Advice on my first build, off-grid cabin

[donsolar]

Firstly, greetings to all members. Been browsing this place for years, always appreciated the no-nonsense and helpful atmosphere.

I've recently bought a piece of land with a small building (30m2) on it and am setting up a solar array to primarily run a fridge/cooler (Dometic CFX 35 or 50W). Other uses include one or two LED light bulbs and occasional phone/laptop charging. I also have a large power oak battery and 100W panel in/on my van for charging phone/laptop/tools etc.
This is in a very sunny part of Portugal (7kWh/m2/day in July, 2 in December)
I would like to stay on 12V DC as it's my first build and this cabin is intended as an electronics-free zone for people who believe they react negatively to electricity in their surroundings. Many "EMF sensitivity experts" recommend lower voltage as less disturbing, or that an inverter creates "dirty power".

At a very safe estimate, the freezer should use 106kWh a year (I've used 1 Amp per hour but a realistic average is 0.7). Adding 95kWh for 2 light bulbs, 18kWh for a laptop (used very rarely: (60Wx2hours)x150days) and 2kWh for 2 iPhones, it would seem that 300kWh should cover me. I've calculated this to be 820Wh per day (300/365=0.82).

I have been scouring the net for a rough list of items and would like to run stage 1 of my planning by this forum.

2x 390W Trina Vertex 390 solar panels (380€) from local classified ads
1x Victron MPPT 150 / 60 (420€) from local classified ads
2x 200Ah batteries or one 300Ah - Lead acid or LiFePo4? Any recommendations? Not looking to spend over 1500€.

I'm aware I need a fuse box, connectors, cables.

I've entered these values and my location into the JRC off-grid performance simulator (PVGIS) and received the following values:

Percentage days with full battery [%]:	90.71
Percentage days with empty battery [%]:	0.24
Average energy not captured [Wh]:	2214.67
Average energy missing [Wh]:	133.74

Does this seem acceptable to you?
Anything I should re-think before continuing my planning?
Thanks in advance.

 

[Bobert]

Firstly, greetings to all members. Been browsing this place for years, always appreciated the no-nonsense and helpful atmosphere.

I've recently bought a piece of land with a small building (30m2) on it and am setting up a solar array to primarily run a fridge/cooler (Dometic CFX 35 or 50W). Other uses include one or two LED light bulbs and occasional phone/laptop charging. I also have a large power oak battery and 100W panel in/on my van for charging phone/laptop/tools etc.
This is in a very sunny part of Portugal (7kWh/m2/day in July, 2 in December)
I would like to stay on 12V DC as it's my first build and this cabin is intended as an electronics-free zone for people who believe they react negatively to electricity in their surroundings. Many "EMF sensitivity experts" recommend lower voltage as less disturbing, or that an inverter creates "dirty power".

At a very safe estimate, the freezer should use 106kWh a year (I've used 1 Amp per hour but a realistic average is 0.7). Adding 95kWh for 2 light bulbs, 18kWh for a laptop (used very rarely: (60Wx2hours)x150days) and 2kWh for 2 iPhones, it would seem that 300kWh should cover me. I've calculated this to be 820Wh per day (300/365=0.82).

I have been scouring the net for a rough list of items and would like to run stage 1 of my planning by this forum.

2x 200W Renogy solar panels (380€) https://amzn.to/3zSXXGe
1x Victron MPPT 100 / 20 (157€) https://amzn.to/3d5bwJR
2x Victron AGM 6v 240Ah https://bit.ly/3vEQVlW

I'm aware I need a fuse box, connectors, cables.

I've entered these values and my location into the JRC off-grid performance simulator (PVGIS) and received the following values:

Percentage days with full battery [%]:	75.79
Percentage days with empty battery [%]:	2.42
Average energy not captured [Wh]:	971.23
Average energy missing [Wh]:	289.37

Does this seem acceptable to you?
Do I even need AGM, or would LiFePo be a better bet? I can keep them from freezing, but would like to store them in the building.
Anything I should re-think before continuing my planning?
Thanks in advance.

