Designing a Photovoltaic System

Analyze Electrical Loads:

To begin with it is important to understand the meaning of the word, ‘load.’  A load in the electrical sense means a circuit that is closed (i.e. switch flipped ‘on’) and is drawing electricity from a source (i.e. battery or power grid).  In this sense, a light bulb is a load on a battery because it is drawing power from the battery.  However there are other meanings of the word ‘load’ that are used in the solar industry.  In the context of construction, a load is a weight that weighs down and/or lifts up on a structure.  In this sense, the complete solar array is a load that the structure of a roof must be able to support.  In addition, the maximum rate of wind speed recorded for the geographical location of the site must be considered as an upward load that could cause the modules to fly off the roof!

The first phase of design for a photovoltaic system involves researching the precise electrical usage of a residence or business.  Utility companies usually have data online for consumers to look up.  “For reference, a typical inefficient North American home uses 25-30kWh/day… Highly efficient homes may use 6-10kWh/day.”  (Home Power Magazine p.156, Aug-Sept 2013).  At this stage, decisions can be made to change habits that will conserve energy, and a photovoltaic system may not be necessary.  Examples include changing light bulbs and appliances, as well as turning off entire power strips when leaving the house.

The chart below is an example of a load analysis for a highly efficient home:

 

Load Qty. xWatts xHrs./Day xDays/Wk. ÷7=Avg. Daily Wh
Refrigerator 1 507 3 7 1,521
Fans 2 100 8 5 571
Computers 2 80 8 5 457
Lights 8 25 6 5 107
Clothes Washer 1 320 1 2 91
Total Power 1,032W Total Energy 2,747Wh

In addition to shining a light on the patterns and behaviors of household electrical consumption, the data also gives us a number to design for.  In this sense, we build a solar energy system backwards, starting with the electricity that is needed, and then proceeding to choose the right modules, racks and balance of system (B.O.S.) components.  PV systems are usually measured in kilowatt hours (kWh).  We can easily convert the number above to kWh by moving the decimal over 3 places, which gives us 2.74kWh/day.

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About danblake222

*MSc "Integrative Ecosocial Design" with Gaia University *BA "Ethnography & Photography" with Minor in Spanish Linguistics from UCSC *Artist in Residence at Omega Institute *Handyman, House Painter, Writer, Photographer, Permaculture Designer, Gardener *Photography portfolio at DBLfoto.com

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