Small off-grid cabin system design (in reverse?)

[retrop]

Like most people new to solar, I've read so much info I'm starting to confuse myself. Would appreciate some input as to the best direction to go with this situation. The details:

I have an off-grid cabin in the mountains in the central part of Utah. The cabin is mainly accessible from late May to early Nov and by snowmobile in the winter. Currently, the cabin is setup with propane mainly used for heat, hot water and cooking. I have a generator which also runs off the propane and currently supplies all energy for the cabin. Eventially, I want to create a system where I do not have to run the generator much but have it as a backup. I've been looking at the best way to stair-step my way into a full system, meaning I can't do it all at once, but would like to take steps to eventually get there.

The problem: the cabin is north - northeast facing. It sits back in very tall trees, almost a U-shape around the cabin. It does receive good sun light in the morning thru the mid-afternoon, but then starts ending up in the shade by mid-afternoon. The only ideal place to locate a solar panel array would be right in the view from the front of the cabin, which i do not want to do.

One of the big positives to the situation is I have an ideal setup for a micro-water generator, which is ultimately my plan, but I won't be able to take on that project for a couple of years. There is also a chance to include a wind generator as well. I'm thinking I could potentially use water, wind and solar for energy generation at some point. Ultimately water for the summer months (the water is not available in the winter) and then wind/solar for smaller needs in the winter.

For the upcoming summer, I want to be able to run a small refrigerator, freezer and Starlink, along with a computer and lights, but still need to research and monitor the sunlight to see how effective solar might be. I've calculated I will need about 6500 watts (or 6.5kWh if I did the math right) for each day to run these items. My plan is to create a 48v system. The system is single phase - 120v. No need for 220. I'd like to create the beginnings of my system by getting an inverter/charge controller along with a lithium battery (120 amp or there abouts) and use this for my needs this year. I plan to charge the battery while I run the generator in the evenings. After I monitor the sun this summer to decide if solar will work, I'll go down the road of how many panels and what I will need. I will also add batteries as I decide how much storage I eventually will need.

I am curious if there is an inverter/controller out there which I could get and eventually grow the system around the inverter, be that water, wind and/or solar? I'm really just needing to get the inverter and battery at this point to have continual power throughout the day but would like to get something that will work with what I eventually want to do.

I'd appreciate any thoughts or direction you might have!

 

[Mattb4]

Yes there are inverter/chargers that would handle your needs. Likely anything 3kW upwards. From the list of your loads it appears that it could be just a 120vAC model. To charge your battery from PV, wind, or hydro will require an appropriate charge controller. To grow your system simply would mean matching the amount of batteries to the ability of your supply to fully charge them whether that be generator, wind, sun or water.

For instance you mention 120ah 48vDC battery to start. Depending on battery chemistry this will give you from 2.5kWH up to 5kWh of usable storage. To charge it back up in 5 hours your charge controller will have to output 12 to 25 amps. There will be some losses going from your supply to battery to inverter and off to load so add that in.

Likely you will need lead acid batteries due to their ability to accept charge in freezing conditions if you can not maintain your cabin warm.

 

[Rednecktek]

You'll need to know not only how mank Kwh of battery you need (battery capacity) but also full draw load of all your stuff (inverter rating) and base from there. If you've got an inverter generator, most AIO's can take input from that nicely.

A plus to having space is you can put a ground mount array just about anywhere with long enough wires. If you don't want to block the view from the patio, is there somewhere a bit further away that will work?

There are a few models if 3kw/48v AIO's that can do 120v and solar and generator. If you need more amps then you can stack them in parallel and double your solar and your amperage. There are even some 6kw 120v units if you want to get more load right out the gate. Once you get over 6kw, most units are 240v split phase so you have to have slightly different wiring in the breaker panel.

If/When you get hydro or wind going, they'll have their own kind of controller setup that goes straight to the batteries and the AIO won't care.

 

[HarryN]

If I am understanding things, your cabin is located in an area that routinely will be less than 30 F, a substantial portion of the year.

The cabin is heated, but really only when you are there.

There is enough snow for solar panels to be covered and not able to supply much power - routinely for days at a time.

So as much as I like LiFe batteries, it is worth strongly considering to use AGMs instead. Much more low temperature capable for charging and discharging compared to LiFe.

Probably the midnight solar line is best for supporting both solar and wind in terms of charge controllers.

 

[HarryN]

As a practical matter - my suggestion is to start by building a small 24 volt system at home to play with, and then work on building the 48 volt system. Even if you don't actually purchase a 24 volt inverter for the learning system, it will help a lot with building up the higher spec requirements 48 volt system.

Even building something as simple as this little test system example will give you a head start on scaling up and costs relatively little to do.

