Off Grid system design help

jkoprowski

New Member
Joined
Jun 26, 2021
Messages
5
Hello,

I have been reading this forum for a while now and am very thankful for all of you who are here sharing information. I believe that I am posting this in the correct place, forgive me if not. So here it is... I am actively designing an off grid system that I plan on purchasing over the next few months. I really want to do this right so I am asking for your guidance and experience in the design. I have experience with Lifepo4, inverters, chargers, bms, electrical engineering; so I wouldn't consider myself a beginner, however I also do not consider myself an expert. I completed an energy audit and determined that I will need between 450-600kwh per month. I live in south Florida and average 4.5-5 hours of sunlight per day. Wind generation isn't really an option due to the slow winds speeds here. We designed and built a very efficient Tiny house and have it currently running on a Honda 2200 generator on 22 acres. The only difficulty with off grid in FL is the requirement for AirCon, hence the need for 450-600kwh. So I would like to design a solar system to run everything most of the time.

Initial ideas:

Victron Energy Quattro 8K (either 240v or 120v depending on charging solution)

Custom Built Lifepo4 battery bank:
Ideas: Source (32) Amy supplied 280ah Lifepo4 cells for 48v 560ah (26,880wh)

Charging:
Either Fronius or Sunny 8k PV inverter (AC charging)
or
Victron smart solar charge controllers (DC charging)

*I am not decided here, due to the desire to still be able to use my Honda 2200 (120v) if needed

PV:
I live near Miami and they have a large panel supplier who routinely has 380-420 watt panels.

Please help me design this system correctly. I do not have a lot of experience with Solar panels yet, and dont want to overlook some solutions that I simply just dont know about yet. Thanks in advance.
 

MichaelK

Photon Sorcerer
Joined
Mar 21, 2020
Messages
1,484
Your 450-600kWh numbers look a bit high, though in the ballpark.

I'm running one 8000 BTU air-con that's consuming about 750W while the compressor is running. What I find is that on nights I have to run the air-conditioner, the frig is also running at full steam, so I"m actually using about 1000W per hour at night. That's with my 48V system. I have my arrays on rotating mounts, so I can produce 1000W till about 6pm.

What exactly will you want in your tiny-house? One central air-con, or smaller niche units? Using my system as a model, expect to be running the air-con for ~10hr on battery, or approximately 10,000Wh. That's a lot! Assuming you don't want to drain your battery dry each and every night, and you don't want to use more than half of your usable capacity (70%) then 35% of your battery is about 26880Wh X 0.35 = 9400Wh, which appears to be about right. In reality, the real-world consumption is lower, with the air-con cycling on and off as it maintains temperature after midnight.

Assuming you can get those 380W panels, you could build a rotating array frame like mine out of unistruts and schedule 40 pipe. Assuming you put 6 panels (2280W) on one frame, you should at a minimum produce about 5sunhours of power per day in summer, and more like 8-9 sunhours when rotating East to West. That works out to be between 11.4kWh and 18.2kWh per day, depending on how enthusiastic you are about array alignment. With two of these arrays you'll have what approximates my primary array capacity.

The Fronius and Sunny inverters you mention I believe are grid-tie inverters that are not useable in this application. What you want is an off-grid, or hybrid inverter, like the Outback Radian, or the Schneider XW+, designed to work with batteries. I have the XW+6848. These are component inverters, so you will need a separate charge controller to feed the battery. I have Midnight200. You will also like having the AC and DC power centers designed to work with them. They hold all the breakers/switches you need to build a "to code" system. Both the Radian and XW produce split-phase 120/240VAC, so they can be wired into a standard main electrical panel. They also have an ACin set of terminals, so you may wire in a generator feed for direct charging in cloudy weather.

When you wire your panels, you'll need to pay close attention to their Voc and the limit of your controller. To do the math, let's pretend that your 380W panel puts out 9.5A at 40Vmp. The Voc is 50V. If you strung all six panels in series, that would give you 240Vmp and 300Voc. That's way over the limit of almost all charge controllers. So, let's wire the array in 3S2P, so you have two 9.5A strings at 120Vmp. The Voc would be ~150V, but around freezing the Voc will peak around 170V. So, no budget controllers. You'll need a controller that can handle 200Voc. Look at the Epever Tracer 10420AN, or Midnight200 charge controllers.
With the array mentioned above the math would be (380W X 6 panels X 0.85Fudgefactor)/50V charging = 38.8A. With two arrays that's 77.5A, so you could make it work with Epever's 80A controller.
 

Attachments

  • IMG_0778.JPG
    IMG_0778.JPG
    1.8 MB · Views: 16

jkoprowski

New Member
Joined
Jun 26, 2021
Messages
5
Hi Michael,

thank you for taking time to reply, very helpful input! To answer some of your questions:

power usage: I placed a kWh meter between my generator and the tiny house and measured 17.7kWh usage in 24 hrs on the typical worst day of the year (hot, and most energy usage). However I do want to add a variable speed well pump in the near future

Air conditioning: I am running a 12,000btu mini split that uses 5.5a or 660 watts when running normally.

refrigerator: I’m running a 12/24v refrigerator that is using maybe 1200 watts for the entire day. (Really happy with it, it works great)

inverters: I’ll look into those inverters because you may be right about those two I mentioned not being able to work off grid.