Lifepo4 is better if available. They have much better efficiency and last longer. The also have almost double the usable amp hours for the same rating. It’s better to avoid the other lithium chemistries as they aren’t as safe as lithium Iron Phosphate.

 

[donsolar]

Lifepo4 is better if available. They have much better efficiency and last longer. The also have almost double the usable amp hours for the same rating. It’s better to avoid the other lithium chemistries as they aren’t as safe as lithium Iron Phosphate.

Awesome, thanks for the info. I think I'll have to build my own to get a 12V >400Ah LiFePo4 under 800€, or can you point me towards a (European) source for such a thing?
Otherwise I've seen the cells from Shenzhen and I'm willing to give it a go....

 

[MichaelK]

Sticking with 12V is really a mistake. I started that way, learned my lesson, and now try to guide others away from 12V. It is really only appropriate if your application has wheels. Dirty power however is real concern, so focus your attention on a good "sine-wave" inverter.

Looking at your power needs, I'd suggest that only 400W of solar will be inadequate. Looking at my own workshop system, where I keep my freezer, it is self-sufficient on about 2.0kWh per day. On your best days, 400W of solar might make that, but not always. Especially if you are going to have lead-acid batteries, their worst enemy is chronic undercharging, so I would look at doubling your solar input.

Renogy is very overpriced, and you can do better. I have Renogy panels in my array portfolio, and I can tell you from experience that they are the poorest performers of all the brands I have.

Try to shop locally rather than ordering via the internet for your panels. Here in the US we have "Craigslist.com" for sellers marketing their stuff. What's your Portuguese venue? Just a few months ago I bought some REC260W grid-tie panels for 65$US. Three would be 195$ which is around just half as many Euros as those Renogy panels for almost double the watts. Cash and carry is the most economical way to go. Bring a voltmeter with you if you have any questions about the quality of panels you are buying.

 

[mvonw]

Sticking with 12V is really a mistake. I started that way, learned my lesson, and now try to guide others away from 12V. It is really only appropriate if your application has wheels. Dirty power however is real concern, so focus your attention on a good "sine-wave" inverter.

Looking at your power needs, I'd suggest that only 400W of solar will be inadequate. Looking at my own workshop system, where I keep my freezer, it is self-sufficient on about 2.0kWh per day. On your best days, 400W of solar might make that, but not always. Especially if you are going to have lead-acid batteries, their worst enemy is chronic undercharging, so I would look at doubling your solar input.

Renogy is very overpriced, and you can do better. I have Renogy panels in my array portfolio, and I can tell you from experience that they are the poorest performers of all the brands I have.

Try to shop locally rather than ordering via the internet for your panels. Here in the US we have "Craigslist.com" for sellers marketing their stuff. What's your Portuguese venue? Just a few months ago I bought some REC260W grid-tie panels for 65$US. Three would be 195$ which is around just half as many Euros as those Renogy panels for almost double the watts. Cash and carry is the most economical way to go. Bring a voltmeter with you if you have any questions about the quality of panels you are buying.

Always consider the future.. I agree that 12v is for vehicles nowadays. When I started, 12v was all that was really available, now great 48 volt batteries and inverters are practical. I just built a large (massively overbuilt) system for my off grid cabin. One thing I know is that energy use never goes down, now I have starlink, security cameras, and remote control A/C heating for the batteries. I also am considering resale someday. I put in an electric car charging plug, even though I don’t yet have an electric car. I’ve already had one friend visit who charged his car off my solar array. I’m also in a cold climate region and I need to capture a lot of sun in few hours a day, so my situation is different.

 

[donsolar]

Sticking with 12V is really a mistake. I started that way, learned my lesson, and now try to guide others away from 12V. It is really only appropriate if your application has wheels. Dirty power however is real concern, so focus your attention on a good "sine-wave" inverter.

Looking at your power needs, I'd suggest that only 400W of solar will be inadequate. Looking at my own workshop system, where I keep my freezer, it is self-sufficient on about 2.0kWh per day. On your best days, 400W of solar might make that, but not always. Especially if you are going to have lead-acid batteries, their worst enemy is chronic undercharging, so I would look at doubling your solar input.

