I showed that exact one to our client and he very nicely asked what the difference was between a 3.0 in Edmonton and a 5.0 in Phoenix when it came to generation of energy. I couldn't really give a good answer except to tell him that the capture area needed to be a lot bigger.
I want to apologize upfront if this has been answered already, I did not read through all the posts.
The chart indicates average isolation hours you can expect per day all year from a solar array providing the array is properly oriented through the seasons and there is no obstructions from trees, buildings, mountains, power poles, (etc). It also does not take in consideration for the systems inefficiencies like PWM charge controllers
In other words, lets say you have a 1000 watt array and you were going to put it into an area that that was marked with a 4, you could expect 4000 watt-hours per day from your array if you adjusted it per season (4 isolation hours x 1000 watts = 4000 watts.
These types of charts are based on annual averages typically 10 years. It makes the complicated calculations of power output a little easier. They take in considerations for cloud cover, early morning and late evening limited outputs and such. These charts are a good reference to get you pointed in the right direction. To get a real good answer, you need to do a whole lot of home work to find out what you would get specifically at each solar site. Most home systems typically end up with only a few panels and such because they can not afford a full blown system to meet their needs. The chart can be used to approximately show your customer about how much energy they will get out of the solar array over time.
