How to Calculate the Correct Power Supply Size for LED Signs
Undersizing a power supply is one of the most common — and most expensive — mistakes in LED sign installation. Get it wrong and you are looking at premature PSU failure, dimming under load, or a sign that cuts out at the worst possible moment. This guide walks through the complete sizing method: counting your LED load, applying the 20% headroom rule, selecting voltage, and choosing the right IP rating for your installation environment.
Why power supply sizing matters
A constant voltage LED power supply converts mains AC to low-voltage DC. It is rated for a maximum continuous output — say, 60W at 12V DC. If your LED load draws more than that, the supply either current-limits (causing dimming), shuts down under thermal protection, or fails outright.
Even if the load is exactly at the rated maximum, the supply runs at 100% duty cycle. This increases internal temperatures, accelerates capacitor degradation, and shortens service life significantly. In a sign that is illuminated 12–16 hours per day, the difference between a well-sized and a marginal supply can be years of service life.
The 80% rule: Never run a power supply above 80% of its rated output during normal operation. Always size up so the continuous load sits at or below 80%.
Step 1 — Calculate total LED wattage
Start with the wattage of each individual LED unit — module, strip segment, or fitting. Multiply by the number of units in the circuit. This gives you the total connected load.
Formula
Total load (W) = number of LED units × wattage per unit
Finding wattage per unit
Always use the datasheet figure, not a rough estimate. For the two professional modules Glowfy stocks:
| Module | Wattage per module | Voltage | Lumens per module | IP Rating |
|---|---|---|---|---|
| GL K8 PRO | 1.2 W | 12 V DC | 120 lm | IP68 |
| GL K2 PRO | 0.72 W | 12 V DC | 70 lm | IP68 |
Both modules use Philips 2835 LED chips and carry CE & RoHS certification. Available for next-day delivery from UK stock in Birmingham.
Step 2 — Apply the 20% headroom rule
Once you have the total connected wattage, add 20% to find the minimum power supply rating. This keeps the supply running at or below 80% of its rated output under full load.
Formula
Minimum PSU rating (W) = Total load (W) × 1.2
Round up to the next standard power supply size (e.g. 30W, 60W, 75W, 100W, 150W, 200W, 300W).
Multiple signs from one supply? Add the total load for all signs connected to that supply, then apply the 20% rule to the combined figure. Each circuit should be individually fused at the supply.
Step 3 — Choose 12V or 24V
The voltage of your power supply must match the voltage of your LED modules exactly. Mismatch causes either under-driving (dim output, poor colour rendering) or permanent damage from over-voltage.
| 12V systems | 24V systems | |
|---|---|---|
| Best for | Channel letter modules, short LED strip runs (up to 5m) | Long LED strip runs, larger cabinets, reduced voltage drop |
| Voltage drop | More pronounced over long runs | Half the current at the same wattage — better for long runs |
| Typical PSU sizes | 12W – 300W | 30W – 600W |
| Glowfy module support | GL K8 PRO · GL K2 PRO (both 12V) | Not applicable for current Glowfy modules |
| Cable gauge | Heavier gauge needed for long runs | Lighter gauge acceptable at same wattage |
For Glowfy modules: All current Glowfy LED modules (GL K8 PRO and GL K2 PRO) operate at 12V DC. Use a 12V constant voltage power supply.
Step 4 — Select the correct IP rating for your power supply
The IP rating of your power supply must match its installation environment. A supply rated IP20 will fail in an outdoor cabinet within months. Even a small gap in a sign tray can allow condensation to reach an inadequately rated supply.
| Installation environment | Minimum IP rating | Suitable for |
|---|---|---|
| Fully enclosed indoor cabinet | IP20 | Interior signage with no moisture risk |
| Indoor cabinet with ventilation | IP44 | Humid environments, food retail, kitchens |
| Under canopy / recessed outdoor | IP65 | Protected outdoor locations |
| Exposed outdoor sign cabinet | IP65–IP67 | Fully exposed to rain and weather |
| Submerged or wash-down locations | IP67–IP68 | Specialist applications only |
When in doubt, size up. An IP67 supply costs marginally more than IP65 but eliminates the risk of moisture ingress in marginal installations. See our full guide: IP Rating Explained.
Worked examples
Channel letter sign — 60 × GL K8 PRO modules
Step 1: Total load = 60 × 1.2W = 72W
Step 2: Minimum PSU = 72W × 1.2 = 86.4W
Step 3: Voltage = 12V DC
Step 4: IP rating = outdoor cabinet → IP65 minimum
Select a 100W, 12V DC, IP65 power supply.
Mixed installation — 40 × GL K8 PRO + 30 × GL K2 PRO
GL K8 PRO load: 40 × 1.2W = 48W
GL K2 PRO load: 30 × 0.72W = 21.6W
Step 1: Total load = 48W + 21.6W = 69.6W
Step 2: Minimum PSU = 69.6W × 1.2 = 83.5W
Step 3: Voltage = 12V DC
Step 4: Under-canopy installation → IP65
Select a 100W, 12V DC, IP65 power supply.
Large tray sign — 200 × GL K2 PRO modules
Step 1: Total load = 200 × 0.72W = 144W
Step 2: Minimum PSU = 144W × 1.2 = 172.8W
Step 3: Voltage = 12V DC
Step 4: Exposed outdoor = IP65–IP67
Select a 200W, 12V DC, IP67 power supply — or two 100W supplies on separate circuits.
Pro tip for large installations: For loads above 150W, consider splitting the load across two smaller supplies on separate circuits rather than using a single large supply. This provides redundancy — if one supply fails, half the sign remains illuminated rather than going completely dark.
