It is not a guesswork for sure. Actually it is very simple. By changing possible ranking factors on your website and analyzing how it influences SERP changes you can come to conclusion which of that factors are more or less important. So by time you get more and more info how that Google works.
Next step is to get more quality info and that info includes more extensive research on a big number of sites and sharing experience with other people, using tools which automatically do some analysis for you and so on...
As an electrician you know facts and deal according to them. The problem with google is that factors are changing. People are always trying to find a way to rank better with no efforts and google is fighting against that.
Once upon a time there were meta keywords and google used them to find out what about is that page. People realized that and spammed it to death. So google removed it as a ranking factor. Then people started stealing content from other sites to make huge website without efforts. Then google said "we will penalize that"... And so on...
Many people know how google works but noone knows it 100%. Just when you think you know almost everything it is already changed. So, one part of that is thinking in advance also. So when you get "opposite" answers to your question one of the reason may be that you get "old data" or just wrong answers.
So if you would ask me what the most important factors are I would say:
- Quality content (unique, complete, media rich)
- Users experience and engagement (on website and social media)
- Authority gained by backlinks (perfectly if gained by points 1 and 2)
So if you are electrician and your website has just phone number and address on it for sure you will be ranked low. If you add more info about yourself you will rank better. Add some images of your work and someone may share it. Add some how to, or diy articles and you will get some audience. And all of that may produce some natural backlinks. Be active on social media...
After all it comes to some mathematical things so you can use moz crawl test and page grader to see what is wrong or what you miss, fix it and that's it - simple 
