We tend to think of Hawaii as sunny and warm all the time, but it's actually just an "above average" place for solar generation. Honolulu gets about 90 sunny days and 181 partially-cloudy days.[1] Phoenix on the other hand gets 211 sunny days and 85 partially-sunny days.[2]

It's not just about sunrise and sunset, but UV index and cloud cover. Here's some actual data on actual solar generation:

http://www.nrel.gov/gis/solar.html

This shows that even on the sunnier Oahu and Maui, solar generation is more like Nebraska than California.

The fact is that sunny days drop about 30% in the winter and UV index is down.[3] I wasn't able to find real-world MWH in Hawaii per month, but I'm guessing it's about a 50% drop overall with some days actually at ~0. Certainly not 0 for the month, but the fact remains that you need backup power for low-solar periods with more capacity if you want to leverage excess power in high-solar periods.

1 http://www.currentresults.com/Weather/Hawaii/annual-days-of-... 2 http://www.currentresults.com/Weather/Arizona/annual-days-of... 3 http://www.weather-and-climate.com/average-monthly-Rainfall-...