First dip into solar

After a long planning and building period I am about to set up solar for our first off-grid cabin. Here are some details:

Twelve roof-mount 235W (Trina solar TMS-235, used panels from SanTan, but all checked so far seem to be in spec, and I have lots of extras). Site is too wooded to install anywhere else. Roof pitch 14 degrees (3:12) installed at 38 degrees latitude, Azimuth 170 degrees (it was supposed to be 180 but apparently my phone's compass is rubbish). Installing as two panels vertical, 6 horizontal. Wiring not yet certain and is flexible, but for what its worth morning and evening shading will likely occur evenly as vertical panel pairs. Summer I have unshaded sun from about 9 AM to 3 PM. Winter sun will probably be partially shaded on bottom panels, but only by bare branches so probably not important. NREL calculator gives me a maximum 403 kWh in July and a minimum 180 kWh in December.

Winter temps reach below zero and I will not have any constantly heated locations for the next few years, so Li batteries are not an option. I plan to use 12V 90 Ah deep cycle AGM batteries at a maximum cost of $0.84/Ah, wired in 48V banks.

Any general suggestions anyone would care to make are of course appreciated, but my main interest is the best way to set this up to maximize my battery life and power availability while minimizing cost and allowing later repurposing. Somewhere down the road I will add a minisplit system and at that time can switch to Li batteries, but need to keep from having to replace the AGM before then. The current building will require minimal power, but I want as much power available as is practical as this will be the power plant for some short-term and long-term experiments, test builds, and temporary setups I intend to try out over the years. Once the batteries are upgraded, any components will be farmed out to smaller, remote outbuildings.

I seem to find conflicting information as to best charging rates and how my power availability affects the maximum number of AGM batteries I can effectively have without harming longevity with a poorly chosen charging rate.

A related issue is I would like to have more battery than is necessary as I would have guests stay there who I can not expect to use power responsibly, but if that would harm battery lifetime I could instead set up a small, limited power circuit on its own battery so they can not drain the main bank (possibly using a timer to charge the battery once a day near peak solar?).

Any advice is appreciated.

Do you use the cabin in the winter as well as summer?

If only used in the summer you can use LiFePO4 and just discharge to 50% and leave them disconnected. They can be stored half charged at -20f without issue. They have to come up to 32 or bettery 40 to charge.

You can also insulate the heck out of them and use the self-heating version. These take power from the solar panels and first heat to a damage free temp, then charge.

If none of these are what you need there is another chemistry that is similar, I forget the exact name but I am sure someone will come along as suggest it.

Or someone will be here to help with AGM.

Why not use Solar Trackers vs roof mount? Way more efficient, easy to manage and maintain, can tilt for snow build up prevention, cheaper to mount than roof, less panels needed.

I am not a fan of AGM batteries, and I am definately not a fan of small AGM batteries that need to be paralleled. Am helping out a neighbor right now about replacing his trashed AGMs because of too many parallel strings.

I'd say you would be far better off with a single LARGE Ah string. Take a look at this battery, which is what I myself have for my own system right now. (Twelve X 4V)


This single string at 48V would be the equivalent of six parallel strings of your batteries. BTW, maintenance is very low. I need to add distilled water to them only quarterly, and maybe 4 months in the winter, when evaporation seems lower.

As a general rule, AGMs like a charging rate of 1/5th of C (0.2C), whereas traditional lead-acid prefers 1/8th (0.125C). To determine how much solar to charge with, use the following formula. I use a 85% adjustment to compensate for real-world solar output.

(C20 Ah X charging rate X charging voltage)/85% = solar needed

For the example of my own battery, at 568Ah, since it is flooded lead-acid, I'll use the 0.125C charge rate. Since charging starts at a low of 50V, and goes up from there, I'll use 50V as the charging voltage. So, the math looks like (568Ah X 0.125C X 50V)/0.85 = 4176W of solar. In fact though, I am slightly overpaneled at 4500W (15 300W panels), and my battery is very happy.

You haven't mentioned anything about what electronics you want to use, nor the loads you want to support. I'll guess and say this is a regular cabin that will have a refrigerator/freezer, television, lights, and a computer? Based on my own daily usage, I'll say you need at least 5kWh of power, with that going up as you bring in big-ticket items like air-conditioners, or well-pumps. I use 20-22kWh of power per day when I'm pumping irrigation water for my orchard trees, and 10+kWh on hot summer days when I'm running the air-conditioner upstairs.

