When calculating the size of your system it is important to know how much power your system is anticipated to generate. The PV Watts link below helps to determine how much power you can expect to generate using the exact location and orientation of where you plan to put the solar array.
Here you enter your address, the kilowatt size of your system and anticipated power reduction factors. In any solar array the best possible setup would a south facing roof with a 34° pitch with no shading. This will allow your array to provide maximum output, weather permitting of course. In the PV Watts site the reduction factors account for the soiling, shade, roof pitch or array angle, electrical losses (leave these at default) and the Azimuth or the direction the array will face. The PV Watts site has information that help you determine your reduction factors and it is very user friendly. It is even not too bad to access from a mobile device but much easier to navigate on a laptop or desktop computer.
In the photo above it helps to explain how to calculate system size. For our system we are getting 24 Silfab premium 310 Watt solar panels. They will be mounted on the South facing (180°) roof with an approximate 30°-35° pitch, the roof is never shaded by trees. The PV Watts website allows for a number of variables that affect power generation. A premium panel has a higher efficiency than a standard panel and this refers to how efficient a panel is at converting sunlight to energy under standard operating conditions. The data sheets for solar panels will provide all pertinent information about the panel, including the standard operating range. In the midwest soiling is rarely a factor due to frequent rain so set this to 0 for Michigan. To get system size we take 24 panels times 310 watts each, totaling 7440 Watts. For our 7440 Watt array, using the PV Watts calculator we can anticipate approximately 10,500 watts production per year. This obviously depends on weather and can vary slightly higher or lower. Based on our consumption of 980 watts per month we will cover roughly 90% of our 11,772 Watts yearly energy needs. Now if we account for the reduced consumption of 939 Watts since we have winterized our hot tub we have now bumped up to just over 93% of our energy needs.
Higher wattage systems cost less per watt. As we progressed through our investigation of solar panels we received quotes from several vendors. With each vendor we provided our monthly usage of 980 watts. The first vendor offered 20 Solar World 290 watt standard panels totalling a 5800 watt system. The annual production would have been roughly 8000 watts or just about 70% of our annual usage. This system was quoted at $5.24 per watt. Another vendor quoted 31 JinkoSolar 295 watt premium panels totalling 9145 watts that would cover about 98% of our energy needs. This system was $2.82 per watt. The system we are getting is 24 Canadian made Silfab premium 310 watt panels totalling 7440 watts at $3.07 per watt. Less panels and lower wattage raised the cost per watt. I found it interesting that each vendor varied the system size starting with the same monthly usage.
It is important to remember that solar arrays can be as few as 1 panel to even larger than 24 panels. How many used depends on your electric usage, how much of your electric needs you want to cover and your budget. There are many different panel manufacturers and various panel wattage's available so finding an arrangement to best fit your needs is easy. No matter how many solar panels you decide on, your system will help you save money and the environment!
NOTE: Without using PV Watts a quick rough estimate of system production would be to take system size and divide it by 70% or 0.7. This gives you a ball park figure to work with. The ideal and most accurate way to determine your system production is to use the PV Watts website or another similar site that takes into account all the factors discussed.
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