Renogy is very overpriced, and you can do better. I have Renogy panels in my array portfolio, and I can tell you from experience that they are the poorest performers of all the brands I have.

Try to shop locally rather than ordering via the internet for your panels. Here in the US we have "Craigslist.com" for sellers marketing their stuff. What's your Portuguese venue? Just a few months ago I bought some REC260W grid-tie panels for 65$US. Three would be 195$ which is around just half as many Euros as those Renogy panels for almost double the watts. Cash and carry is the most economical way to go. Bring a voltmeter with you if you have any questions about the quality of panels you are buying.

Thanks Michael.
I'm in two minds about 12V. I would like to stick with it as I am moving from the city to, amongst other things, get away from EMF and the temptations of constant electronics. I have to limit the amount of electricity available to myself to the necessary or I'll spend too much time on the laptop, plus my skin and psyche reacts badly to electronics nearby. I will be building several outhouses, including a studio/workshop, with bigger systems for the loads needed, which will be dedicated "high voltage" areas. As the cabin is really just a place to sleep, I'd like to keep it as low power as possible.
On the other hand I can't guarantee I won't be needing an AC in the future (this summer we had a whole week where it consistently hit 45 degrees Celsius in the afternoon) and of course the wife may appreciate a food blender....

I agree with you re: 400W of panels not being enough, I will be purchasing 800W of panels.

I appreciate your tip regarding looking for used offers: I didn't see anything cheap enough to be worth driving out recently but found an offer just now about 1 hour away for 3 Risen SYP200S-M. He wants 90€ per Panel, but I'm sure I could get all three for 250. Do they look good to you?

 

[donsolar]

Always consider the future.. I agree that 12v is for vehicles nowadays. When I started, 12v was all that was really available, now great 48 volt batteries and inverters are practical. I just built a large (massively overbuilt) system for my off grid cabin. One thing I know is that energy use never goes down, now I have starlink, security cameras, and remote control A/C heating for the batteries. I also am considering resale someday. I put in an electric car charging plug, even though I don’t yet have an electric car. I’ve already had one friend visit who charged his car off my solar array. I’m also in a cold climate region and I need to capture a lot of sun in few hours a day, so my situation is different.

I hear you on planning for the future, definitely. This system is purely for the bedroom, as this cabin is not big enough for anything else. In future I will certainly build bigger systems for other parts of the property where more power is needed. I'd like to keep the sleeping quarters electronics-free as much as possible though.
It also seems a bit pointless to me to be transforming the 12V to 48V when the only things I will be doing in this building are running a camper-fridge (dometic cfx 50w), an led lightbulb and occasional phone charging. I was under the impression these applications run on 12V easily without extra costs or adapters.
Building an overbuilt system is the last thing I want if I'm honest - I'd rather plant more trees or spend the extra cash on a nicer shower cabin.

 

[LywWyr]

Higher voltages reduce copper wire costs .

 

[MichaelK]

my skin and psyche reacts badly to electronics nearby. I will be building several outhouses, including a studio/workshop, with bigger systems for the loads needed, which will be dedicated "high voltage" areas. As the cabin is really just a place to sleep, I'd like to keep it as low power as possible.

Is your sensitivity related more to electrical flow, or the 50Hz field around it? Do you react differently to being near DC appliances as compared to AC ones? I usually try to dissaude people from purchasing DC appliances because of their steep price premium, but in this case, I would consider it for health reasons rather than power conservation.

3 Risen SYP200S-M. He wants 90€ per Panel, but I'm sure I could get all three for 250. Do they look good to you?

They have a relatively high Voc, at 45V, so even two in series would be closing on the V limit of your controller. Does your location get frost in the winter? At 0C the voltage is 1.12X higher than at 25C, so 90Voc becomes 100Voc right around freezing. The price is a bit higher than what I would be willing to pay. The Euro is ~ at parity to the dollar right now, isn't it? So, 65$ is about 65E? I'd consider somewhere around 65-80E for a 250-290W panel a good deal. I'd say that 90E for only 200W is too much.

