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Replacing a 250W HPS: How Much LED Output Do You Actually Need?

Learn how to compare a 250W HPS with LED using net luminaire output instead of directly matching the HPS lamp lumen rating.
Illustrative calculation for evaluating a 250 W HPS luminaire replacement with LED

A lamp's rated lumen output is not, by itself, a sufficient basis for selecting a replacement luminaire.

A 250 W high-pressure sodium (HPS) lamp may be listed with an initial luminous flux of approximately 27,000 lm. That figure can be useful for identifying the existing lamp, but it does not automatically define the output of the complete installed luminaire or the output required from an LED replacement.

This worked example shows how assumptions affect an initial comparison and what must be verified before the result is used in a project. The numerical values below are illustrative, not a universal conversion rule for every 250 W HPS fixture.

Step 1: Identify the basis of the 27,000 lm figure

First confirm that 27,000 lm is the lamp's rated output and identify the source and test conditions for that value. It should not be treated as measured output from the complete installed luminaire unless the supporting documentation says so.

For a real project, record the lamp type, luminaire model, available photometric data and the condition of the existing equipment. If the complete luminaire's output is already documented, use that information rather than substituting a generic efficiency assumption.

Step 2: Calculate an illustrative luminaire output

Suppose, for illustration only, the existing fixture has an assumed optical efficiency of 55%. Applying that single factor to a 27,000 lm lamp rating gives:

27,000 lm × 0.55 = 14,850 lm

This is the estimated output under the stated assumption. It is not a verified measurement of the installed fixture.

The original calculation used a further 30% loss factor, giving:

27,000 × 0.55 × 0.70 = 10,395 lm

That second calculation is appropriate only if the additional 30% represents a distinct loss that has not already been included in the 55% factor. Without a clear definition and supporting data, the two factors should not be combined as though they were verified properties of the actual luminaire.

Step 3: Compare the LED datasheet on the same basis

Assume a candidate LED luminaire is listed at 10,000 lm. Before comparing it with the HPS estimate, confirm that the LED figure is the declared output of the complete luminaire and that the HPS estimate was derived on a comparable basis.

Item Illustrative value Evidence required
HPS lamp output 27,000 lm Lamp datasheet and rated conditions
Assumed fixture efficiency 55% Luminaire data or a justified engineering assumption
Additional loss factor 30% in the original example Evidence that this is a separate loss, not already included
Candidate LED output 10,000 lm Exact LED model datasheet and output basis

The table is a screening aid. It does not establish that the LED candidate is equivalent or suitable for the installation.

Step 4: Verify the lighting result

Even a valid comparison of luminaire lumens does not demonstrate equivalent illuminance. The actual result depends on the optical distribution, mounting height, spacing, orientation and the geometry of the area.

Where the project requires a lighting calculation, evaluate the proposed LED arrangement using appropriate photometric data and the specified work plane and lighting criteria. Do not approve the replacement solely because its lumen figure is close to an estimated HPS value.

Step 5: Complete the hazardous-area checks

For a hazardous-area installation, the candidate LED luminaire must also meet the project's Ex requirements. Verify the exact model and certificate scope, complete marking, protection concept, gas or dust group, EPL or zone suitability, temperature limitations, ambient range and any special conditions of use.

Lighting performance and hazardous-area certification are separate acceptance checks. Passing one does not demonstrate compliance with the other.

Information to collect before choosing the LED

  • Existing HPS lamp and luminaire model.
  • Manufacturer's lamp and luminaire data, including photometry where available.
  • Mounting height, spacing, orientation and area geometry.
  • Required illuminance and applicable lighting-design criteria.
  • Candidate LED datasheet and photometric file.
  • Project hazardous-area specification and certification requirements.

Conclusion

The 27,000 lm rating of a 250 W HPS lamp does not automatically mean that a 27,000 lm LED luminaire is required. However, reducing that lamp figure using assumed factors does not automatically establish the correct replacement output either.

Use documented luminaire data wherever possible. State assumptions explicitly, avoid applying overlapping loss factors, and verify the proposed LED arrangement against the required lighting performance and Ex requirements.

For the general explanation of lamp lumens versus luminaire lumens, see Why HPS Lamp Lumens Cannot Be Compared Directly With LED Lumens.

For work-plane illuminance and lighting-design checks, see Why LED Retrofit Should Start With Lux, Not Lumens.

Engineering note: The example values are illustrative. They must not be used as design or acceptance values without verification against the actual equipment and project documentation.