Bare, bare bones solar system - how to skip inverters and converters?

I just read through this current thread, it's great, but I have a variation on the discussion:

https://permies.com/t/172655/set-solar-critical

We have a 3000 gallon water tank on a small hill, so we can get gravity fed water. We have a rocket stove and lots of firewood so we can cook.

We've lived off of small (2k or 3k watt) generators for 4-5 day stretches, powering "the essentials" and getting by okay. Not luxurious, but okay. Here's what we've decided our "essentials" are (your mileage will almost certainly vary):

- a few efficient electric blankets to keep us (and our parrot), warm enough at night.
- a few lamps
- our laptops and our "modem"

It seems to me that these things can, or already do run on low voltage DC. So the question is this:

Can we do a solar panel and battery system and skip the whole inverter / converter loop? For example, I know my laptop's adapter provides my laptop with 18v, DC. Sounds a lot like what solar batteries provide, no?

So I'm wondering how to find the kinds of things I've listed above that are designed for low voltage DC power? Maybe some RV devices?

Does it sound correct that this would be a good way to get the most utility out of solar batteries?

Yes, you can setup tiny systems to run on DC.

No, 18v for you laptop isn't the same as solar battery (mostly likely 12v). But you can buy "car adapters" for most laptops that you can run off your battery system.

Ok, it sounds like i'm on the right track, thanks.

So I'm wondering what to duck-duck-go about to find out what I can get that can minimize the whole inverter / converter loop and how exactly to do that..

For example, let's say a solar battery outputs 18v DC. I would think I'd want to put some sort of surge protector-y sort of gizmo between that solar battery and my laptop.

That's the sort of thing I'd like to find out about!

Consider going 24V, as you can use smaller wire. Also, DC conversion is a bit more efficient in the downward (buck) direction than boosting.

makes sense (24v). do you know of any references or sources that can help me put together such a system?

There should be all sorts of good tips out there. This guy uses LiFePo batteries, which on the surface look more costly, but since they can be discharged far lower than lead acid the price per unit usable storage is actually equal or better On top of that, they will last longer and they often have BMS built in, which means one less component for the system.

The typical options for battery systems are 12v, 24v, and 48v.

For a small simple system I would choose 12v.

Personally, I see no reason for a 24v system. If you want a bigger system and smaller wire size go 48v. I don't see enough 24v products to justify it.

For example, let's say a solar battery outputs 18v DC.

No, solar batteries don't output 18v, you need a car charger.

You can use a DC Fuse block with automotive fuses as your power distribution point.

Bert Bates wrote:I just read through this current thread, it's great, but I have a variation on the discussion:

https://permies.com/t/172655/set-solar-critical

We have a 3000 gallon water tank on a small hill, so we can get gravity fed water. We have a rocket stove and lots of firewood so we can cook.

We've lived off of small (2k or 3k watt) generators for 4-5 day stretches, powering "the essentials" and getting by okay. Not luxurious, but okay. Here's what we've decided our "essentials" are (your mileage will almost certainly vary):

- a few efficient electric blankets to keep us (and our parrot), warm enough at night.
- a few lamps
- our laptops and our "modem"

It seems to me that these things can, or already do run on low voltage DC. So the question is this:

Can we do a solar panel and battery system and skip the whole inverter / converter loop? For example, I know my laptop's adapter provides my laptop with 18v, DC. Sounds a lot like what solar batteries provide, no?

So I'm wondering how to find the kinds of things I've listed above that are designed for low voltage DC power? Maybe some RV devices?

Does it sound correct that this would be a good way to get the most utility out of solar batteries?

Yes and no. You can source all of the equipment you need in DC but what seems to happen is that you deal with a lot of dc  manufacturers that make far fewer products so they are much more expensive and don't have to meet the safety standards required of anything that plugs into your wall. If you add up the cost of replacing everything you currently have with dc equivalents its just less expensive to add an extra solar panel, a slightly larger battery and a small inverter to deal with the inefficiencies. I know maybe I sound biased but I lived through the DC only era of solar and it was not great. The loads you are talking about are only a few hundred watts total so the losses are really not much.
For what its worth...
cheers, David

I gave up on 12VDC years ago and I couldn't be happier.  Currently I'm running both 24 and 48V systems oriented towards AC only, and find everything to be totally seamless.

You CAN make 12VDC work, and some people do, but the compromises they make to make it work are not acceptable to me.  You can get a 12V frig, a 12V TV, and 12V coffee makers, but they are all oriented to the RV market, for which you pay a hefty price premium.

