How to Size a Power Supply: Continuous Load, Peak Load and Headroom

Choosing the right wattage is the single most common sizing question we get. Undersize it and the supply runs hot, drops out or fails early; oversize it wildly and you have paid for capacity you never use. Here is the method we recommend.

1. Add up the continuous load

List every load the supply has to run at the same time and total their power in watts. If a device is rated in amps, multiply by the supply voltage (W = V × A). This figure is your continuous load — the steady current the supply must deliver all day without complaint.

2. Allow for peak and inrush

Motors, pumps, lamps and large capacitor banks pull far more at switch-on than they do running — often several times their steady current. Check whether your supply needs to ride through that surge, or whether a supply with a defined peak power rating (many of ours hold a higher output for a short window) is the better fit.

3. Derate for temperature

A supply’s full rating usually applies up to around 50 °C; above that it must be derated, often losing 2.5 % of output per °C. If the supply lives in a warm enclosure, read the derating curve at your ambient, not the headline figure.

4. Leave some headroom

Sizing a supply to run flat out at 100 % is asking for a hot, short-lived part. Aim to load it to roughly 70–80 % of its rating: it runs cooler, lasts longer and leaves room for the load to grow.

A quick rule of thumb

Continuous load, plus peak allowance, divided by about 0.75 for headroom, read against the derating curve at your worst-case temperature. Still unsure? Tell us the load and we will recommend a part, or add a few candidates to a quote to compare.