MBP Battery Consumption and Charging Speed app?

I'd like to know if there is an app that shows you how fast (how many Watts) the MBP is consuming and how fast it is charging (Watts and Voltage).

I know that there is the Activity Monitor but it only shows an Energy Impact that doesn't translate to Watts.

The 15" model has a 76Wh battery, and the charger is 87W. Is it correct to say that if I charge a 0% battery MBP while it is turned off it should take a little bit less than 1 hour? Also if I charge that with my smartphone chargers that output 18W at max (QC2 and QC3) is there a way to know if it is charging using the full power (and the voltage/amperage)?

Your best bet would be to try coconutBattery. It provides a whole heap of information that you'll find useful, and from memory includes both Discharging with and Charging with measurements in watts!

As for your other questions, it's really best to just ask one question per question, even though they may be related. However, in a nutshell:

• No, you cannot assume the 87W USB-C Power Adapter will charge a 76 watt hour battery in less than an hour. There are a lot of factors that can affect charging times, and typically this means you'd be waiting longer than an hour. However, in theory and under the best case scenario (e.g. your MBP is powered off and the charger is working at the optimal 20.2V and 4.3A and has absolutely no efficiency loss) it could in theory be fully charged in about 52 minutes. If we assume the same scenario but with a typical efficiency loss of 20%, it will take a little longer than an hour (63 mins to be exact) to charge your battery. Of course, in real life many factors can affect this.
• I would not recommend using a smartphone charger to charge your MBP. Yes, in some cases this will work, in many it won't. However, while it may be okay to do this on a one-off basis because you've forgotten your MBP charger, it is not something I would recommend doing on any sort of ongoing basis. Eventually it will most likely damage your smartphone charger, although this will depend on its quality and design.

NOTE: Calculations used in my answer assume a 2016 or 2017 15" MacBook Pro.