Dear forum members,
this forum and Youtube channel is a gold mine. Thank you for sharing so much information about solar and off-grid systems.
I have been reading a lot of threads and posts which helped me a lot designing my off-grid system. I am finalizing the sizing of my solar array and would love to get your feedback and pointing the possible errors I have made.
My system in a nutshell:
- charge controller: Growatt SPF3000-24V with a Max open circuit voltage of 145V (30-115VDC operating), max solar power of 2kW and max output power of 3kW: https://www.growatt-america.com/show-42-589.html
- battery: 280Ah 24V-8S LiFePo4 from alibaba - 6.7 kWh 0-100% (full capacity used in emergency) or 5.5 kWh usable 10-90%
- BMS: Overkill Solar 24V 100A
- Used solar panels I am planning using: SunTech 285W (VOC 44.8V, max power Current 7.95A, Voltage temperature coefficient -0.34%/degC): https://store.santansolar.com/product/suntech-285w
Everything related to battery storage, protection etc. is taken care of and I am feeling safe about it but I can elaborate if you have questions while I will not provide much more details in this post.
My main goal is to power my network equipment and my house ventilation (just a ventilation, not heat/cold) 24/7 while using the surplus to charge my EV. Network + ventilation. I have made energy consumption measurement will a KillAWatt and came up with a need of 4-5 kWh per day (depending on the season). The surplus in storage capacity in summer can be put in the car while it should provide some reserve for cloudy/dark days. I will of course also use this system as power backup but power outages are so rare in my area that I dont expect needing this function very much. I have a 4.5 kW generator I could use to power the Charge controller topping off any solar power I could get and run the fridge and other critical equipments. My heat pump needs less than 2kW which could also run on it if needed.
With all these boundary conditions, here's what I am planning to do and where I need your critical view on my design.
I was initially planning on using 9 of those panels in a 3s3p configuration, however I realized I would blow the 145 Voc limit under freezing temperatures, hence went down to 2s4p or 2s5p. Let's do the maths with a 2s4p and keep a 2s5p as power reserve option for winter time.
A 2s4p would output 1930W considering a panel temperature of 60 degC (not sure if high enough) with an operating voltage of 72V at this temperature. The max Voc would be 104V at -20degC/-20degF and would not exceed 145V at -40degC either (coldest temp in Portland ever recorded was -19degC). In winter I will obviously max out my MPPT charger on a sunny day at 2kW.
To size my cable going from the array to my Growatt, I took the highest current which occurs at max panel/outdoor temp as I have the lowest voltage but still pulling almost 2kW in ideal conditions and equals to 32A. My distance to my charge controller will be 15ft max one trip. I found different calculators online and got required AWG ranging from 2 to 10 in copper. Taking AWG6 as starting point, I expect something around 60V at max panel temperature and max power, running 32A in 6AWG gets me to 0.37V Vdrop on a 15ft one-way cable or 0.6%. That would make me lose only 12W in my cable which I find very acceptable. Do you agree with this calc? Taking 8AWG would get me to 0.58Vdrop or 18W and 10AWG 0.93Vdrop and 29W. What would you do? I know that cost is a factor but what would be a good compromise? I dont think I will upgrade to a split phase system on the same solar array, If I do so, I will probably add a second 2kW solar array and double my storage as well, hence 32A@60V is the max I will push in these cables.
Regarding solar array isolation, I was planning on adding a 50A circuit breaker. Shall I add a fuse?
Regarding the 2s4p or 2s5p: I feel the 2s4p is rightly sized for summer but will probably be weak during darker/cloudy days. To make things easier to read, I prepared a table showing main metrics for both configurations:
I feel the 2s5p is a bit overkill but cold days are typically darker as well in Portland at the same time and I cannot decide hom much power I will get from a 2s4p while theoretically pulling 2701W of max power. Any idea on how to find the best compromise?
this forum and Youtube channel is a gold mine. Thank you for sharing so much information about solar and off-grid systems.