Victron is a Tier-1 brand, but I'd look at a more capable unit, maybe in the 150V/40A range. I would always plan for future expansion. People find that their initial estimations are overly optimistic, and add more panels as reality sets in. More amps and a higher voltage limit will let you take advantage of panel opportunities that present themselves in the future.

 

[donsolar]

Is your sensitivity related more to electrical flow, or the 50Hz field around it? Do you react differently to being near DC appliances as compared to AC ones? I usually try to dissaude people from purchasing DC appliances because of their steep price premium, but in this case, I would consider it for health reasons rather than power conservation.

These are some good questions, and unfortunately I don't have many answers. I have not had the chance to experiment with different systems yet. I would like to find this out in future. I can only tell you that the new flat I've moved into has "bad" wiring (I frequently pick up the renegade hum of the mains circuitry with audio equipment, a tell-tale sign for me) and since then my sleep routine and skin have been making their unhappiness known. Diet/exercise etc have stayed the same. I've tested for mould, carbon monoxide etc and found nothing. On the other side of my bedroom wall, on the street, is the grid/mains inverter box (unsure of correct term) which takes the mains electricity from the grid and converts it to feed it to the block of flats. I can hear the hum when it powers up a few times a night.

I have also read this great guide to building a 12V home system for EMF sensitive peoples. It gave me the impression DC may have less of an impact upon my wellbeing, and confirmed my suspicion that inverters are a source of "dirty" power.

Recently, I spent 2,5 weeks in the cabin - with only the power oak battery, one 100W solar panel, my phone and a camping light - and all my symptoms vanished. A rash on my back I've had for half a year vanished within a few days. Of course this could also be due to less pollution, healthier way of living. Only time will tell. But for now I'm pretty set on staying with DC, due to these reasons.

Thanks for your understanding.

They have a relatively high Voc, at 45V, so even two in series would be closing on the V limit of your controller. Does your location get frost in the winter? At 0C the voltage is 1.12X higher than at 25C, so 90Voc becomes 100Voc right around freezing. The price is a bit higher than what I would be willing to pay. The Euro is ~ at parity to the dollar right now, isn't it? So, 65$ is about 65E? I'd consider somewhere around 65-80E for a 250-290W panel a good deal. I'd say that 90E for only 200W is too much.

Victron is a Tier-1 brand, but I'd look at a more capable unit, maybe in the 150V/40A range. I would always plan for future expansion. People find that their initial estimations are overly optimistic, and add more panels as reality sets in. More amps and a higher voltage limit will let you take advantage of panel opportunities that present themselves in the future.

Ok, so a larger controller is needed. I think I'd use at least 3 200W panels, maybe even 4. I doubt it will freeze, minimum temperature stays around 5 on average. Nonetheless I guess I'll be looking at a controller with 200V to be safe and allow upgrades. So If I'm doing 600W I need 50A, if I do 800W I need 66A. I see a 250/70 would set me back around 800€, maybe I can get one used for 500€. Fair enough, these things are important.

Do you have any recommendations for batteries? I'm struggling a bit there.

Again, thanks for your advice.

 

[Danke]

I have no idea if it would do you any good, but some people swear by the positive effects of “earthing” or “grounding”

 

[MichaelK]

I frequently pick up the renegade hum of the mains circuitry with audio equipment, a tell-tale sign for me) and since then my sleep routine and skin have been making their unhappiness known. .......... I can hear the hum when it powers up a few times a night.

Recently, I spent 2,5 weeks in the cabin - with only the power oak battery, one 100W solar panel, my phone and a camping light - and all my symptoms vanished.

OK, it sounds to me that what you are describing is intolerance to 50Hz AC. That's the "hum" you refer to. So it might not be voltage, and you may be perfectly fine with a higher voltage. Though pricy, there are DC versions of a lot of stuff, including refrigerators, and even air-conditioners, all operating on 12/24/48VDC. You'll just have to shop around for it. Try checking out trucking depots where the big transport trucks park.

Ok, so a larger controller is needed. I think I'd use at least 3 200W panels, maybe even 4. I doubt it will freeze, minimum temperature stays around 5 on average. Nonetheless I guess I'll be looking at a controller with 200V to be safe and allow upgrades. So If I'm doing 600W I need 50A, if I do 800W I need 66A. I see a 250/70 would set me back around 800€, maybe I can get one used for 500€. Fair enough, these things are important.

