Ask to view the Tesla app, that will show generation for whatever period you look at, gives a real world peek at it. If there is one inverter and not panels with a built in inverter, check for shade, as a one inverter setup will lose generation when some panels are shaded, whereas if each panel had its own inverter only the shaded panels will have reduced generation

Other than that, if panels look in sound condition, that's probably it 

If you do buy the house, make sure all the logins for the inverter, Tesla PW2 and monitoring platform for inverter are provided. Check the state of health of the battery on the PW2 via the app. Get warranty paperwork for entire solar setup from current owners on settlement. 

Thanks both for your input. Yes there are umpteen devices that will require credentials handed over, in addition to the solar. Sign of the times.

Is it not possible to reset that sort of device and establish new credentials?

Surely that would be preferable to using someone else's credentials?

In this case the house comes with the existing network included. For simplicity, if it was me selling, i would reset the credentials and provide a list of URL or IP addresses and temporary credentials rather than leaving it to someone to workout how to reset multiple devices. But yes I would assume all the devices could be reset.

Make sure you sight two important documents:

• COC (Code of Compliance) from the installer certifying that all parts and installation meet electrical code
• ROI (Record of Inspection) from an independent solar inspector

As others have mentioned, ensure you understand the network connectivity and credentials for ongoing access.