I've got a 2017 MacBook Pro which comes stock with an 87W USB C Power Adapter.
I want to purchase an additional power adapter, but I don't require an official Apple one. I notice that Anker make one with 60 W power output over USB C.
Other than charging my laptop at about 2/3rds the speed of the Apple Power Adapter, (2/3rds = ca. 60/87 w), would there be any effect / damage on my machine?
The USB-C plug is not the problem, but the charger is.
It is underrated, so it will never manage to completely charge your battery while the Mac is in use, or supply enough when needed.
With that said, you can use it to charge battery overnight to use it during the day.
To verify it is best to take occasional look at the battery charge condition in About this Mac.
This indicated value will be negative if charger is not plugged in or not sufficient.
It will be positive if charger is sufficient.
No damage to your Mac will occur in any case.
Imagine a water reservoir (battery) with water gushing out of a sluice (CPU usage), and a bucket (low-rated charger). You're putting water into the reservoir with the bucket, but it may not be equal to the rate of water coming out of the sluice.
The reservoir level will fall, but at a slower rate than if you weren't filling it with the bucket.
The Apple-supplied 87W charger has enough power to supply your laptop's power needs, even at max CPU, AND to charge the battery at the same time.
A lower-rated charger cannot always supply enough power at the maximum rate that the laptop needs, and also charge the battery at the same time. It will recharge the battery fully when the lid is closed, and you're not using the laptop.
I have a 13' MacBook Pro which has a 61W Power Adapter supplied by default. For this I have bought an Anker 30W rated charger with which I never ever had any issues charging the notebook (even when doing CPU intensive stuff), and I also have a cheap 15W (5Vx3A) charger in my car as well, which still slightly charges the notebook, unless I'm doing something CPU intensive. I sometimes also use my mobile phone charger rated 10W (5Vx2A) as well to charge my notebook, in that case it always discharges, but at a much slower rate than if I hadn't plugged it in at all. It can still charge the notebook if I leave it on overnight though.
Although your notebook does have a higher power consumption by default, as it's the 15' variant, based on my experience with the smaller version I don't imagine using it with only a 60W charger will cause too many issues, even when you are really trying to push the computer to the limits. However please note that the one in your post (which is exactly the same as I have) is actually only 30W on the USB-C port - the other 30W is shared on the 4 plain USB ports. You might try to check the values in the About your Mac section as Buscar said to see your notebook's power consumption, especially if you do heavy CPU/GPU intensive tasks.
Note: I've been using this setup for more than a year now without seeing any performance degradation of the batteries.
Your Mac will be fine. It will charge slower. While it is working hard, the battery might even discharge slowly if 60 watt isn’t enough, but without the charger it would discharge a lot quicker.
The only problem is that the charger may not be built to run at 100% for long times. Anker has a good reputation, just don’t push that charger too hard.
Sure. But using an Apple adapter is always recommended over 3rd party ones. Yes, Apple's adapters are expensive but they are of high quality and are the best for your MacBook.
