What Power Supply Efficiency Means and Where the Heat Comes From
Efficiency (η) is the fraction of input power that becomes useful output power. The remainder is dissipated as heat inside the supply. The relationship is straightforward:
Heat dissipation (W) = Input power − Output power = Output power × (1/η − 1)
Example: a 100 W supply with 91% efficiency dissipates 100 × (1/0.91 − 1) ≈ 9.9 W of heat. A 100 W supply with 76% efficiency dissipates 31.6 W — three times more for the same useful output. That difference accumulates inside the cabinet, raises ambient temperature for every other component, and accelerates ageing.
This is why automation engineers specify high-efficiency supplies: less heat in the cabinet → lower temperature → longer service life.
Efficiency of Common Series in Our Catalogue
The table shows typical efficiency for the most widely used DIN-rail series, measured at rated load and 230 V AC input.
| Model (example) | Series / Brand | Output Power | Efficiency (typical) | Heat at 100% Load |
|---|---|---|---|---|
| NDR-480-48 | NDR / Mean Well | 480 W | 92.5% | ≈ 39 W |
| HDR-60-48 | HDR / Mean Well | 60 W | 91% | ≈ 5.9 W |
| LRS-100-48 | LRS / Mean Well | 100 W | 91% | ≈ 9.9 W |
| RSP-500-48 | RSP / Mean Well | 500 W | 90.5% | ≈ 52 W |
| NDR-240-48 | NDR / Mean Well | 240 W | 90% | ≈ 27 W |
For comparison: an older design at 76% efficiency running the same 240 W output would dissipate ≈ 76 W — almost three times more heat.
Efficiency and Lifespan: Why Temperature Degrades Capacitors
The dominant ageing mechanism in a switching power supply is degradation of the electrolytic capacitors. Their service life follows the Arrhenius rule: every 10°C increase in operating temperature roughly halves the capacitor's lifespan.
If a capacitor is rated for 10,000 hours at +85°C:
- At +75°C it lasts ~20,000 h
- At +65°C — ~40,000 h
- At +55°C — ~80,000 h
Choosing a higher-efficiency supply is therefore not just an energy-saving decision — it directly extends the MTBF of every component in the cabinet.
Practical guideline: if the panel already contains other heat sources (variable-frequency drives, servo amplifiers, relays) — specify DIN-rail supplies with the highest available efficiency, even if they cost slightly more.
What Derating Means
Derating is the reduction of permissible output load at elevated ambient temperature. The manufacturer guarantees rated output only up to a defined temperature — typically +50°C. Above that, the load must be reduced.
A typical derating curve for most Mean Well and Delta series looks like this:
- Up to +50°C — 100% rated output.
- From +50°C to +60°C — gradual reduction to 50–70% (series-dependent).
- Above +60–70°C — operation prohibited or severely restricted.
The specific derating curve is always included in the product datasheet — look for a section labelled "Derating" or "Output Load vs. Temperature."
Why a Hot Cabinet Demands a Larger Power Margin
The standard sizing rule is to select a supply with 20–30% headroom above the calculated load. In a hot cabinet, this margin may not be sufficient.
Sizing procedure for elevated temperatures:
- Determine actual load in watts.
- Add 20–30% for inrush and ageing margin.
- Establish the maximum cabinet temperature on the hottest day or at full equipment load.
- If temperature exceeds +50°C, find the derating curve for your candidate supply and check whether it can still deliver the required load at that temperature.
- If not — select a higher-rated model or improve cabinet airflow.
Example: cabinet at +60°C, load requirement 200 W. The NDR-240-24 (240 W rated) can safely supply only ≈ 168 W at +60°C per its derating curve. That is insufficient — the NDR-480-24 or an added ventilation opening is the correct choice.
Ventilation and Clearances: Low-Cost Ways to Reduce Cabinet Temperature
Beyond selecting a more efficient supply, straightforward installation measures lower ambient temperature and extend service life:
- Clearances between supplies. Maintain at least 25–40 mm above and below each supply (check the datasheet) for free convective airflow. Supplies mounted flush against each other restrict cooling and heat each other.
- Correct orientation. Most DIN-rail supplies are designed for horizontal mounting with ventilation slots at top and bottom. Rotating the supply can significantly reduce cooling effectiveness.
- Cabinet ventilation openings or a fan. Even a simple thermostat-controlled fan can reduce internal cabinet temperature by 10–15°C — which, by the Arrhenius rule, doubles capacitor life.
- Mount power supplies at the bottom of the cabinet. Hot air rises — with the supply at the bottom, its heat dissipates upward naturally without warming the control components above it.
How to Select a Supply Accounting for Efficiency and Derating
A step-by-step process for getting the selection right:
- Calculate the load — total current of all consumers (or use our article on how to calculate power supply wattage).
- Add 20–30% margin for inrush and component ageing.
- Estimate cabinet temperature — if above 45°C, check the derating curve.
- Compare efficiency between candidates — at higher wattages a 1–2% efficiency difference translates to tens of watts of heat per year.
- Confirm OTP is present — in a hot environment it becomes especially important.
Have an unusual environment or a non-standard load profile? Send us your load list — we will recommend the right power supply within one business day.
