15 W VS 25 W VS 45 W
15 W, 25 W and 45 W refer to the rated power capacity (VA/W class) of an AGL isolation transformer — not the intensity of the connected lamp. The correct rating is always selected according to the connected AGL light fixture and the approved circuit design, not by wattage preference alone.
At a Glance
| Feature | 15 W | 25 W | 45 W |
|---|---|---|---|
| Rated power class | 15 W | 25 W | 45 W |
| Relative capacity | Low | Medium | High |
| Typical use | Lower-power AGL fixture | Medium-power fixture | Higher-power fixture |
| Suitable lamp load | Depends on design | Depends on design | Higher load capability |
| Physical size | Generally smaller | Medium | Generally larger |
| Cost | Generally lower | Medium | Generally higher |
| Losses | Generally lower | Medium | Generally higher |
| Main advantage | Compact, economical | Balanced capacity | Higher load capability |
What Does 15 W / 25 W / 45 W Mean?
The rating indicates how much electrical load the transformer is designed to supply under its specified operating conditions. In transformer specifications, the rating is often expressed in VA (volt-amperes) rather than W. For any particular AGL transformer, always use the manufacturer's stated VA rating and applicable operating conditions rather than assuming from the class name alone.
Each Class in Detail
15 W
Intended for a relatively low-power lighting load.
- Smaller power capacity
- Suitable for lower-power fixtures where specified
- Generally compact
- Lower cost compared with higher-rated units
25 W
Provides greater load capacity than the 15 W class.
- Medium power capacity
- Selected where the fixture needs more than a 15 W unit can provide
- Balanced size and cost
- Common middle-tier selection
45 W
Provides the highest capacity of the three classes.
- Highest power capacity
- Used where the connected fixture requires it by design
- Generally larger physically
- Generally higher cost and losses
Do Not Assume Higher Wattage Means Brighter Light
45 W transformer ≠ 45 W light output. The transformer rating represents its power-handling capability — it does not automatically mean the connected lamp will operate at higher intensity. Actual light output depends on fixture design, lamp/LED technology, operating current, CCR setting, transformer characteristics and the fixture's electrical requirements.
Transformer Rating Must Match the Fixture
The correct selection flows from the fixture, not from wattage preference:
- AGL light fixture defines the required electrical load
- Required electrical load determines the transformer rating
- Transformer rating is then selected as 15 / 25 / 45 W accordingly
Do not select a transformer simply because its wattage is higher — the manufacturer's specified transformer for that fixture should always be used.
AGL Series Circuit Example
A simplified series circuit can contain different transformer ratings where specifically permitted by the system design — but the transformer type and rating must always be compatible with the specific fixture and AGL circuit design.
15 W Branch
Lower-power AGL fixture on the series circuit.
25 W Branch
Medium-power AGL fixture on the series circuit.
45 W Branch
Higher-power AGL fixture on the series circuit.
Detailed Parameter Table
| Parameter | 15 W | 25 W | 45 W |
|---|---|---|---|
| Power capacity | Low | Medium | High |
| Relative load capability | 1× | 1.67× | 3× |
| Typical physical size | Smaller | Medium | Larger |
| Typical cost | Lower | Medium | Higher |
| Suitable fixture load | Lower | Medium | Higher |
| Light intensity | Not determined by rating alone | Not determined by rating alone | Not determined by rating alone |
| Selection basis | Fixture requirement | Fixture requirement | Fixture requirement |
The 45 W unit has 3 times the nominal power rating of the 15 W unit and 1.8 times that of the 25 W unit. Actual physical dimensions and losses remain manufacturer-specific and should not be assumed from wattage alone — a higher-rated transformer does not automatically consume its full rated power; actual losses depend on construction, loading and operating conditions.
Common Misunderstandings
The wattage class describes power-handling capability, not lamp intensity. Actual brightness depends on fixture design, lamp/LED technology, operating current and CCR setting.
The transformer rating must match the connected fixture and manufacturer's specification — selecting a higher wattage simply because it is available is not the correct basis for selection.
The rating is the maximum designed capacity, not the actual load. Actual losses and heat depend on transformer construction, real loading and operating conditions.
Which Rating Applies Where?
15 W, 25 W and 45 W describe different AGL isolation transformer power capacities — low, medium and high respectively — not different light output levels. The transformer rating must always match the connected AGL fixture and the manufacturer's specification for that fixture.
For an actual airport project, the final rating selection should be based on the applicable current ICAO/national AGL standard, the approved circuit design and equipment requirements — not on wattage preference or a generic comparison alone.
Engineering & Expert Notes
The Rating Is Not the Lamp Wattage
ConceptThe class refers to transformer capacity. Select it from the fixture's actual load, including any LED driver behaviour, and not from the lamp label alone.
Leave Headroom, but Not Excess
MarginToo little margin overheats the unit; too much changes how lightly loaded transformers behave on the circuit. Aim for a sensible fit and document the basis.
Recheck the Transformer When the Fixture Changes
RetrofitMoving from lamp to LED can lower the load. Review whether the existing rating is still appropriate rather than assuming like-for-like.
Stock the Ratings You Actually Use
SparesMatch spare holdings to the installed base and record which locations use which rating.
Frequently Asked Questions
Not automatically. The wattage class describes power-handling capability. Actual light output depends on fixture design, lamp/LED technology, operating current, CCR setting and transformer characteristics.
The choice is driven by the connected AGL fixture's required electrical load and the manufacturer's specification for that fixture — not by picking a higher wattage on its own.
Transformer ratings are often expressed in VA (volt-amperes) rather than W. Always use the manufacturer's stated VA rating and operating conditions for the specific unit.
A series circuit can contain different transformer ratings where the system design specifically permits it, but each transformer's type and rating must remain compatible with its specific fixture and the overall circuit design.
As a general design trend, yes — 15 W tends to be smaller, 25 W medium and 45 W larger. However, exact physical dimensions are manufacturer-specific and should not be assumed from wattage alone.
Not necessarily by default. The rating is the maximum designed capacity, not the actual consumed power. Actual losses depend on construction, real loading and operating conditions.
Taking 15 W as the 1× baseline, 25 W is approximately 1.67× and 45 W is 3× the nominal power rating — 45 W is also about 1.8× the 25 W rating.
Governing Documents
For an actual airport project, verify the current applicable editions of the relevant standards and the specific transformer/equipment datasheets before using this comparison for project design.
ICAO Annex 14, Volume I
Aerodrome Design and Operations — series circuit requirements and isolation transformer rating characteristics for AGL.
ICAO Aerodrome Design Manual
Applicable material covering series circuit design, transformer rating selection and electrical infrastructure guidance.
National CAA / DGCA / AAI Requirements
For Indian airport projects, also verify applicable DGCA requirements, AAI requirements, airport-specific technical specifications, project drawings and contract specifications.
Final Engineering Note
Exact transformer rating, VA class and fixture compatibility can vary with the governing standard, equipment manufacturer and approved airport design. For final design, use the current applicable standard and manufacturer datasheets rather than a generic comparison page.
Note: 15 W and 25 W ratings are typically found on low-power, LED-era isolation transformer lines (e.g. 10–15 W and 20–25 W classes), which are separate from the longer-established 30/45/65/100/150/200/300/500 W series used for legacy tungsten-halogen AGL fixtures. Confirm which product line applies to your fixture before specifying.
15 W, 25 W and 45 W describe different AGL isolation transformer power capacities — the correct rating is always selected to match the connected fixture and manufacturer's specification, not chosen by wattage preference.
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