Sunshine is measured in units of Solar Insolation. Technically, insolation is described kWh/m2 per day.
For instance, Northern California might have 5.5 hours of solar insolation per day. This means if you are living there, you will receive on average 5.5 hours of sunshine on every m2 of ground every day.
This is useful for working out how much energy you can get for your solar panels.
However, this is not a straight measure of daytime hours. Insolation is a measure of how many usable hours of sunshine you get a day.
Even in very sunny states, the amount of usable solar electricity sunlight is about 5-6 hours. This is because the angle of light makes a big difference to how much energy is produced.
Ever wonder why the sun is hottest at noon? This is because there is the highest concentration of sun energy per m2 at noon. The sun is directly overhead, and the light is not spread thinly over the ground.
Usually the best angle of light comes from 10am to 4pm. This is when your solar panels will produce the most energy.
The best way to work out the solar insolation values of your home is to check out a map.