Hello All,
To all the technical people out there who would like to join the solar force or who are struggling to learn the basics of designing a residential solar PV system, here is a quick description of how to design a system.
Please follow these steps:
1. Determine the annual kWh usage for your house.
2. Now lets assume that 1 kW of solar PV produces 110 kWh per month or 1320 kWh/Yr (applicable to the East coast)
3. Divide your annual kWh usage (step 1) by 1320 to figure out the kW system size you would need to produce the desired kWh in an year.
4. Measure your roof using google maps.
5. Layout of the panels would be in portrait mode. Lets assume we are using SolarWorld SW 280 Watt panels. Panel dimension: 39.41″ x 65.94″
6. Lets say your roof has a dimension A” x B”. “A” being the length and “B” being the depth
# Rows along the depth of the roof= B / 65.94
# Panels in a row- along the length of the roof= A / 39.41
7. Now from step 3 you know the #panels you needed to offset 100% of your usage and from step 6 you know the actual #panels you can fit on your roof. If #panels you can fit is more than the #panels you need to offset 100% usage then you would only install the number of panels you need to offset 100% usage.
8. At this point we have figured out the number of panels to be installed. Now we need to figure out the stringing in to the inverter.
The following link takes you to different sizes of residential string inverter, Fronius- http://www.fronius.com/cps/rde/xchg/SID-5ECEF354-FAFFD472/fronius_usa/hs.xsl/2714_11381.htm
To start with the stringing, we need to check the operating voltage range of the inverter which has the maximum and minimum DC input voltages that an inverter can handle.
Maximum # panels in a string = Max operating voltage of inverter / Open circuit voltage of solar panel (Voc)
Minimum # panels in a string = Min operating voltage of inverter / Voc
# Strings = Max DC input current / Solar Panel’s short circuit current (Isc)
Always check the maximum number of allowable string into an inverter. For example, Fronius Primo 5.0-1 208-240 has 4 DC connection terminals, which means we can input 4 strings into that inverter.
9. We need to now determine the rating of over current protection devices. We need an over current protection device between each DC input string and the inverter and then one over current protection between the inverter and the point of interconnection (which could be a breaker in the main electrical panel in case of load side connection and could be a supply side tap in case of supply side connection)
10. For overcurrent protection devices:
DC side (before inverter)- The current of one string in parallel = Short Circuit current of one panel (Isc) x 1.56.
DC side (before inverter)- The current of one string in parallel = Short Circuit current of one panel (Isc) x 1.56.
AC Side- Maximum output current of the inverter x 1.25
11. Point of interconnection: According to the 120% rule, you cannot install a solar feedback breaker greater than 20% of the main electrical panel’s rating. So figure out the Amperes of the service coming to your house (Should be on the main electrical panel) and check if the inverter’s maximum output x 1.25 is greater than or lower than 20% of Main electrical panel Amperage.If it is less than we can install a breaker, if more then we need to do a supply side connection (Tapping into the utility wire between your meter and the main electrical panel).
Hope this helps. Comment on this blog with your questions.
FYI- every inverter manufacturer have their own string sizing tool where in if you plug in the # panels, and panel wattage, it would tell you how many strings and how many panels in a string you should design.