I've found myself that the extra money that needs to be spent to make DC work is really better spent investing in larger batteries and more panels.

The big problem that most people have is that they don't know what they need to produce in terms of Watthours (Wh) before they throw stuff together.  Then they spend even more money correcting their initial mistakes.

What you should be doing first is making an itemized list of everything you want to power and how much they each will consume.  Once you have an idea of what needs to be fed, you have an idea of what you need to build to keep it fed.  Here are suggestions as to where you can start....

Computer/router: 150W/hr X 2 hours = 300Wh
LED lights: 50W X 4 hours = 200Wh
Electric blanket: 100W X 8 hours = 800Wh
12V inverter on 24/7:  20W X 24hours =480Wh

Added up, that's ~1800Wh.  Maybe a bit less if you turn the inverter off at times.  Let's round that up to 2.0kWh per day.

Assuming you don't want to drain the battery more than 50% the size you need at 12V is (2000Wh X 2)/12V = 333Ah battery.  That is a bit less than a Trojan L-16, which is a fairly large 150lb battery.  They are 6V each, and you need two in series to make a 12V battery bank.  They cost about 350$ each, or 700$ total.

You might get better performance out of a 24V system.  The same math is 2000Wh X 2)/24V = 167Ah battery.  CostCo is selling decent golf-cart battery right now for 110$.  They are 6V, so you need 4 in series.  That would be 440$ total.  So, a 24V battery of the right capacity would be cheaper than the 12V version of the same size.

You want to charge at 1/8th of C, which is 210Ah, so the math is (210Ah/ X 25Vcharging = 656W of solar panels.  I like to derate 85% for real-world sun conditions, so call that 772W of panels.  That would be three 260W residential grid-tie panels.  I picked some of those up for 65$ each last year, but prices have been going up.  On Craigslist right now they are running ~75-80$ right now.

You need a charge controller to handle the current coming off the panels.  Epever makes the Tracer4210AN, which can handle as much as 1000W at 24V.  Since panels are the cheapest component right now, get four of those 260W panels.  Then they can be wired 2S2P for 1040W total.

Finally, the inverter.  Get a sine-wave inverter in the 1000-2000W range.   If you eventually chose to add a refrigerator, this will handle it.

MIchael laid it out really well above. The reality of solar is people keep adding loads so a reasonable amount of overbuilding at first is a good idea as its easier and cheaper to do it right in the beginning then to patch on extra as you go.

thanks so much for the informative post, and for taking the time to walk thru some example math!

Bert Bates wrote:thanks so much for the informative post, and for taking the time to walk thru some example math!

One thing that does get overlooked sometimes is that just because you go the 24 or 48 volt route for your main battery does not mean you have to give up on the 12 volt infrastructure you already have. I recently installed a 24 to 12 volt converter for a client who had a lot of older infrastructure that ran on 12 volt. All his main house lights and a 3 year old 12 volt freezer were running on a separate 12 volt wiring system that predated modern systems. The converter was about $60 canadian for a 40 amp unit plus some fuses and disconnects. It replaced an AC power unit someone had installed which was a huge phantom load. Its made a 300-500 watt hr difference for them per day which is huge on a small system. If they had been starting from scratch I would have gone 48 volts right out of the gate or at least 24. They currently run a 24 volt outback as their main inverter which replaced the trace DR(now a museum piece) you see mounted on the wall. So some old school photos of probably 30 years of off grid history and showing there is life after 12 volts.
cheers,  David

David Baillie wrote:

Bert Bates wrote:thanks so much for the informative post, and for taking the time to walk thru some example math!

One thing that does get overlooked sometimes is that just because you go the 24 or 48 volt route for your main battery does not mean you have to give up on the 12 volt infrastructure you already have. I recently installed a 24 to 12 volt converter for a client who had a lot of older infrastructure that ran on 12 volt. All his main house lights and a 3 year old 12 volt freezer were running on a separate 12 volt wiring system that predated modern systems. The converter was about $60 canadian for a 40 amp unit plus some fuses and disconnects. It replaced an AC power unit someone had installed which was a huge phantom load. Its made a 300-500 watt hr difference for them per day which is huge on a small system. If they had been starting from scratch I would have gone 48 volts right out of the gate or at least 24. They currently run a 24 volt outback as their main inverter which replaced the trace DR(now a museum piece) you see mounted on the wall. So some old school photos of probably 30 years of off grid history and showing there is life after 12 volts.
cheers,  David

Fun stuff!

I've got a LOT of irons in the fire right now. If life wasn't so busy, I think it would be fun to build my own system. But at this point, I think I'd like to get a more or less turn-key solution.

