Except for occassional foggy weather along the central and northern coasts, California has great sunshine almost year round.

The image above shows the relative strength of solar radiation in the state.

Orientation

How do you determine where your home or building site is a good place for a PV system? It's relatively simple.

The property or roof where the PV system will be installed should have clear, unobstructed access to the sun for most of the day and be free from shade.

The best orientation for a PV system is on a south-facing roof.

If your location looks promising, a PV provider can trace the sun's path for you and determine whether your home or business would benefit from a PV system.

Typically, composition shingle roofs are the easiest to work with.

Shading

Shading is a critical issue for PV performance. The PV array needs to be located where it will have access to the sun and where it will not be in the shadow of other building elements (e.g., facades and parapets, mechanical/ plumbing elements (including vent pipes and flues), and landscaping (when at full growth).

Shading constraints from neighboring properties are common as well, especially given the urban setting for many commercial and multifamily buildings. Thus when considering possible shade problems, look both at your site and at buildings that shade your site.

Full or partial shading of the panels inhibits the production of electricity... "because the solar cell with the lowest illumination level determines the operating current for all of the cells wired in that series." (Energy Design Resources web site) Solar professionals describe this effect of partial shading as similar to kinking a garden hose, where the reduced size of the opening allows only a small amount of water through. Because of this effect, a relatively small amount of shading can create a disproportionate reduction in electricity production. Note: Thin film PV products are less affected by shading and overcast days than are crystalline PV modules.

It may be helpful to perform a shading analysis of your site and building outline to calculate (or to optimize) space with good solar exposure. PV professionals can also help you in this area.

How Much Shade-Free Space Do You Need?

Remember the rough sizing requirements for PV: You'll need about 100 sq. ft. for each kilowatt (kW) of system capacity for crystalline technologies and 175 sq. ft. for each kilowatt of thin film PV products.

For example, a 10 kW system with crystalline modules would require 10 kW x 100 sq. ft. per kW = 1000 sq. ft of unshaded area.

A 30 kW system using thin film product (e.g., building-integrated roofing material) would require 30 kW x 175 sq. ft per kW = 5250 sq. ft. of rooftop or other space.

Consider these estimates to be minimums for the shading analysis.