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Guide · 2026-11-11

How to find a device’s watts from its label

Watts are volts times amps. That part is simple. The hard part is picking the right line on the label, because most power adapters print two: an input line and an output line. Neither one is the wattage the device holds for an hour. Both RunTime and the UPS calculator need a load in watts, so it is worth knowing which number to type.

The output line is a ceiling

The output line says what the adapter can deliver. Apple’s USB-C 20 W adapter guide prints USB-PD output of 5 V at 3 A or 9 V at 2.22 A for several models. 5 × 3 is 15 W. 9 × 2.22 is 19.98 W, which Apple sells as 20 W. That is the adapter’s ceiling, not the wattage your phone holds for an hour. This is the figure RunTime’s load list uses for that adapter, and it is labeled as a rating, not a reading.

The input line is not the draw

The same Apple guide prints an input of 100–240 V at 0.5 A. Multiplying 240 by 0.5 gives 120, six times the adapter’s 20 W output. A 20 W adapter is not using 120 W, so that line is not a load to plan with. Starlink’s Standard 4 specification sheet does the same thing: its power supply is listed at 100–240 V and 2.5 A, while the dish’s published average power consumption is 75–100 W. Do not plan a battery from the input line.

A published consumption figure beats both

Some makers print what the device actually uses. ResMed’s AirSense 11 Americas user guide lists the 65 W power supply at an AC input of 100–240 V and 2.0 A, and a DC output of 24 V at 2.71 A. 24 × 2.71 is 65.04 W, the supply’s rating. The same table lists typical power consumption of 56.1 W and peak power consumption of 73.2 W. The typical figure is the one the CPAP guide and RunTime use. When a maker publishes a consumption figure like this, use it.

Watts and VA

ResMed prints its consumption both ways: 56.1 W (111.5 VA) typical and 73.2 W (137.6 VA) peak. On AC, volts times amps gives volt-amperes, and VA can be higher than watts. Battery runtime is a watt-hour question, so RunTime uses watts. A UPS also has a VA rating to respect, so the UPS calculator asks for both. How to size a UPS for a gaming PC explains that check.

Which number to type

  1. A plug-in meter reading over a real session, if you have one.
  2. The maker’s published consumption figure, such as typical or average power.
  3. The output rating, volts times amps, as a ceiling. The runtime it gives is the short end.

Never the input line. If the label prints amp-hours or milliamp-hours for a battery rather than watts for a load, that is a capacity, not a load. How to convert amp-hours to watt-hours covers it.

Sources: Apple USB-C Power Adapter user guide, ResMed AirSense 11 user guide, Americas, technical specifications, and Starlink Standard 4 specification sheet, read 2026-11-11.