These solar insolation maps are provided by the Dept. of Energy, NREL (US National Renewable Energy Lab). They can help you determine the appoximate sun-hours per day in your location. Here's an example:
Let's say you live in Southern California, near the coast. You have an application where you require power year-round. So, you look for the worst months of the year for sunshine, December and January here in the states. Look at the maps for those months and you will see that S. California has a minimum of 4.5-5 hrs per day. Note that the designation in the maps, kWh/m2/day is equivalant to sun hours per day. You would use 4.5 hours for a year-round application near the coast of southern California.
If you were using an RV, just in the spring through fall months,you would look up the lowest hours of sun per day in those months in your location and use that for the sun-hours per day. All the links below open the monthly map in a pdf file in a new browser window.
All prices are in USD.