Quick reference: module count → PSU size
Use this table to select the minimum power supply rating for standard module counts. All figures include 20% headroom. Round up to the next standard size where indicated.
| Module count | GL K8 PRO (1.2W) | Min PSU (K8) | GL K2 PRO (0.72W) | Min PSU (K2) |
|---|---|---|---|---|
| 20 | 24W | 30W | 14.4W | 20W |
| 30 | 36W | 45W | 21.6W | 30W |
| 50 | 60W | 75W | 36W | 45W |
| 75 | 90W | 120W | 54W | 75W |
| 100 | 120W | 150W | 72W | 100W |
| 150 | 180W | 240W | 108W | 150W |
| 200 | 240W | 300W | 144W | 200W |
| 250 | 300W | 360W | 180W | 240W |
Min PSU figures already include the 20% headroom factor. Select the next standard size up if the calculated figure falls between standard ratings.
Common mistakes when sizing LED power supplies
1. No headroom — running at 100% capacity
The most frequent cause of PSU failure in the field. A 60W load on a 60W supply means maximum internal temperature, accelerated capacitor wear, and thermal shutdown on warm days. Always apply the 20% rule.
2. Using voltage from memory rather than the datasheet
Specifying a 24V supply for 12V modules — or vice versa — is a destructive mistake. Double-check the datasheet for every module type before ordering the supply.
3. Ignoring voltage drop in long cable runs
Even a correctly sized power supply can cause dimming if the cable run between supply and modules is too long or the conductor cross-section is too small. See our guide on voltage drop and power injection for full calculations.
4. Using an indoor-rated supply in an outdoor or humid location
An IP20 supply in an outdoor sign cabinet will fail within a single winter in the UK. Condensation and moisture ingress destroy driver boards rapidly. Always match IP rating to installation environment.
5. Connecting multiple incompatible voltages to one supply
Mixing 12V modules and 24V strip on the same supply output, even through a converter, creates unpredictable load behaviour. Use separate, correctly rated supplies for different voltage systems.
6. Forgetting future expansion
If a client may add letters or panels to a sign in the future, budget for that capacity now. Replacing an undersized supply six months after installation — including access, rewiring and callout costs — is far more expensive than selecting a slightly larger supply at build time.
Troubleshooting
Problem: Sign cuts out after a period of use and comes back on when it cools
Likely cause: Thermal protection triggering on an overloaded or inadequately ventilated supply.
Fix: Measure total load with a clamp meter. If above 80% of rated output, upgrade the supply. Ensure the supply housing has adequate ventilation clearance.
Problem: LEDs are noticeably dimmer at the far end of a run
Likely cause: Voltage drop across a long cable run, not necessarily a supply sizing issue.
Fix: Check voltage at the far end of the run. If below 11.5V on a 12V system, refer to our guide on voltage drop and power injection. A second injection point is usually the fix.
Problem: Power supply is warm or hot to the touch under normal use
Likely cause: Normal below 60°C surface temperature; above that suggests the supply is running near its limit.
Fix: Measure load. If near rated capacity, either reduce the load or upgrade to a higher-rated supply. Check ventilation clearance around the unit.
Problem: LEDs flicker intermittently
Likely cause: Loose connection, insufficient conductor cross-section, or a failing supply capacitor.
Fix: Check all terminal connections first. Measure output voltage under load — a healthy supply should hold 12V ±0.5V. If voltage is fluctuating, the supply is failing and should be replaced.
Problem: Supply has failed after water ingress
Likely cause: IP rating insufficient for installation environment.
Fix: Replace with a supply of the appropriate IP rating for the location. Seal all entry points for cables. Do not re-use a supply that has been exposed to moisture — internal corrosion compromises long-term safety.
Frequently asked questions
What size power supply do I need for LED signs?
Add up the wattage of all your LED modules, then multiply by 1.2. For example, 60W of LEDs requires at least a 72W supply — round up to the next standard size (75W or 100W). Always use the module datasheet for wattage, not a rough estimate.
Why do I need 20% headroom on an LED power supply?
Running a supply at 100% of rated output causes it to run hot, shortens its lifespan, and risks thermal shutdown during warm weather or after extended use. The 20% headroom keeps it in the safe operating zone, extends service life, and ensures consistent output voltage under all conditions.
Can I use one power supply for multiple LED signs?
Yes, provided the combined wattage does not exceed 80% of the supply's rated output and all signs operate at the same voltage. Fuse each sign circuit individually at the supply for protection.
What voltage power supply do I need for Glowfy LED modules?
Both the GL K8 PRO and GL K2 PRO run on 12V DC. Specify a 12V constant voltage power supply. Using the wrong voltage will either under-drive the modules or permanently damage them.
Do I need a waterproof power supply for outdoor LED signs?
Yes. Any supply installed in an outdoor cabinet should carry a minimum IP65 rating. For fully exposed locations, choose IP67. Even a protected cabinet can experience condensation ingress — do not use IP20 supplies in any external location.
How many LED modules can I run from a single 100W power supply?
At 80% load capacity, a 100W supply can handle 80W of LEDs. Using GL K8 PRO modules at 1.2W each, that is 66 modules. Using GL K2 PRO modules at 0.72W each, that is 111 modules.
Not sure which supply to specify?
Our technical team can review your module count, installation environment and cable runs and recommend the correct supply specification. Available Monday–Friday, 8:30am–6:30pm.
Trade pricing
Buying LED supplies at trade scale?
A Glowfy trade account gives you volume pricing on GL K8 PRO and GL K2 PRO modules from your first order. Free to join, no minimum order, and a dedicated account manager who understands sign making.
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