How to Size an LED Power Supply: Wattage, Margin & Inrush
Undersize an LED power supply and it runs hot, dims early, and dies in a year. Oversize it wildly and you pay for capacity you never use. Between those two is a 20-minute calculation that most buyers skip.
Step 1: total the real load
Add the wattage of every fixture the driver will feed. Use the fixture's input wattage, not its "equivalent" rating — a "50W equivalent" downlight may draw 9W.
- LED strip: watts per metre × metres (check the strip label: 4.8W/m, 9.6W/m, 14.4W/m are common).
- Panels and troffers: the rating plate on the fitting.
- Signage modules: modules × watts per module.
Example: 12 metres of 14.4W/m strip = 173W.
Step 2: add the 20% margin
Multiply by 1.2. That 20% is not padding — it covers three things that all shorten driver life:
- Heat. A driver at 100% load in a 40°C ceiling runs far hotter than the same unit at 80% load. Every 10°C over the rating roughly halves capacitor life.
- Mains variation. Input dips and surges are absorbed more comfortably with headroom.
- Future additions. The strip someone adds next year.
173W × 1.2 = 208W → pick a 240W or 250W model, the next standard rating above your number.
Step 3: check inrush before you size the breaker
Switching power supplies draw a large but very brief surge when energised — typically 30-60 A for a few milliseconds, sometimes more on high-power units. If you put ten drivers on one switch or contactor, that surge adds up and can trip the breaker or weld the relay on day one.
- Many small drivers? Stagger them with a time-delay relay, or use a contactor rated for the summed inrush.
- One big driver? Use a type-C or type-D breaker rather than type-B.
- Ask for the inrush figure — it is on the datasheet, and a supplier who cannot give it is guessing.
Step 4: derate for temperature
Most drivers are rated for full output at 40°C ambient or below. Above that, derate — typically 2-3% of output per °C, but check the curve for your model. Practical cases:
| Install | Typical ambient | What to do |
|---|---|---|
| Open indoor ceiling | 25-35°C | Standard 20% margin is enough |
| Sealed ceiling void | 45-55°C | Add 30-40% margin, or move the driver into open air |
| Outdoor in direct sun | 50-65°C | Shade the unit, or step up one size and use a potted IP67 model |
| Inside a sealed sign box | 45-60°C | Vent the box, or use a vented IP65 case |
Worked example: shopfront sign, 8 modules
Sign modules: 8 × 1.5W = 12W. Strip accents: 4 m × 9.6W/m = 38W. Total = 50W. Margin ×1.2 = 60W. Installed in a sealed box that reaches 50°C in summer → add another 30% → 78W. Choose a 100W unit. It will run at roughly half load, stay cool, and outlast the sign.
Five sizing mistakes
- Using "equivalent wattage" instead of real draw.
- Sizing to 100% load with no margin.
- Ignoring ambient temperature in a sealed enclosure.
- Putting ten drivers on one switch without checking inrush.
- Forgetting the future — the extra strip, the second sign.
What we need from you
Send the load in watts, the output voltage (12V or 24V), the install location and rough ambient temperature, and how many units you need. We will recommend the right rating — and tell you when a smaller, cheaper model is genuinely sufficient.
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