Design safer systems with a right‑sized supply everywhere. Compare rails, input current, and thermal loss. Get a practical recommendation and export it instantly now.
| Scenario | Rails / Total | Efficiency | Margin + Headroom | Recommended PSU |
|---|---|---|---|---|
| 12 V PLC + sensors | 12 V @ 2.0 A (24 W) | 85% | 25% + 10% | 45 W |
| 5 V SBC + peripherals | 5 V @ 3.0 A (15 W) | 90% | 20% + 10% | 25 W |
| Mixed rails control box | 12 V @ 2 A + 5 V @ 1 A (29 W) | 88% | 25% + 15% | 60 W |
These examples assume steady-state loads. Add extra margin for motors, heaters, and large capacitive inrush.
Start by listing every load and its steady current. For rail-based designs, compute each rail wattage with P = V × I, then sum rails to obtain total load power. Example: 12 V at 2.0 A is 24 W, 5 V at 1.0 A is 5 W, giving 29 W total. This calculator supports three rails and keeps the arithmetic consistent.
Power supplies run cooler and last longer when not operated at the limit. A practical planning approach is to add margin for future expansion plus headroom for startup surges. If your 29 W load uses 25% margin and 15% headroom, the required rating becomes 29 × (1 + 0.25 + 0.15) = 40.6 W. The tool then rounds up to a common standard size.
Efficiency links output power to what the source must deliver. At 85% efficiency, a 29 W load draws about 29/0.85 = 34.1 W from the input. The difference, 5.1 W, becomes heat inside the supply and enclosure. Use this result to size ventilation, heatsinking, and cabinet temperature rise assumptions.
For AC-fed supplies, power factor affects current. With 230 V AC, 34.1 W input, and PF 0.70, estimated current is 34.1/(230 × 0.70) = 0.21 A. With PF 0.95, current drops to 0.16 A. Lower current can reduce breaker stress and wiring loss, especially in multi-supply panels.
Datasheets often specify output derating above a temperature threshold or at altitude. If you apply 20% derating, a previously required 40.6 W rating becomes 40.6/(1 − 0.20) = 50.8 W, pushing selection to 60 W. Derating is also useful when supplies operate in sealed boxes or near hot components.
The calculator estimates input current and suggests a conservative fuse rating using a 125% factor. It also provides a simple wiring gauge hint for short copper runs. Exported CSV and PDF outputs help you document assumptions, share values with reviewers, and keep consistent power budgets across revisions. Record chosen standard size to simplify procurement and spares.
Many control systems use 20–30% margin plus 10–20% headroom. Higher headroom is common for motors, heaters, radios, and large capacitive loads that create inrush or startup peaks.
Efficiency determines how much extra power the supply draws to deliver the required output. Lower efficiency increases input watts and therefore increases input current, wiring losses, and heat inside the enclosure.
Apply derating when ambient temperature is high, airflow is restricted, altitude is significant, or the supply operates near hot components. Use the manufacturer’s derating curve whenever it is available.
It uses a conservative 125% multiplier on estimated input current and rounds up. Real designs must also consider inrush current, time-delay fuse selection, branch circuit rules, and regional electrical standards.
Enable each rail you need and enter its voltage and steady current. The calculator sums rail watts automatically. If you already have a single total watt figure, disable rails and enter total load power.
Only if you include them through headroom or margin. For short surges, increase headroom. For long peaks, increase the load estimate. Always verify with the supply’s transient and overload specifications.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.