When I was studying for the exam, the most important element was using free NABCEP Certification study resources. What took a huge amount of time was identifying the best resources and organizing everything. The test is segmented in separate segments, so you'll need to organize your studying for each segment. Here's the best video resource I found for each segment of the test. The outline of the exam is published in the NABCEP Job Task Analysis. 

1. Verifying System Design
2. Managing the Project
3. Installing Electrical Equipment
4. Installing Mechanical Equipment
5. Completing System Installations
6. Conducing Maintenance and Troubleshooting 

This is the best resource I found on the key formulas that you need to understand. 

Code is very important, especially for section 1,3,4, and 5. 

System design is critical

This video was very useful just for thinking about managing your time during the test. 

I've installed a 300kW of solar PV and I created this site after studying for the NABCEP PV Installers exam. I created a set of free cliff notes to help me easily skim through all the content and found the best study guide I could find was HeatSpring's NABCEP PV Installer Certification. If you want to get 10% off this course, you can review the course and get the discount here. 

What helped me most to study was going through full examples, so I wrote out entire designs. Starting from the basics, to array size and estimating power product, to inverter, string, and conductor sizing. Here's a few articles I wrote on this: 

1. Learn the basic terms of solar PV
2. Learn how to size the array and estimate power production
3. Learn how to size the inverter, size the strings and conductors 
4. Download the NABCEP Study Guide Cliff Notes for Free
​5. Buy the 6 week NABCEP PV Installer Certification Prep Course and get 10% off