No, Lenovo a7700 does supports an 18-watt fast charger but still, you can charge this device with an 18-watt charger. The device will be charged with 5-watt input because this device is designed with 5 watts charging support only.
How to check devices is charging fast or not?
For checking any device/ smartphone charging fast or not you can be checked by the following steps:
Here we are comparing two chargers 5 watts normal/box charger and 20-watt fast charger.
So, let start

Step 1:
Install ampere apps from Google play store or you can just link here to go to apps link.
Step 2: plug your normal /box Charger in your smartphone Phone and open an ampere app and let the app measure charging speed (in mAh). Then take a screenshot or note down the charging speed.
Step 3:
Now unplug normal/ box Charger from your smartphone and plug the fast charger and again measure charging speed from ampere app then take screenshots or note down charging speeds.
Step 4: Now, finally you can compare does your device is charging fast or not.
How to buy an 18-watt fast charger?
You can purchase 18 watt fast chargers from many online and offline stores but will we suggest you go with Xiaomi 18 watt fast charger.
You can purchase this charger from the Xiaomi official store or offline store.