Small test fixture

Sometimes it is easier to show people things in photos than to describe them. So I made a little test fixture from left over parts, nothing fancy. In this case, some people asked about methods of dealing with pre-charging the capacitors on the input stage of inverters, and I had suggested that...

diysolarforum.com

 

[retrop]

Yes there are inverter/chargers that would handle your needs. Likely anything 3kW upwards. From the list of your loads it appears that it could be just a 120vAC model. To charge your battery from PV, wind, or hydro will require an appropriate charge controller. To grow your system simply would mean matching the amount of batteries to the ability of your supply to fully charge them whether that be generator, wind, sun or water.

For instance you mention 120ah 48vDC battery to start. Depending on battery chemistry this will give you from 2.5kWH up to 5kWh of usable storage. To charge it back up in 5 hours your charge controller will have to output 12 to 25 amps. There will be some losses going from your supply to battery to inverter and off to load so add that in.

Likely you will need lead acid batteries due to their ability to accept charge in freezing conditions if you can not maintain your cabin warm.

Thanks for the reply. I didn't put together that the way to add different input sources would be a charge controller specific to the type of input. I was so focused on an "all-in-one" inverter/controller I missed probably a more effective way to handle additional input sources as the situation arises. Also, you are correct, I probably do need to really think thru the type of battery(s) I'm going to use. It was below zero a couple of times I was up there this winter. I find myself making rookie assumptions that just getting what is perceived as the best makes everything work. That's both incorrect and expensive. Appreciate your thoughts!

 

[retrop]

If I am understanding things, your cabin is located in an area that routinely will be less than 30 F, a substantial portion of the year.

The cabin is heated, but really only when you are there.

There is enough snow for solar panels to be covered and not able to supply much power - routinely for days at a time.

So as much as I like LiFe batteries, it is worth strongly considering to use AGMs instead. Much more low temperature capable for charging and discharging compared to LiFe.

Probably the midnight solar line is best for supporting both solar and wind in terms of charge controllers.

Your assumptions are correct, temperature in the winter is a concern I missed as I was starting to think this thru. I am definitely going to think thru the type of battery to use. Got caught up in LiPO is the best, so use it without taking into consideration all aspects. There is definitely enough snow to cover the panels, 5-7 feet on average. I'm not overly concerned about the winter as those are just weekend trips, but if I could make it work it would open up other options for the winter. Thanks for your observations!

 

[retrop]

You'll need to know not only how mank Kwh of battery you need (battery capacity) but also full draw load of all your stuff (inverter rating) and base from there. If you've got an inverter generator, most AIO's can take input from that nicely.

A plus to having space is you can put a ground mount array just about anywhere with long enough wires. If you don't want to block the view from the patio, is there somewhere a bit further away that will work?

There are a few models if 3kw/48v AIO's that can do 120v and solar and generator. If you need more amps then you can stack them in parallel and double your solar and your amperage. There are even some 6kw 120v units if you want to get more load right out the gate. Once you get over 6kw, most units are 240v split phase so you have to have slightly different wiring in the breaker panel.

If/When you get hydro or wind going, they'll have their own kind of controller setup that goes straight to the batteries and the AIO won't care.

Thanks for the reply, great info! I have given thought to placing the array in a different location but backed off for two reasons, one was a concern about line loss over a distance (which is yet to be determined) and the other was just the cost of wire. I really need to do some analysis of the sun patterns in different areas relatively close to the cabin to determine if there is an adequate place to locate the array. Good project this summer.

I do have an inverter generator so that's good. As I replied to another, I did not put together the idea of using different charge controllers to handle the input. That approach really makes it much more simple to plan for the future, just add the type of controller needed when and if the need arises. What I really need to do is figure out which AIO would be good for my situation and then add to it when I figure out what direction I'm going to head.

 

[retrop]

As a practical matter - my suggestion is to start by building a small 24 volt system at home to play with, and then work on building the 48 volt system. Even if you don't actually purchase a 24 volt inverter for the learning system, it will help a lot with building up the higher spec requirements 48 volt system.

Even building something as simple as this little test system example will give you a head start on scaling up and costs relatively little to do.

Small test fixture

Sometimes it is easier to show people things in photos than to describe them. So I made a little test fixture from left over parts, nothing fancy. In this case, some people asked about methods of dealing with pre-charging the capacitors on the input stage of inverters, and I had suggested that...

diysolarforum.com

This is great advice! I'm going to do this. Going to address your comment in a new post as well.

 

[retrop]

So, after some great feedback, I believe I'm guilty of some rookie assumptions and believing just getting the percived best solves all problems. Setup really is based on many factors and while many are the same, each situation needs to be looked at objectively.