OK, here's the math you need to use. Assume that a 12V battery charges at 13V+, a 24V one at 25V+, and a 48V one at 50V+. Let's say you buy the 50A model and run it at 24V. The basic math is 50A X 25V = 1250W of solar. Here is one caveat though. You can expect to almost never see rated output from a panel, because those numbers were derived in a controlled test chamber, so you need to incorporate a derating (I call it the fudgefactor) into your math. I like to use 85%, which came from empirical measurements. So, that 1250W with derating becomes 1250W/85% = 1470W. So, you can safely mate that controller to 1470W of panels.

Do you have any recommendations for batteries? I'm struggling a bit there.

I'm a lead-acid guy, so don't expect me to recommend anything Lithium because I have no experience with them. The quality brands in the US that I have used are Trojan and Rolls. I would recommend both. I'm not a fan of AGM. I think they are overpriced, and shorter lived then flooded batteries. Give them distilled water, and high charge levels, and lead-acid will give you many years of good service.

 

[12VoltInstalls]

that an inverter creates "dirty power"

A pure sine inverter is cleaner than many grid lines and most generators.

seems a bit pointless to me to be transforming the 12V to 48V when the only things I will be doing in this building are running a camper-fridge (dometic cfx 50w), an led lightbulb and occasional phone charging. I was under the impression these applications run on 12V easily without extra costs or adapters.

would like to stay on 12V DC as it's my first build and this cabin is intended as an electronics-free zone

I run a fridge, coffeemaker, laptop, phone chargers and ran the furnace all winter on 12V solar (with a bit of supplemental during the dark periods.
Under 2000W - or better, under 1500W- 12V is fine. There’s no blanket setup where low voltage at lower output causes a random bus to crash in Sri Lanka killing dozens.
12V is simple for you and your vehicle can recharge 12V in an emergency (though that isn’t a great method in practical use).

Does this seem acceptable to you?

Yes, but more panels

Do I even need AGM, or would LiFePo be a better bet?

LiFePo can freeze or be used below freezing. But they will die quickly if charged below freezing. AGM costs a lot for the lifespan of cycled batteries ?
Get LiFePo, or flooded deep cycle if you will monitor them well. I’d get LiFePo because they can stay partly charged without damage.

Anything I should re-think before continuing my planning?

USB is 5V- you’ll need converters for that.

Use 12V LED lights

 

[donsolar]

OK, it sounds to me that what you are describing is intolerance to 50Hz AC. That's the "hum" you refer to. So it might not be voltage, and you may be perfectly fine with a higher voltage. Though pricy, there are DC versions of a lot of stuff, including refrigerators, and even air-conditioners, all operating on 12/24/48VDC. You'll just have to shop around for it. Try checking out trucking depots where the big transport trucks park.


OK, here's the math you need to use. Assume that a 12V battery charges at 13V+, a 24V one at 25V+, and a 48V one at 50V+. Let's say you buy the 50A model and run it at 24V. The basic math is 50A X 25V = 1250W of solar. Here is one caveat though. You can expect to almost never see rated output from a panel, because those numbers were derived in a controlled test chamber, so you need to incorporate a derating (I call it the fudgefactor) into your math. I like to use 85%, which came from empirical measurements. So, that 1250W with derating becomes 1250W/85% = 1470W. So, you can safely mate that controller to 1470W of panels.


I'm a lead-acid guy, so don't expect me to recommend anything Lithium because I have no experience with them. The quality brands in the US that I have used are Trojan and Rolls. I would recommend both. I'm not a fan of AGM. I think they are overpriced, and shorter lived then flooded batteries. Give them distilled water, and high charge levels, and lead-acid will give you many years of good service.

Thanks for the advice! I've worked out a second draft of my equipment, will update the first post to reflect this.
What are your reasons for preferring lead-acid over LiFePo4? I'm trying to decide between the two.

 

[donsolar]

A pure sine inverter is cleaner than many grid lines and most generators.