Your mounting system looks great, nice job, I’m going to need some blueprints when the time comes. I hope this helps answer some of your questions. Let me know what else pops into your mind.

thanks
 

OffGridInTheCity

Solar Addict
Joined
May 23, 2021
Messages
545
Location
Southern Oregon
It sounds to me like you're in the ballpark - but I can share actual numbers from my system in Southern OR USA.

I have a 12.8kw pv array (45 x 285w panels) which generates 80kwh high and 20kwh low (winter). After inverter losses etc I get 84% of that in actual 240/120v that the house consumes - e.g. 67kwh in summer and 17kwh in winter. I mention this to try to provide a top level picture - as you can roughly just scale this up/down for your own results.

For example then - to get 17kwh in summer - you'd need roughly 25% of a 12.8kw = 3.2kw. That's would be on the order of 10 panels at 350w each kind of thing. Note: I like PV watts https://pvwatts.nrel.gov/pvwatts.php and you can use it to get a good idea of the array size -> power for your specific location by month.

Next - battery bank. I'm off-grid and have an 81kwh battery bank and it averages 40% DOD. Again, by extrapolation at 25% a 20kwh battery bank would run around 40% DOD. You could do 80% DOD on a 10kwh battery bank. So let's say you need on the order of a 10kwh battery bank. You're projection of 26kwh would let you size up to 6kw PV array... and still maintain an average 80% DOD.

Power. My goal is to run the whole house including cooktop, dryer, heat-pump, ... everything. My yearly average consumption is 2,400kw / hour and it 'could' fit in a 12,000w inverter if I didn't run everything all at once but I went with dual - so 24,000w of inverters.
Your goal is roughly 25% (again) so you could function at around a 6000kw inverter but might want to leave room to double that in the future. So maybe GroWatt 5000w - and you can 2 or 3 as you grow.

Consumption: I'm off-grid and use ATSs to consume the power. When batteries charge up the ATS auto-switches to solar power and then back to grid when batteries run down - each day. This keep me away from the power company - but it means I can't sell excess back to the grid - so if I don't consume the power it's lost. Also I have a UPS system to smooth the twice daily switching. Sounded like you're thinking off-grid... and it works great for me - but that's not for everyone. This is a *key decision* that will affect the equipment choices.

Recap of 'gross' extrapolations:
- 3-6kwh PV array. 6kw+ if you want hope to run continuous cloudy weather. **This is the big variable - PV Watts will give you some good guidance to chew on. I'm biased and don't think anyone can ever have too many panels - but my wife thinks their ugly. Sigh :)
- 26kwh battery bank - good for up to 6kw array
- 6000w inverter - leave room to expand. For off-grid, a GroWatt or MPP Solar PIP will have ATS + UPS built in - very convienient.
 
Last edited:

Wellbuilt

Solar Addict
Joined
Sep 14, 2020
Messages
777
Wow you guys are burning a lot of power .
I’m lucky if I am using 2/300 watts per hour
Winter or summer ?
It was 85o today and I had the windows open trying to warm the house up .
It stays 65/67o in side all summer
It just a little to cool for shorts and a tee shirt watching tv .
 

jkoprowski

New Member
Joined
Jun 26, 2021
Messages
5
It sounds to me like you're in the ballpark - but I can share actual numbers from my system in Southern OR USA.

I have a 12.8kw pv array (45 x 285w panels) which generates 80kwh high and 20kwh low (winter). After inverter losses etc I get 84% of that in actual 240/120v that the house consumes - e.g. 67kwh in summer and 17kwh in winter. I mention this to try to provide a top level picture - as you can roughly just scale this up/down for your own results.

For example then - to get 17kwh in summer - you'd need roughly 25% of a 12.8kw = 3.2kw. That's would be on the order of 10 panels at 350w each kind of thing. Note: I like PV watts https://pvwatts.nrel.gov/pvwatts.php and you can use it to get a good idea of the array size -> power for your specific location by month.

Next - battery bank. I'm off-grid and have an 81kwh battery bank and it averages 40% DOD. Again, by extrapolation at 25% a 20kwh battery bank would run around 40% DOD. You could do 80% DOD on a 10kwh battery bank. So let's say you need on the order of a 10kwh battery bank. You're projection of 26kwh would let you size up to 6kw PV array... and still maintain an average 80% DOD.

Power. My goal is to run the whole house including cooktop, dryer, heat-pump, ... everything. My yearly average consumption is 2,400kw / hour and it 'could' fit in a 12,000w inverter if I didn't run everything all at once but I went with dual - so 24,000w of inverters.
Your goal is roughly 25% (again) so you could function at around a 6000kw inverter but might want to leave room to double that in the future. So maybe GroWatt 5000w - and you can 2 or 3 as you grow.

