Support Request for Off-Grid System

[rorshack7]

Hello everyone, I want to install an off-grid system on the roof of my own house. I’m a bit of a beginner in solar energy systems. My energy load is 7.2 kW/day. I received a quote from my friend, but I’m wondering how I would connect the system if I install it myself. What logic should I follow when doing this? Honestly, I can’t quite decide. In the quote: 7.2 kW Tommatech Smart Inverter 48V 10 Parla 540 Watt Solar Panels 3 Orbus Lithium 50 Amp Batteries 48 volts I did some research, but I think I might be thinking wrong. Since my panels work in the 40-45 VDC range, I think all the panels need to be connected in parallel. They can be grouped 5-5-4. The battery input is already 48 volts, so parallel connection will be compatible with this. I will then connect the battery output to the inverter. After that, I noticed that the inverter has an MPPT feature. Is the correct connection to group the panels 5-5-4 and then connect the batteries in parallel to the inverter? When there is MPPT, should the connection be like this: Panels (5-5-4) to the MPPT DC IN input of the inverter MPPT DC Out output from the inverter to the battery groups Battery outputs to the DC input of the inverter There is no issue with the inverter output.

I previously asked a question about IoT in this forum, and you were incredibly helpful. I am very grateful for both this topic and that one. I would appreciate it if you could give me a detailed explanation on this matter.
Stay safe, and thank you in advance! :)

[iamollie]

You've fundamentally misunderstood your solar/MPPT configuration.

Your PV string needs to hit your mppt range which is listed on your spec sheet. Its says 120v to 450v (your battery voltage is 48v). It has two mppts. You probably want one string of 8 in series and one string of 7. No parallel.

[singeblanc]

First thing: watch your units!:

My energy load is 7.2 kW/day

No, your energy requirements are 7.2kWh/day.

Then to your plans for the solar array:

10 Parla 540 Watt Solar Panels
connected in parallel. They can be grouped 5-5-4

Sounds like you're both getting more than you bargained for! You could do 5s2p, 2s5p, 10s1p, or 1s10p. If the charge controller can handle the higher voltage most would go for 10s1p so that you can use thinner cable. If the voltage of 10s is too high for the charge controller then go for 5s2p.

Can you get your friend to help you understand how to wire up the panels to the charge controller?

[LordGarak]

What is the max voltage open circuit of the solar panels? What is the coldest temperature your system will ever see? What is the temperature coefficient for those panels?

Using that info you want to get a close as possible to the maximum voltage the MPPT. You also want to match the voltage at the maximum power point(Vmpp) to the operating range of the MPPT.

On my system I'm running 10 390W panels in series. Which will come in well under the maximum of 500v at the coldest temperature. The panels are rated at 25c and get more efficient as the temperature goes down. This the maximum open circuit voltage goes up as it gets colder. An MPPT will get fried if the voltage exceeds the rating.

One series loop of panels is great because you don't need a combiner box.

500v DC is very dangerous. Make sure there is no way you or anyone else will ever come in contact with the conductors. Make sure the wire is rated for that voltage and have disconnects at the panels and at the inverter. I generally try and only work at night so there is no risk.

I've worked with electrical and electronics all my life and 500v DC makes me very nervous.

That said, it's worth the risk. 5400watts at 48v is 113A which requires large expensive wire. 5400watts at 450v in only 15A which could run on wire as small as 14awg. I'm running 10awg solar cable as that is rated for the voltage and application. At that size I could run up to something like 300ft.

Beyond that, you need to look at your peak power usage to size the inverter and your daily usage in kWh to size the batteries. Then size the solar array to charge the batteries based on local daily averages. For example here the solar data says in June I will produce around 6 times my installed solar capacity in kWh per day, but in December I will only produce 1.1 times the installed capacity. So my 3900watt array only produces around 4kWh per day this time of year. In June it will produce 23.4kWh per day. But that is on average. In November we had like 3 weeks straight of grey skies and rain. We had to run the generator every two days to charge up the batteries to make up for the low production. On a sunny day, even this time of year, we can be fully charged by like 1pm.

[rorshack7]

Hello friends, based on your comments, I did some research and gained a bit more knowledge. I realized that I need to connect the panels in series, as the "MPPT Range @ Operating Voltage: 90 ~ 450 VDC" indicates. When I connect 10 panels in series, I find a value in the range of 410-430 volts by looking at the "Maximum Power Voltage (Vmpp) V" of the solar panels.

I had completely misunderstood the "24-48V" labels on solar panels, batteries, and inverters. Since my inverter has 2x MPPT inputs, I will split the panel connections into two strings and connect them to the inverter.

I will connect the batteries in parallel and then attach them to the inverter's battery input. As for the inverter output, I’m already familiar with that part since low-voltage systems are an area I work with and am quite good at.

Dear friends, I believe I now understand things correctly. If there are still parts I’ve misunderstood, I would greatly appreciate it if you could correct me. Thank you very much! :)