I have a couple new questions based on the feedback:

My system is single-phase 120v with a demand of around 6.5 kWh a day and I don't ever see that changing. Is there really any need to go 48v or would 24v be adequate? Could always expand, but if all things stay equal, is there really a need?

Seems like a better approach to batteries might be to go with AGM due to low winter temperature concerns. What negatives would there be to this approach other than battery life and weight?

Appreciate any thoughts

 

[Mattb4]

Higher voltage less amps. This can favor the 48vDC to allow for smaller cable for heavier and more loads. 24vDC however is a bit less cost to get into and a bit safer to work with. Some folks that have gone the 24vDC route (I am one) can feel trapped by the sunk costs preventing easy switch over to 48vDC as they grow their systems. Typically you will find less options for higher current inverters in the 24vDC segment.

Is there a need in your case for 48vDC over 24vDC? From your description of your loads it sounds like either will do it.

AGM will work if you keep in mind that they are more suited for occasional usage. They also should only be discharged to 50% as an average for increased longevity.

When it comes to solar and possible voltage drop from long distance cable runs. Many SCC's are now able to run much higher array voltages. Voltage of over 400vDC can really reduce cable size even for fairly long runs.

 

[HarryN]

So, after some great feedback, I believe I'm guilty of some rookie assumptions and believing just getting the percived best solves all problems. Setup really is based on many factors and while many are the same, each situation needs to be looked at objectively.

I have a couple new questions based on the feedback:

My system is single-phase 120v with a demand of around 6.5 kWh a day and I don't ever see that changing. Is there really any need to go 48v or would 24v be adequate? Could always expand, but if all things stay equal, is there really a need?

Seems like a better approach to batteries might be to go with AGM due to low winter temperature concerns. What negatives would there be to this approach other than battery life and weight?

Appreciate any thoughts

I have built both 24 and 48 volt systems, AGM and LiFe types.

The big advantage of 24 volt systems, especially for a DIYer is the ease of finding components and end use devices that are 24 volt compatible compared to 48 volt.

There are refrigerators, lights, chargers, USB chargers, fans, etc on the market that can run directly off of 24 volt which can potentially allow you to run the cabin with the inverter off sometimes.

There are a number of options for inverters in 24 volt - many available in 48 as well up to 2 - 3 kW, and you can always add on a second one for some small loads if really needed.

There are 120 vac - 24 volt chargers widely available, so you can use or not use a combo inverter / charger or keep them as separate entities. The advantage of keeping them separate is a higher tolerance to power quality coming in from the generator if it is acting up. The advantage of a combo unit is - price.

There definitely are 48 volt versions of things on the market, but the amount of time that you will need to invest in research to pull it off with 48 volt will triple vs using 24 volt. IMHO, the main advantage of 48 volt is the ability to make things more compact - and that aspect is definitely true.

I would not worry too much about wire distance and voltage loss. Just as an example, you can buy a victron 150 / xxx charge controller - which sets the max input voltage at 150 Voc.

Plan to use 100 volts of the solar array for useful power, and another 20 volts of the array to deal with line loss. I realize that it is not a perfect solution, but it is a cabin in the middle of nowhere for occasional use, not a perfectionist NASA project.

Approach the project as a way to "reduce generator run hours", not eliminate them.

@RV8R has some experience in this cabin in the middle of no where concept.

 

[seneysolar]

Just my 2 cents... im a fan of 24V, its great for smaler systems. If your demands arent too high, Its a good balance of battery quantity and cable sizing.

Im also a lead acid fan! Though not so much AGM, they just dont like to be deep cycled. If they are in a shed or garage, Id recommend 6V flooded lead acids, dont get 12v batteries. I like the Trojan SPRE-06-415 for cold climate off grid. If in a living space and you absolutely need AGM, look at Full River DC400-6. Whatever you go with, look at the spec sheet and cycle life so you know what to expect. Put 4 of either in series for 24V.

As an example, I have these at my camp. Running a 4k low frequency Aims off grid year round for 7 years now. Turn panels vertical for winter on a pole mount to keep snow off. Shut inverter off when i leave but the solar cc stays on. A pair of 350w panels in series to a 30A charge controller keeps them topped off.

 

[RV8R]

So, after some great feedback, I believe I'm guilty of some rookie assumptions and believing just getting the percived best solves all problems. Setup really is based on many factors and while many are the same, each situation needs to be looked at objectively.

I have a couple new questions based on the feedback:

My system is single-phase 120v with a demand of around 6.5 kWh a day and I don't ever see that changing. Is there really any need to go 48v or would 24v be adequate? Could always expand, but if all things stay equal, is there really a need?