I'm thinking ~800 watts of panels, 2-4 kwh of storage, and maybe 800w (what's the unit here, max output?)

I'm open to adjust these, but they feel about right.

I'm wondering if anyone has recommendations for turn-key solutions? The closest I've found is a company called "eco-worthy". They have an 800w, 3.2 kwh, ~800 w of panels, 2x12v 100ah lithium battery system for $1950.

Thoughts?

Bert Bates wrote:
I'm wondering if anyone has recommendations for turn-key solutions? The closest I've found is a company called "eco-worthy". They have an 800w, 3.2 kwh, ~800 w of panels, 2x12v 100ah lithium battery system for $1950.
Thoughts?

Garbage.  Stay away from Eco-worthy.  Have been spending a lot of time online trying to help an Eco-worthy customer on another board.  It's low-end junk that barely functions.  Don't base all your decisions on W/$.

My advice is to stay completely away from "turn-key solutions".  My personal observation is that the "kits" out there are primarily to bundle the low-budget items to get them out the door that people otherwise would pass by.

You will get better quality and FAR better performance with individual components that you assemble yourself.  Look through some of my previous posts that outline a basic 24V cabin system that you can put together for ~1950$.  Buy your panels locally instead of ordering online.  Shipping doubles the cost of panels.  High-voltage residential grid-tie panels give you the best bang for your buck.  Skip 12V panels.  Here is a link..... https://permies.com/t/205879/limits-solar-energy

Operating a solar system requires some hands-on skills.  Really, in my opinion, the only person that can properly manage a solar system is the person that assembled it.  Assembling your system wire by wire is how you get those skills.  Buying a "turn-key" system is a good plan for failure.

Here I've cut and pasted the specs of a "24V cabin system" from the link above, but edited the equipment somewhat to make costs line up more with prices I am seeing right now, and with what you want to spend.  With fixed mounts, expect this basic system to make you >2.5kWh/day in winter, and >5.0kWh/day in summer.  With rotating mounts, double those numbers.
4 golf-cart batteries, now 110$ at Costco: 440$
4 260W grid-tie panels at 80$ each: 320$
1 40A MPPT charge controller: Epever Tracer 4210AN is 125$ right now on Ebay
24V sine-wave inverter:  Samlex makes a budget 1500W unit for 510$ that is UL listed.  
wiring, breakers, electrical boxes: 500$
total:  1895$

Michael said: "Operating a solar system requires some hands-on skills.  Really, in my opinion, the only person that can properly manage a solar system is the person that assembled it.  Assembling your system wire by wire is how you get those skills.  Buying a "turn-key" system is a good plan for failure."

Okay, cool. Let's take your advice a step further: Do you know of any good books (or other training materials), that would walk me thru assembling my own system? Because again, sometimes I enjoy diving into a complex new area and going thru a big learning curve. But right now I don't have the time for that, and I'd like to get a system up and running.

Bert Bates wrote:
Okay, cool. Let's take your advice a step further: Do you know of any good books (or other training materials), that would walk me thru assembling my own system? Because again, sometimes I enjoy diving into a complex new area and going thru a big learning curve. But right now I don't have the time for that, and I'd like to get a system up and running.

There are plenty of books from Amazon you can chose from.  An online forum I frequent is probably an even better resource.  One I recommend is https://diysolarforum.com/.
Problem with most published books is that solar is advancing so fast, that most publications are obsolete by the time they get into print.

Bert Bates wrote:Michael said: "Operating a solar system requires some hands-on skills.  Really, in my opinion, the only person that can properly manage a solar system is the person that assembled it.  Assembling your system wire by wire is how you get those skills.  Buying a "turn-key" system is a good plan for failure."

Okay, cool. Let's take your advice a step further: Do you know of any good books (or other training materials), that would walk me thru assembling my own system? Because again, sometimes I enjoy diving into a complex new area and going thru a big learning curve. But right now I don't have the time for that, and I'd like to get a system up and running.

Midnite solar put out a pdf that is chock full of good information...
https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.midnitesolar.com/pdfs/DIY_Manual_-_Dec_2021.pdf&ved=2ahUKEwjo2tjou6X9AhVhj4kEHd5kCfAQFnoECA0QAQ&usg=AOvVaw1wuFvzznHMhEdHRBvRzJZi

David, thanks so much for posting that Midnight DIY manual.  Will have to print out a couple of copies to pass along to my solar neighbors.  It really re-enforces what I've been trying to tell them.  Think it will sink in better if a major company is saying the same thing.