Explanation for wrong charging wattage / slow charging?

I have a 2013 Retina MBP and a fairly new 85 W Magsafe 2 charger. But when I connect this charger to my Macbook and check in Battery Health, it claims that it is receiving only 49 W.

Does anyone know why the laptop should be drawing such a low supply from the Magsafe 2?

85 W = maximum power used by your charger on the power supply side on AC.

49 W = actual power received by your MacBook internal power managment on DC.

Where is evaporating the missing 36 W?

• Within AC -> DC transformation.
• Joule effect within all conductors. This explains why your charger is heating (and incidentaly heating the atmosphere of our hosting Earth). This consumption is high because it is depending on the square of `I` (`RI²`) and on low voltage `I` is high.
• A part is diverted by the PMC to power different parts of the computer based on its energy saving state.

The shown charging wattage is correct. The whole wattage of the MagSafe is divided into charging the battery and running your Mac. So probably your Mac needs ~35 W and the rest of ~50 W is used to charge the battery.

You may estimate the needed wattage of your Mac by temporarily unplugging the MagSafe.

The power usage (and the realtime Usage in mAh) provided by the battery will change to certain amounts. After replugging the charger this will change back to the charging wattage.

On my MacBook Pro I get the following:

On Battery: ~70 W (- 6510 mAh)
Charging:      ~6 W (         -        )

Estimated total wattage: ~76 W (not taking into account that the power consumption is slightly lowered in battery mode)

After quitting a power consuming app running in the first attempt I get

On Battery: ~40 W (- 3980 mAh)
Charging:    ~32 W (         -        )

Estimated total wattage: ~72 W

• Beware: 70 W ⇒ 5830 mA (without the h, which denotes a total capicity which regularly increases over time).---- 40 W ⇒ 3330 mA. – dan Dec 22 '15 at 15:22
• @danielAzuelos I didn't wanna say that 40 W = 3980 mAh. It's just what's that app is showing as "realtime usage" – klanomath Dec 22 '15 at 15:31
• Understood. This mAh number will vary over time: its the actual or max. charge level. – dan Dec 22 '15 at 15:34
• @danielAzuelos Still one of the answers is wrong... and the app is at best mediocre ;-) – klanomath Dec 22 '15 at 15:39