What Wattage Converter Do I Need?

Start with the actual AC input label and manual. USB output wattage is not the same as the AC load drawn from the wall.

For AC, volts multiplied by amps gives apparent power in VA. Real watts also depend on power factor. Check both W and VA limits where the converter specifies them.

Illustrative arithmetic: a known 240W input load times 1.25 is 300W; a 250W load gives 312.5W before rounding. These are calculations, not tested converter recommendations.

The separate example 120V times 2A gives 240VA, not automatically 240W. With the same arithmetic margin it is 300VA; do not substitute that result for missing power-factor or startup information.

Manufacturers can impose larger starting-load allowances, exclude electronic controls or motors, limit continuous use, or prohibit converters entirely. The Conair Model 303 manual is one explicit no-converter example.

At a mismatched high-power heating load of 1500W or more, this site stops and recommends a suitable local-voltage appliance. The 1500W threshold is a PlugVoltage policy, not a universal electrical standard.

Sources and limits

What the evidence covers

A source supports only the facts, socket types or models listed below. A model example does not verify every device in its category. Confirm your own label and outlet.

1. OpenStax: Power in an AC Circuit, section 15.4 · checked: 2026-09-04

   For sinusoidal AC, average real power is RMS voltage times RMS current times the power factor. Multiplying the RMS voltage and current alone does not necessarily give real watts. Power factor discussion and RMS average-power equation before Example 15.3. An electrical principle, not a measured power factor, startup demand or converter approval for any appliance. Use manufacturer ratings for the actual load.

2. SKROSS: World to USB series manual, version 1.2 · checked: 2026-09-04

   AC current limits and USB output are separate. In the AC20PD examples a single USB-C port reaches about 20W; simultaneous USB-A + USB-C use shares 15W. PDF page 1, English technical ratings and safety instructions. These figures apply to the named series. The manual prohibits stacking adapters and multiplug sockets; other models require their own manual.

3. Conair: 1875 W Model 303 hair dryer instructions · checked: 2026-08-21

   Conair Model 303 specifies 110–125V AC, 60Hz and prohibits use with a voltage converter. PDF page 1, important safety and operating instructions. This label excludes Japan’s 100V. It does not describe every hair dryer.

Worked examples

Converter sizing examples

The watts × 1.25 figure is a PlugVoltage estimate, not a verified converter requirement. Confirm startup demand, permitted loads and duty cycle with both manufacturers. The site does not recommend conversion for mismatched high-power heating appliances.

120V x 2A label AC: 120V × 2A = 240VA. W = VA × PF. The watts × 1.25 figure is a PlugVoltage estimate, not a verified converter requirement. Confirm startup demand, permitted loads and duty cycle with both manufacturers. The site does not recommend conversion for mismatched high-power heating appliances.

1500W hair dryer 1500W: PlugVoltage STOP. The watts × 1.25 figure is a PlugVoltage estimate, not a verified converter requirement. Confirm startup demand, permitted loads and duty cycle with both manufacturers. The site does not recommend conversion for mismatched high-power heating appliances.

65W USB-C output 65W is usually DC output to a laptop. Do not use it to size a mains voltage converter.

Step by step

1. Read the actual input voltage, frequency, current and power ratings.
2. Check permitted loads, startup demand and duty cycle in both manufacturer manuals.
3. Use the sizing calculation only as a preliminary estimate; do not use it to override a manufacturer restriction or the high-power stop.

Related guides

Guidance only — not professional electrical advice. Always confirm against your device's label before plugging in. Local wiring (especially in hotels and older buildings) can vary.