Seems like a better approach to batteries might be to go with AGM due to low winter temperature concerns. What negatives would there be to this approach other than battery life and weight?

Appreciate any thoughts

Hey LFP are great ,,, Other than they are more complicated “BMS” & the Cold .

I have a remote cabin in Canada that we typically use April to November, & then I pull out my 75lb Rolls AGMs.

S6-275AGM-RE | Rolls Battery

rollsbattery.com

There are a few reasons why I went AGMs & all AGMs are not manufactured equally nor do they have equal performance specs;

1. Cold weather operations
2. Simpler
3. Conservative electrical energy needs

We have 2 6v AGMs ( 12vdc @ 250Ah battery bank ), a Victron 100 | 50 MPPT, & 600W array ( 60v @ 10amps ).

We also have the same Rolls AGMs in our van & if we go LFP in the van, the cabin will get the van AGMs probably for a 24vdc system ,,, the Victron MPPT is good for 12vdc or 24vdc batteries, but the array is;

12vdc = 700W max
24vdc = 1400W max

So I can double ( or more ) the array.

 

[SoggyShoes]

If you are going with lead batteries putting 6V in seies to get the desired voltage is better because in 6V deep cycle batteries the plates in the batteries are thicker. So they are more durable and will last longer.

Lithium is different because they need a BMS and you really want a BMS to be able monitor all the cells in the battery.

One other option is to dig a basement or dry vault that is well below the frost line so your batteries never freeze.
Many years ago we did this for someone to have a wine cellar on Mt. Hood. No light and always 55 degrees.

 

[retrop]

After reading the comments and doing a little more research (a lot more), I'm leaning toward a 24v system. Seems more in the arena of what I am trying to do and my abilities as a rookie. I only need 120v. I also like the idea of the 6v lead acid batteries, makes sense to me. Low temperature is definitely a factor.

I'd like to know if my initial calculations make sense as I look at setting up a 24v system. I'm needing about 6500w a day. Just focusing on 1-day's need, if I divide 6500/24v = 270 amp hours. Factoring in the need to stay above 50% on the charge, I would need to double the needed amp hours to 540 for a one-day supply of power, not including any additional power generated. Am I on the right track so far?

Also, just to answer a few questions in earlier posts, the location I will install this system is in a standalone 6x12 ft cinderblock shed with a concrete floor. This is where my generator is stored and the panel to which the cabin is already wired. There is plenty of room for the components, so I'm not overly concerned about space.

 

[Rednecktek]

Yup, you've got the basic math down. You'll want to do a couple banks of the 270ah or 4 banks of 135ish Ah batteries in parallel so the voltage drop across the batteries is spread out and doesn't drop too much.

 

[RV8R]

After reading the comments and doing a little more research (a lot more), I'm leaning toward a 24v system. Seems more in the arena of what I am trying to do and my abilities as a rookie. I only need 120v. I also like the idea of the 6v lead acid batteries, makes sense to me. Low temperature is definitely a factor.

I'd like to know if my initial calculations make sense as I look at setting up a 24v system. I'm needing about 6500w a day. Just focusing on 1-day's need, if I divide 6500/24v = 270 amp hours. Factoring in the need to stay above 50% on the charge, I would need to double the needed amp hours to 540 for a one-day supply of power, not including any additional power generated. Am I on the right track so far?

Also, just to answer a few questions in earlier posts, the location I will install this system is in a standalone 6x12 ft cinderblock shed with a concrete floor. This is where my generator is stored and the panel to which the cabin is already wired. There is plenty of room for the components, so I'm not overly concerned about space.

As @Rednecktek stated you have the math right, & that is theoretical. Theory & Practical operation can vary somewhat, but your plan & estimated use will keep you in the top 50% SOC.

Battery bank configuration will make a difference.

A big advantage of LFP is their ability to take the big charge current until they are practically full. So with FLA & AGMs, you charge time has to be factored in with the internal resistance of the battery bank. As you reach closer to 100% with AGM the charge current drops off as the SOC % rises.

The big issue is “How are you gonna charge the batteries” & the battery bank configuration to effectively receive that charge.

So I believe you meant you need 6500Wh each day. So the theory on that is “power & time”. It would be good to run estimated scenarios ( math ), to see “How you are gonna theoretically charge your batteries”. If Solar, weather scenarios also should be thrown into those theoretical calculations.

The “Internal Resistance” of the battery bank (series & parallel) configuration might give you a better efficiency when the batteries @ a higher SOC are taking amperage “paralleled” more than series. Wiring configurations & all connections being equal & voltages across the battery bank being equal also somewhat important.

The above is an attempt to spotlight “theory” & “actual” differences where theory is assuming the system is built to 100% “perfect” & then there is reality / actual.