The charge controller you get will determine the strings of panels you can put together. When you say winter temps drop below zero, do you mean F or C? Will make a difference in the solar string math.

When you say your land is wooded, how much space are we talking about here? What I myself am using is rotating ground mounts with solar strings running at 120VDC. With regular 10 gauge copper wire, I can run that ~130' without any measurable voltage drop. Are there clearings within your woods where you could position ground mounts, and run the power back to your main building? The higher your array voltages, the longer distances you can run them without voltage drop.

I've experienced the exact same thing with visitors to my land. They don't seem to get the idea that conservation is important. So far, I've done the monitoring myself, without having to be too much of a ogre. One of the perks of having a BIG battery bank. For your own place, it might be appropriate to create a smaller satelite system with it's own small battery bank, and it's own inverter. Is this a second out building that you could put it's own solar panels on? You could make it with cheap golf-cart batteries, but don't worry about them if the power fails. People seem to learn faster about conservation after the lights have gone out. Some inverters can be set to shut off once the battery reached a certain voltage. Setting it a bit too high would shut things off faster, without damaging the batteries. Guests sometimes have to learn the hard way.

You could also use a mini-split to keep it warm when you are not there that will give you a battery drain so LiFePO4 would be fine inside. Charge up in day, drain down at night

I'd seriously consider some sort of vehicle to home/load setup in this instance. Maybe a Hysolis can be brought along, shared with guests...

To answer a few questions here are some more details:

It will be used 4 seasons, but should be thought of less as a cabin and more as a wood tent. We have a lot of work to get the property setup for our eventual retirement, and this a base of operations to work from while we do the next build. We only have two main seasons - too hot to work in mid-day and too cold to sleep in a tent at night separated by brief seasons of too wet to do anything, so this is more about making those hot, cold, and wet times livable while we wait for the usable part of the day to go do some work. Even when not working, it is more a base for outdoor living, than a living space on its own. It is under 175 sq. ft, heavily insulated, propane heat in winter but only while occupied, either just fans in summer or an A/C run only when enough excess solar is there to run directly from the panels, no fridge, no microwave. At most just a few lights, fans, a laptop, a hot plate, and chargers for my battery tools. Usually in use 1-2 days a week, with a full week few times a year.

Solar trackers and ground mount of any kind is out of the question. This particular part of the property is wooded and we want it to stay that way. The only nearby clearing is our next build site, so it is unavailable. Eventually the current building will become storage or a playhouse, but the water source will be nearly and the panels will then be used to power that, irrigation, communications, and other central services. Any power generation costs will not go to waste and even with the inefficiencies of the mounting should be sufficient for needs. Snow cover is not a big concern - we only have 15-20 days a year with 1" of cover and only 3 or so with 5" or more. There are some pretty nasty roads to get there, so we would avoid the property on any snowy day anyway. When not occupied, power usage will be minimal and even 5-10% panel efficiency should keep things running fine, as long as that does not damage the batteries.

Power storage is the important question. It will be years before we really decide what we want to do at the property and until we can make meaningful choices about how much power we need and how much we need to store vs. how much can be used immediately when available (i.e. pumps that run only when the sun shines), expensive battery systems with 10 year lifetimes are just not a reasonable expense, and would just delay us on getting started on the next build, which is the real push. What I really need is a system that will run reliably for 2-3 years, can provide the power I need to keep the wood tent livable and run some experimental setups on the side while costing as little as possible, then can be discarded or broken up to run lights in a seldom used outbuilding, a powered driveway gate, or a weather station (uses will be spread out over a 50 acre lot, which is why I think multiple small strings are better than one large one and AGM better than FLA). That Rolls battery mentioned looks great, but realistically it is a $6000 battery system that may not turn out to fit my needs two years from now. Maybe its the right choice later for the home that will follow, but I think not now.

So I think my original choice is the best - a few small strings of small batteries. The question is how to best arrange that and what chargers and inverter systems I use to maximize battery life so I can hopefully use them long enough to get to the next building with the next system.

Incidentally my choice of 48V is driven purely by wanting to test out some 48V DC appliances for the next build. I am not tied to that. My choice of multiple small strings is so I have the freedom to set up temporary test systems in other configurations.