from the tenderer with the real geometry.
Two high-bay-specific points that a floor-lux specification misses.
Vertical illuminance is what picking depends on
In a racked warehouse, the visual task is on the face of the rack — labels, barcodes, product at every shelf height. That is a vertical surface, and horizontal floor illuminance says little about it. Two schemes with identical floor lux can differ enormously on rack faces depending on whether the distribution is aisle-oriented.
Where racking exists or is planned, specify vertical illuminance on the rack face, at the highest and lowest picking levels, and require the calculation to model the racking as an obstruction. A calculation run on an empty box is worthless for a warehouse — the racks are most of the room.
Beam distribution follows the geometry, not the height
The habit is to pick narrow optics for high mounting. The real driver is the aisle geometry: narrow aisles between tall racking need a distribution that gets light down into the aisle rather than onto the top of the racks, which usually means an aisle-specific linear distribution rather than a symmetric round high bay. Open floor at the same height wants the opposite. Get the layout before choosing the optic.
The gains that are not energy
Retrofit business cases are usually built on watts, and in high bay the operational changes are frequently worth more.
- No restrike delay. HID lamps need minutes to restrike after any interruption — a brief power dip stops work until the lamps recover. LED returns instantly. In a production environment this alone can justify the project, and it is rarely in the model.
- Occupancy control becomes possible. You cannot switch HID off for an empty aisle because of the restrike. LED can be dimmed to a background level and raised on detection. In a warehouse with intermittently occupied aisles this is typically a far larger energy saving than the fitting efficiency difference, and it exists only for the LED option. Credit it in the controls field of the payback calculator.
- Maintenance access disappears. Relamping at height needs a lift, an operator, and usually a production stoppage. Removing that obligation is a recurring saving, and the access cost is the same line that makes the retrofit itself expensive.
- Cold stores gain twice. LED efficacy improves at low ambient while HID struggles to start and run. Confirm the driver's rated minimum ambient, and check the compliance window at cold start — see driver matching, because a cold store is precisely where the cold-Vf failure appears.
Three things that get worse if you are careless
Glare
An HID high bay is a large, deeply shielded reflector. Many LED high bays are compact arrays of small, very bright emitters. The mechanism in the UGR note applies with full force: same lumens, much higher source luminance, far more discomfort — particularly for anyone working at height on a forklift or mezzanine, who looks along the plane of the fittings rather than down. Ask for the shielding angle and the aperture luminance, and be sceptical of a UGR figure for a high bay, since the tabular method's assumptions fit this geometry poorly.
Uniformity after count reduction
The commonest value-engineering move is fewer, brighter fittings. Average illuminance holds; uniformity collapses; the space reads as striped. Fix the spacing-to-height relationship in the specification, or require a uniformity figure that the layout must satisfy, rather than only an average.
Emergency lighting assumptions
Where an existing scheme included dedicated luminaires specifically because HID could not restrike after a supply interruption, the emergency strategy may have been built around that. Changing the general lighting technology is a good moment to have the emergency scheme reviewed by whoever is responsible for it — not to assume it still holds.
What to put in the tender
- Maintained illuminance on the working plane and vertical illuminance on rack faces at stated heights, with uniformity for each.
- Existing installation survey results attached, so tenderers are pricing against measured conditions rather than the original design intent.
- Photometric calculation required, with racking modelled as obstruction.
- Shielding angle and aperture luminance stated; glare assessed for elevated working positions.
- Controls strategy explicit — aisle occupancy detection, background level, daylight linking where there are rooflights.
- Driver minimum ambient and cold-start behaviour where the space is unheated or refrigerated.
- Access and disposal priced as a separate line, since it is common to both routes and usually dominates.
Sources and further reading
- EN 12464-1 — illuminance, uniformity and glare requirements for industrial work places, including the treatment of vertical surfaces.
- CIBSE Lighting Guide 1, The Industrial Environment — high bay layout, aisle lighting and the visual tasks involved.
- IES LM-79 / LM-80 — measured luminaire photometry and package lumen maintenance, for comparing candidate products on evidence rather than equivalence.