How to calculate the correct transformer for your LED strip light installation — covering power supplies from 30W to 300W
1. About 9.6W/m LED Strip Light
LED strip lights are flexible circuit boards populated with surface-mounted LEDs, available in a range of power densities. The power density — expressed in watts per metre (W/m) — determines how bright the strip is and how much power it consumes per metre of length.
The 9.6W per metre LED strip is a popular mid-range specification widely used in under-cabinet lighting, cove lighting, shopfitting, signage backlit displays, and architectural feature lighting. It offers a strong balance between brightness and power efficiency, and is available in both 12V DC and 24V DC versions.
Because the wattage per metre is fixed and known, calculating the power supply requirement for any installation is straightforward — you simply need to know the total metres of strip being installed.
2. The Basic Calculation Formula
The core formula for sizing a power supply for LED strip is:
Total Wattage = Total Metres × 9.6W
Minimum Power Supply Size = Total Wattage ÷ 0.8 (80% load rule)
The 80% load rule is a non-negotiable best practice in LED installation. A transformer continuously running at full rated capacity generates excess heat, degrades faster, and is significantly more prone to failure. Running at 80% or less keeps the unit cool, extends its service life, and provides a buffer for any minor load variations.
⚠ IMPORTANT: Always size your power supply so that your LED load does not exceed 80% of the transformer’s rated output. A 100W transformer should carry no more than 80W of strip. Overloading a transformer will shorten its life and may cause it to fail prematurely.
3. Step-by-Step Worked Examples
Example 1: Under-Cabinet Installation — 2.5 Metres
Step 1: Calculate total wattage → 2.5m × 9.6W = 24W total
Step 2: Apply 80% rule → 24W ÷ 0.8 = 30W minimum transformer
Result: A 30W transformer is the correct minimum selection for 2.5 metres of 9.6W/m strip.
Example 2: Cove Lighting — 6 Metres
Step 1: Calculate total wattage → 6m × 9.6W = 57.6W total
Step 2: Apply 80% rule → 57.6W ÷ 0.8 = 72W minimum transformer
Step 3: Select next standard size above 72W → 75W or 100W
Result: Select a 75W transformer (if available) or a 100W transformer for comfortable headroom.
Example 3: Shopfront Display — 15 Metres
Step 1: Calculate total wattage → 15m × 9.6W = 144W total
Step 2: Apply 80% rule → 144W ÷ 0.8 = 180W minimum transformer
Step 3: Select next standard size above 180W → 200W or 2 × 100W
Result: Use a 200W transformer, or split across two 100W transformers feeding separate 7.5m runs each.
Example 4: Large Architectural Installation — 25 Metres
Step 1: Calculate total wattage → 25m × 9.6W = 240W total
Step 2: Apply 80% rule → 240W ÷ 0.8 = 300W minimum transformer
Step 3: Select transformer → 300W single unit or 2 × 150W
Result: A single 300W transformer covers this load exactly at 80%. Alternatively, use two 150W units feeding two 12.5m runs for better voltage drop management.
✔ TIP: When splitting a large installation across multiple transformers, aim to keep each run under 10 metres for 12V strip and under 15 metres for 24V strip to minimise visible voltage drop.
4. Quick Reference: Metres Per Transformer
The table below shows the maximum metres of 9.6W/m LED strip that can be safely powered by each standard transformer size, with the 20% safety headroom already applied.
| Transformer | Max Load (80%) | Max Metres | 12V Current | 24V Current | Typical Use |
| 30W | 24W | 2.5m | 2.0A | 1.0A | Under-cabinet, display |
| 40W | 32W | 3.3m | 2.7A | 1.3A | Small cove, display |
| 60W | 48W | 5.0m | 4.0A | 2.0A | Medium cove, shelving |
| 75W | 60W | 6.25m | 5.0A | 2.5A | Medium installation |
| 100W | 80W | 8.3m | 6.7A | 3.3A | Cove, shopfitting |
| 120W | 96W | 10.0m | 8.0A | 4.0A | Feature lighting |
| 150W | 120W | 12.5m | 10.0A | 5.0A | Larger cove, facade |
| 200W | 160W | 16.7m | 13.3A | 6.7A | Commercial display |
| 250W | 200W | 20.8m | 16.7A | 8.3A | Large architectural |
| 300W | 240W | 25.0m | 20.0A | 10.0A | Large facade, signage |
Note: Current values shown are for the DC output side of the transformer. 12V installations carry double the current of equivalent 24V installations for the same wattage. Always size output cabling accordingly — refer to The Lighting Zone Wire Gauge Selection Guide.
5. 12V vs 24V Strip: Choosing the Right Voltage
9.6W/m LED strip is available in both 12V and 24V versions. The light output and power consumption are identical — the difference lies in how the strip manages voltage internally, and the practical implications for your installation.
| Factor | 12V Strip | 24V Strip |
| Max run length | Up to 5m per feed point | Up to 10m per feed point |
| Current draw (9.6W/m) | 0.8A per metre | 0.4A per metre |
| Voltage drop | More pronounced over distance | Less pronounced over distance |
| Cable sizing | Heavier gauge required | Lighter gauge acceptable |
| Best for | Short, compact installations | Long runs, large areas |
| Cut interval | Every 3 LEDs (~50mm) | Every 6 LEDs (~100mm) |
✔ TIP: For any installation where individual strip runs exceed 5 metres, always specify 24V strip and a 24V power supply. The halved current significantly reduces voltage drop and allows lighter, more cost-effective cabling throughout.
6. Voltage Drop and Run Length
Voltage drop is the gradual reduction in voltage along the length of the strip as current flows through the copper conductors. On a 9.6W/m strip, the effect becomes visible as a dimming towards the far end of a long run — the LEDs furthest from the power supply receive slightly less voltage and produce less light.
For 12V strip, voltage drop becomes noticeable beyond 5 metres from the feed point. For 24V strip, this threshold extends to approximately 10 metres. Beyond these distances, brightness inconsistency will be visible to the naked eye in typical interior lighting conditions.
To manage voltage drop on longer installations:
• Feed from both ends of the strip run to halve the effective run length from each feed point.
• Use intermediate injection points — connect additional feed wires from the transformer to the midpoint of long runs.
• Split into multiple shorter runs fed from separate transformer outputs or separate transformers.
• Use heavier gauge cable between the transformer and the strip to reduce resistive losses in the feed wiring.
7. When to Use Multiple Transformers
For installations exceeding 10 metres of 9.6W/m strip, using multiple transformers is often the better solution compared to a single large unit. Benefits include:
• Improved voltage consistency — each transformer serves a shorter run, keeping voltage drop within acceptable limits.
• Fault resilience — a transformer failure affects only one section, not the entire installation.
• Simpler cabling — transformers can be positioned closer to their respective strip runs, shortening output cable lengths.
• Better thermal management — multiple smaller transformers distributed across an installation run cooler than one large unit in an enclosed space.
As a practical guideline: for total installations above 15 metres (144W), consider using two or more transformers. For installations above 25 metres (240W), multiple transformers are strongly recommended.
8. Summary: The 3-Step Method
Follow these three steps for every 9.6W/m LED strip installation:
Step 1 — Measure total strip length in metres and multiply by 9.6W to get total wattage.
Step 2 — Divide total wattage by 0.8 to get the minimum transformer size required.
Step 3 — Select the next available standard transformer size at or above that minimum. For runs over 10 metres, consider splitting across multiple transformers.
9. Need Assistance?
For help sizing power supplies, planning your LED strip installation, or selecting the right products, please contact The Lighting Zone:
Email: admin@thelightingzone.co.za
Tel: +27 10 590 6101