I run a fridge, coffeemaker, laptop, phone chargers and ran the furnace all winter on 12V solar (with a bit of supplemental during the dark periods.
Under 2000W - or better, under 1500W- 12V is fine. There’s no blanket setup where low voltage at lower output causes a random bus to crash in Sri Lanka killing dozens.
12V is simple for you and your vehicle can recharge 12V in an emergency (though that isn’t a great method in practical use).

Yes, but more panels

LiFePo can freeze or be used below freezing. But they will die quickly if charged below freezing. AGM costs a lot for the lifespan of cycled batteries ?
Get LiFePo, or flooded deep cycle if you will monitor them well. I’d get LiFePo because they can stay partly charged without damage.

USB is 5V- you’ll need converters for that.

Use 12V LED lights

Wow, this is really helpful. Many thanks.
For your laptop and phone chargers - how have you solved the outlet/adapter issue?
I've updated the OP to include my new panel and controller configuration. Picked up the panels today.
Can you recommend me a good LiFePo brand? I need 300Ah at the least, but I don't wanna spend 2000€.
How long are your cable runs?

 

[efficientPV]

My house is almost all 12V DC, it is almost on every outlet since house was once grid tied. My inverter is out in the shed with all the other stuff and it only turns on when needed. I have to laugh at these power requirements. I have hot water, dishwasher, clothes washer, pump on just a car battery. That recently changed because the battery was very old and I noticed the fridge would turn off after 45 seconds early in the morning. I had to supplement it with a lawnmower battery dated 2011 I was going to return to town recycling. Battery is only needed for startup surge. Everything runs off real time generated power. I did splurge on a 50AH lithium for the house at night so I could binge watch movies at night and it was needed to keep me alive with medical equipment. In the morning when I wake up it is drained down to 13.3V.

 

[12VoltInstalls]

For your laptop and phone chargers - how have you solved the outlet/adapter issue?

12V powered switched USB outlets, laptop is 19V so I use the inverter

 

[MichaelK]

Thanks for the advice! I've worked out a second draft of my equipment, will update the first post to reflect this.
What are your reasons for preferring lead-acid over LiFePo4? I'm trying to decide between the two.

Very simple. I don't believe all the hype. They "SAY" you can drain Li to 100%. They "SAY" you can't discharge lead-acid more than 50%.

But, look at these to graphs size by side. The one of the right is for my Rolls battery.

What the graphs show is that draining an Li battery to 100% drastically shortens it lifespan, just like lead-acid. In fact, depending on the rate of discharge, maybe Li is worse then lead-acid. Even the typical 80% discharge is still quite damaging.

So, I don't believe the hype can justify the extravagant price premium of Li.

 

[donsolar]

My house is almost all 12V DC, it is almost on every outlet since house was once grid tied. My inverter is out in the shed with all the other stuff and it only turns on when needed. I have to laugh at these power requirements. I have hot water, dishwasher, clothes washer, pump on just a car battery. That recently changed because the battery was very old and I noticed the fridge would turn off after 45 seconds early in the morning. I had to supplement it with a lawnmower battery dated 2011 I was going to return to town recycling. Battery is only needed for startup surge. Everything runs off real time generated power. I did splurge on a 50AH lithium for the house at night so I could binge watch movies at night and it was needed to keep me alive with medical equipment. In the morning when I wake up it is drained down to 13.3V.

Which outlets do you have? As in, three-prong plug, cigarette lighter etc? Or is everything just hard-wired to a fuse box?
Yeah, I can imagine you must laugh at my power requirements haha. I've never been someone with many comforts in my domestic life, don't mind hand-washing clothes and dishes for example. But for the average citizen of the 12st Century 800Wh a day must seem laughable :D
I do also have a standalone power oak which will serve as my luxury battery (phone, laptop, kettle etc).

 

[donsolar]

12V powered switched USB outlets, laptop is 19V so I use the inverter

Are these like the cigarette plug/USB adapters available for in-car phone charging?
I'm just wondering whether to bother creating outlets in the cabin. The 12V system is meant to keep the fridge and lights running primarily, with a separate battery for phone/laptop use. I might as well just hard wire the fridge and lights to the fuse box, no?