Consumption: I'm off-grid and use ATSs to consume the power. When batteries charge up the ATS auto-switches to solar power and then back to grid when batteries run down - each day. This keep me away from the power company - but it means I can't sell excess back to the grid - so if I don't consume the power it's lost. Also I have a UPS system to smooth the twice daily switching. Sounded like you're thinking off-grid... and it works great for me - but that's not for everyone. This is a *key decision* that will affect the equipment choices.

Recap of 'gross' extrapolations:
- 3-6kwh PV array. 6kw+ if you want hope to run continuous cloudy weather. **This is the big variable - PV Watts will give you some good guidance to chew on. I'm biased and don't think anyone can ever have too many panels - but my wife thinks their ugly. Sigh :)
- 26kwh battery bank - good for up to 6kw array
- 6000w inverter - leave room to expand. For off-grid, a GroWatt or MPP Solar PIP will have ATS + UPS built in - very convienient.
Thank you for your input. It nice to see some real world numbers. If it wasn't for needing air conditioning my system would be tiny. Its definitely a requirement down here in FL though. I am choosing off grid because of my location. Its looking like between $20,000-$40,000 to run power all the way out to me (around 2000 ft of line). Unless you guys know any tricks with the power companies. I'll look into your equipment recommendations. Thanks again.
 

jkoprowski

New Member
Joined
Jun 26, 2021
Messages
5
Wow you guys are burning a lot of power .
I’m lucky if I am using 2/300 watts per hour
Winter or summer ?
It was 85o today and I had the windows open trying to warm the house up .
It stays 65/67o in side all summer
It just a little to cool for shorts and a tee shirt watching tv .
Yea, I wish i could get away without air conditioning. Where are you located?
 

Bri from IA

New Member
Joined
Nov 27, 2020
Messages
16
I'm a month late to the party, but sharing info just in case. I have an off-grid system and use a SolArk 12k, all in one inverter/charge controller. It has max PV voltage of 500V, so more panels in series and less wiring runs. Output is also 240/120V split phase 50Amps. Max input/output is 12kW, but if running just from battery, i.e. at night, then it's 8kW.

My system is roughly 8.5kW of panels and about 75kWH of battery. I'm running about 75-80% of my house in an off-grid mode, and do hard wiring changes to shift load between solar/utility. Solar doesn't touch the utility at all. Waiting to see how the winter goes, and if I add more panels to go completely off-grid, or if I stay in a mix with the utility. If I went completely off-grid my single biggest load, 5.5kW electric clothes drier would have to be either removed/converted, or I get a second SolArk to run in parallel.

The total cost of my system, including significant costs for a ground-mount rack, is right in the middle what the utility is going to charge you for getting power to your site. But then the utility will also send you a monthly bill, and there's still good tax credits (30%?) for solar. Looks like you'd be money-ahead to go with solar.

You might want to look at getting batteries through Michael Caro on this forum in the group buy section. I had good luck with him.
 

Steve_S

Offgrid Cabineer, N.E. Ontario, Canada
Joined
Oct 29, 2019
Messages
5,586
Location
Rural NE Ontario Canada
My region goes from -35C/-31F to +40C/104F for a heat wave. I gave away my AC System as I did not need it anymore. House design solved the issue because the building itself has a thermally broken shell. There is a 2" passive airflow space between the siding & the walls (Rain-screen installation method) and same for the Roof which is a Passive Cool Roof that vents the hot away from the actual structure. I have yet to see my house hit 26C

Also, a WHITE Roof reflects sun rays away and actually WILL significantly reduce the heat absorption of the entire building. The darker the colour the more it absorbs and passes it into the house. YET look at how many folks out there have Black or dark roofs and just don't get it. They cannot equate why houses in "The Old World" such as Greece, Spain, Morocco etc are painted White or Light Colours including their roofs.

Double Skinned roofs (like cool roofs) date back to the Roman Era ! As do Earth Tubes for heating & Cooling which actually works GREAT, says the guy with two 50' earth tube runs that are piped into one of my side structures. BUT it's tricky and needs serious planning.

41C/105F last week during a heatwave and the house never got above 26C/79F. Only time it does when I have my Woodstove going !

Too many folks reach for a mechanical solution that carries ongoing costs and rarely consider passive natural methods & systems BUT in nations where frugality and independence are valued for many reasons, such things are far more common.

Bottom Line: Conservation is always far cheaper than Generation & Storage, and if it's all passive it carries no ongoing costs (see independence).
 

Wellbuilt

Solar Addict
Joined
Sep 14, 2020
Messages
777
Yea, I wish i could get away without air conditioning. Where are you located?
O I just saw this , I’m in NY state north west of NY city 3 hours .
On the top of a Mountain in the middle of nowhere .
It gets into the 90s here but my roof is r 60 and the walls are r35
the floor is concrete with 2” blue Dow board the concrete holds the heat and the cold for days .
It takes me 3/4 days to warm the place up in the winter and it holds the cold from the floor .
The house has a garage on the south side and 150x10’ porch roof all around so the sun never hits the walls .
 
Top