General Headroom: 30% Is the Golden Ratio
In typical industrial environments, it is recommended to reserve a 30% power margin. That is, if the stable load power is 100W, select a switching power supply rated at 130W or above. This avoids excessive cost while handling grid fluctuations and transient overloads.
Increased Headroom for Special Scenarios
High-Temperature Environments (>40°C): For every 10°C increase in temperature, it is recommended to add an additional 10% headroom, bringing the total margin to 40-50%.
Inductive/Capacitive Loads (motors, capacitor banks): Startup currents are very high; a margin of 50%-100% is recommended.
Long-Term 24/7 Uninterrupted Operation: Reserving over 40% can significantly extend the lifespan of electrolytic capacitors.
The Bottom Line Principle
Under no circumstances should the rated power of the power supply equal the maximum power of the load. Leaving a margin is not only a safety requirement but also reduces ripple and enhances overall system reliability.