## Sunday, 3 January 2016

### How to Determine the quantity of Solar Power Available at Your House?

Solar electricity produced will be a function of how many photons from the sun hit your pv cell, and how many of those photons really get an electron moving to generate a current. The quantity of sunshine(number of photons) that strikes your photovoltaic panel is the deciding element in sizing your photovoltaic system and determining the quantity of solar panels required. The quantity of solar energy is going to be a function of your latitude, tree and cloud cover, plus the time of year, and will differ significantly from area to area.

## How do we Measure Solar Power?

Solar power is a combination of the hours of sunlight and the strength of the sunshine you may expect at your site. Insolation defines the relationship and can be measured in kilowatt-hours per square meter per day which is called irradiance. A solar irradiance of one thousand watts per square meter is the standard for the power received from the sun at noon in the middle of summer. We can define solar irradiance as the quantity of kilowatts per square meter per day. If the sun were to remain directly overhead for 8 hours, the irradiance would be 8..

Solar irradiance will differ considerably throughout the year particularly in northern latitudes. For instance, Chicago will have an irradiance of just 1.6 in January, yet it will be 6.1 in June, and the yearly average will be 4.0. This means that solar energy in December is 70% less than in June. Compare this to Phoenix where the irradiance is 7.8 in June and 3.0 in December or 5.5 for the year. If you are not tied to the power grid, this means your capacity has to be more than 2.3 to 3 times the size that would be determined by June numbers. Values for your location are easily available on the web because NASA’s weather satellites have been obtaining this data around the world for many years.

## How do we Use Irradiance to Plot the Size of Your System.

If we learn how many kilowatts we require to satisfy the energy needs of our home, we are now in a position to size our system. If we are plotting a grid-tie system, you can use the average yearly irradiance for your computation because the goal is to make your payment to your utility company zero for a full year. If we are plotting a off-grid system, we want to pick the irradiance valuation for December since we want to produce adequate electric power all year.

If you divide your needs for the day by the irradiance, you will obtain the amount of watts your system must generate. For instance, if your home in Phoenix calls for 600kw per month, or 20kw per day, your system would need to generate about 3600 watts per hour of total sunlight. You can then divide the system wattage by the output of your solar panels to determine the number of panels required. For this house in Phoenix the system will need 24 solar panels rated at 150 watts.