I have been reading a lot of threads and posts which helped me a lot designing my off-grid system. I am finalizing the sizing of my solar array and would love to get your feedback and pointing the possible errors I have made.
My system in a nutshell:
- charge controller: Growatt SPF3000-24V with a Max open circuit voltage of 145V (30-115VDC operating), max solar power of 2kW and max output power of 3kW: https://www.growatt-america.com/show-42-589.html
- battery: 280Ah 24V-8S LiFePo4 from alibaba - 6.7 kWh 0-100% (full capacity used in emergency) or 5.5 kWh usable 10-90%
- BMS: Overkill Solar 24V 100A
- Used solar panels I am planning using: SunTech 285W (VOC 44.8V, max power Current 7.95A, Voltage temperature coefficient -0.34%/degC): https://store.santansolar.com/product/suntech-285w
Everything related to battery storage, protection etc. is taken care of and I am feeling safe about it but I can elaborate if you have questions while I will not provide much more details in this post.
My main goal is to power my network equipment and my house ventilation (just a ventilation, not heat/cold) 24/7 while using the surplus to charge my EV. Network + ventilation. I have made energy consumption measurement will a KillAWatt and came up with a need of 4-5 kWh per day (depending on the season). The surplus in storage capacity in summer can be put in the car while it should provide some reserve for cloudy/dark days. I will of course also use this system as power backup but power outages are so rare in my area that I dont expect needing this function very much. I have a 4.5 kW generator I could use to power the Charge controller topping off any solar power I could get and run the fridge and other critical equipments. My heat pump needs less than 2kW which could also run on it if needed.
With all these boundary conditions, here's what I am planning to do and where I need your critical view on my design.
I was initially planning on using 9 of those panels in a 3s3p configuration, however I realized I would blow the 145 Voc limit under freezing temperatures, hence went down to 2s4p or 2s5p. Let's do the maths with a 2s4p and keep a 2s5p as power reserve option for winter time.
A 2s4p would output 1930W considering a panel temperature of 60 degC (not sure if high enough) with an operating voltage of 72V at this temperature. The max Voc would be 104V at -20degC/-20degF and would not exceed 145V at -40degC either (coldest temp in Portland ever recorded was -19degC). In winter I will obviously max out my MPPT charger on a sunny day at 2kW.
To size my cable going from the array to my Growatt, I took the highest current which occurs at max panel/outdoor temp as I have the lowest voltage but still pulling almost 2kW in ideal conditions and equals to 32A. My distance to my charge controller will be 15ft max one trip. I found different calculators online and got required AWG ranging from 2 to 10 in copper. Taking AWG6 as starting point, I expect something around 60V at max panel temperature and max power, running 32A in 6AWG gets me to 0.37V Vdrop on a 15ft one-way cable or 0.6%. That would make me lose only 12W in my cable which I find very acceptable. Do you agree with this calc? Taking 8AWG would get me to 0.58Vdrop or 18W and 10AWG 0.93Vdrop and 29W. What would you do? I know that cost is a factor but what would be a good compromise? I dont think I will upgrade to a split phase system on the same solar array, If I do so, I will probably add a second 2kW solar array and double my storage as well, hence 32A@60V is the max I will push in these cables.
Regarding solar array isolation, I was planning on adding a 50A circuit breaker. Shall I add a fuse?
Regarding the 2s4p or 2s5p: I feel the 2s4p is rightly sized for summer but will probably be weak during darker/cloudy days. To make things easier to read, I prepared a table showing main metrics for both configurations:
| 2s4p | 2s5p | unit | |
| Array max power at min temp | 2701 | 3376 | W |
| Corresponding array size factor based on 2kW MPPT | 1.35 | 1.69 | - |
| Array max power at max temp | 1953 | 2441 | W |
| Corresponding array size factor based on 2kW MPPT | 0.98 | 1.22 | - |
I feel the 2s5p is a bit overkill but cold days are typically darker as well in Portland at the same time and I cannot decide hom much power I will get from a 2s4p while theoretically pulling 2701W of max power. Any idea on how to find the best compromise?
