What is solar insolation?

What is solar insolation?
The solar insolation stands for the quanity of the electromagnetic energy /solar radiation that can reach to the surface of the earth,Simply said it means how much sunlight we can get .
Why is the knowing of insolation level helpful?
If we know the insolation levels of a particular region.We can make the determination which size of the solar collector we need. Generally speaking, a zone with rich insolation level will cost less collectors than an area with poor insolation levels.By knowing the insolation level of your region,you can calculate collector size and energy output more accurately. 
What units are used to express Insolation levels?
Different countries use different units to express insolation level. But the most common unit is kWh/m2/day, which means the amount of solar energe that a square metre of the earth's surface in a single day can absorb. Of couse the figure is a average value for the temperature difference in a days' length. 
The conversions of the different units based on surface area are:
The original energy conversions are:
1kWh = 3412 Btu = 3.6MJ = 859.8kcal
Is my region's insolation level low, moderate or high?
The following example shows how is an region's insolation level. The average annual value of 5 is considered as a very high figure,although it is not considered very high during the summer months.Because in some hot ,sunny place ,such as central Australia, the annual average insolation is only 5.89. average insolation is 5.89.
By comparing to the following following two extreme locations,you can know your location’s insolation level.
Average annual insolation levels:
Central Australia = 5.89 kWh/m2/day - Very High
Now can I know my insolation level, how to calculate my collector's size?
Please click here to visit the collector sizing page.
Click on the links below to view insolation levels for your region.
Africa Asia Pacisic Canada USA Sth America Europe
By visiting the NASA website below,you can find your area's insolation levels 
The lattitude and longitude is basic information to find an area's insolation level, but firstly you should have a free registration at the site.