Insolation is a measure of solar radiation energy received on a given surface area in a given time. The name comes from a portmanteau of the words incident solar radiation.
"It is commonly expressed as average irradiance in watts per square meter (W/m2)" (source - Wikipedia)
Solar panels and the power they produce via sunlight varies throughout different geographical locations around the world. Most solar design is based on Peak Sunlight hours (PSH). One peak sunlight hour is the suns ability to deliver 1000 watts per meter square (1000w/m2) for 1 hour.
During summer and winter this will vary and an average is used. For example- Sydney, Australia is located 33° 53'S latitude and 151°10'E longitude and historic data shows sydney averages 5 PSH, which is 5 hours the sun will deliver 1000w/m2. During summer you can receive 6.0 - 6.5 PSH and during winter around 4.0 - 4.5.
Solar streetscape design all their systems based on the minimum PSH ( middle of winter ) for the particular geographical location our system is being installed. For sydney we base our design on around 4.0 PSH which ensures the solar panels will always deliver over the consumed lamp amperage during mid winter which is a worst case senario.
The world insolation Maps show Minimum Winter time PSH. Local sunlight and weather data should also be considered when designing your solar lighting systems.