how do I find the wattage of a power adaptor on a Mac or iphone?

androcles1953 wrote:

"To fast charge, you need an iPhone 15 model with a USB-C cable like the one that came in the box or an earlier iPhone model with a USB-C to Lightning cable and one of these adapters• :

Apple 18W,* 20W, 29W, 30W, 35W, 61W, 67W, 87W, or 96W, or 140W USB-C Power Adapter"

…

I have loads of USB adaptors around the house, and some charge the Macbook but more slowly. I tried using the System Information app to show what wattage was being used, but it always says 8w.

iPhones generally do not need as much charging power as MacBooks do. Something that's sufficient to fast-charge an iPhone might not even charge a MacBook at regular speed, let alone fast-charge it.

For instance, on some of the 16" MacBook Pros, the only way to fast-charge is using the 140W adapter and a MagSafe 3 connection, because the version of USB-C Power Delivery that those MBPs support doesn't allow negotiating power delivery of >100 watts over USB-C, and those MBPs need more than 100W to fast-charge.

I'd suggest looking up the Technical Specifications for your particular Mac notebook and seeing what sorts of power adapters Apple shipped with it.

androcles1953 wrote:

now that we have USB C on Mac laptops and iPhones etc, how important is it to use a proper Apple adaptor on the Macbook rather than a generic USB C adaptor? Will it charge more slowly?

As long as it is a quality adapter that supports USB-C Power Delivery, and has enough wattage, I don't believe that the brand (Apple vs. third-party) would make a difference.

The 140W Apple adapter might be a special case.

I don't know how it negotiates power delivery of >100 watts when used with a MagSafe cable, but the negotiation might involve something non-standard – something that Apple was doing before versions of USB Power Delivery which support >100W came along.

It is always best to use an original charger, those are optimized for charging apple products. Always make sure to use no more then the wattage that your device supports

See this link for further reference

Fast charge your iPhone - Apple Support