Energy legs

We recently investigated adding solar power to one of our projects; a small retail building in California.  I learned quite a bit and I thought I would share some of the  information I learned.

Parapets

Many jurisdictions require screening for rooftop mechanical units.  A common way to screen mechanical units is to add parapets around the entire perimiter of the building.  Tall parapets will limit the area of roof that will be fully exposed during the day and therefore limit the amount of viable roof area used for solar power generation.   The parapets will shade the roof equal to 2 times the parapet hieght.  For instance, if your parapet is 3 feet, it will shade 6 feet of roof.  See the figure below for more information.

Usable Area and Energy Production

Net effective roof areas

A rule of thumb for the usable square footage (after you take into account parapets and other obstructions), would be to take 60% of the available area.  This reduction accounts for spacing of panels, mechanical ducts, walking areas, chords and other obstructions.

For the solar panel manufacturer we were talking with, thier product would produce 1kW of energy per 100sf.  This is the magic number that the solar industry wants to raise; if one manufacturer can produce more energy per sqaure foot for the same price, they have an advantage over their competitors. 

kW and kWh

Full solar production is during 10am and 4 pm (geographic assumptions include northern hemisphere, west coast, etc.).  On average, there will be 3 hours of full exposure in the winter and 7.5 hours in the summer in which the solar panels will be effective.  Averaged out over one year, that is 5.2 hours per day of available production. 

For instance, if your net effective roof area was 1000 sf, and you happened to building in the same region as our building, you would be able to produce 1000sf*1kW/100sf*5.2 hours/day = 52 kWh of energy. 

This brings up another topic, what is power and what is energy.  This website summed it up best (for me anyways):

Power may be measured at any point in time, whereas energy has to be measured during a certain period.

Disclaimer

This information is very specifc to the one job we worked on, in the one area and the production numbers was one specific product.  The technology is changing so fast, that these costs, available production, etc. might be completely out of date in a couple of months.  However, the tips presented above will stay the same.  Watch out for screening, your entire roof area will not be effective in generating power, solar panels will only produce power for a limited amount of time each day, and it will vary throughout the year. 

For more information, here is a good treehugger.com article on how solar works.