I recently set up my own off grid solar system in a tiny cabin that I designed and had built.
I used the Solar Controller+Inverter Combos recommended on this site (Growatt) but ultimately switched to ones by Renogy. The Growatt one I purchased has a wifi adapter and app - wifi requirement was a deal breaker when remote. The renogy one has a Bluetooth adapter and app, plus feels better designed (though actually I think the internals are all pretty similar).
There are a couple surprise gotchas I didn’t know:
1) These Controller+Inverter combo units require a battery to turn on. This means: if your battery runs completely out of charge, you have to get electricity from somewhere to charge your battery a bit before the Controller+Inverter will turn on and charge your battery again. I was pretty annoyed at how not “off grid” this felt.
2) Temperatures below freezing are bad for batteries. I purchased a Renogy 48v smart battery with self heating. Unfortunately less sun in winter means less energy, and heat production is energy intensive. I’m not at the cabin 100% of the time so I have to take the battery with me when I leave to avoid spoiling/freezing it. Still not sure how I’ll ultimately get around this.
> Still not sure how I’ll ultimately get around this.
If it's portable, your option works well.
For year round off grid use, there's something to be said for sealed lead acid, or flooded lead acid - they don't care about being cycled while cold (capacity is down and the voltage sags a good bit), and as long as they don't get too deeply discharged, they don't freeze.
A single panel, facing south, absolutely vertical, will shed snow and if you have no other loads, is usually enough to offset controller loads and self discharge over time to keep the batteries fully charged (and therefore not freezing) over winter.
There's no way you'll generate enough heat on electricity to keep a battery bank (or cabin) warm in the winter most places. I've got 5kW of panel on my office, and still can't manage that for a small, well insulated shed.
I live on 25 ha of forest, have an axe and saw, and heat with wood. Let me tell you: you want to use the saw for everything but splitting. Axes are fine for burly cavemen.
Anyway, in a cold climates like mine an acre would not be enough to keep you warm because trees grow slow when it's winter six months out of twelve. It takes a lot of wood to keep you warm, and one tree gives surprisingly little wood. After a handful of years you'll have clearcut your measly acre and still have another couple of decades to go before it becomes harvestable again. I'd say you want at least 10 acres if you manage it well and the soil and drainage are at least half decent.
They grow faster since they already have a root structure, the size is consistent and the wood is generally straigter (the japanese have a similar technique for extremely straight cedar logs)
By the time you get to 100 acres you can basically heat your house with the stuff that blows over in storms and the remainder can yield a nice cash crop every couple of years, giving space for the other trees to grow. Personally I'd prefer a chainsaw for anything but the most incidental cutting, and a hydraulic woodsplitter (rented) to get the job of preparing firewood for a while year done in a short time.
An acre of trees should be enough to give you an average of 4 kW of thermal power year round, way more than you probably need. Though at 1 W/m^2 how much industry an area of trees could support was the binding constraint on industry for a lot of recorded history. Especially there was always pressure to turn forests into farmland.
Could get by on much less than an acre if it were managed, but better to have more land and be able to keep it as unmanaged forest. That'll also produce fresh game and give some buffer on the lumber supply for lean years. How much rain falls is strictly out of my hands.
Seems to me that if you want to solar heat a cabin, a solar panel is going around the horn. Design the cabin to be a solar collector, like a greenhouse.
You’d still need backup heat production (heat pump, or wood burning, or fossil fuel, or electrical off batteries) for the inevitable month long ‘gray and cold’ stretches, or you’re gonna freeze.
Single purpose but not reliably there heat capture methods add cost and maintenance.
It’s often more cost effective to have a general purpose method that while not as efficient in one case is generally more overall (cost) efficient.
Pretty sure you don’t live in Central Europe or areas like northern Michigan?
There are weeks with zero direct sun, just low cloud cover, scattered snow, and cold cold cold.
I was in Michigan once when Lake Superior froze over, shore to shore.
Unless the place is so well insulated you can keep it warm purely with body heat, you need supplemental heating in climates like this. The sun is not always shining.
As that thread confirms, you do need backup in a cold climate, even with a ton of insulation. Amounts of insulating including R50 and R70, which is pretty mind blowing (and are difficult and expensive to accomplish in practice due to thermal bridging/‘leaks’).
Some folks in less severe climates (like New Hampshire) seemed to get by with baking and other managed appliance heat. That does require you spend time doing, and orient your activities around, things to produce heat to stay warm. Some will find that palatable, others won’t.
Posters in Colder climates (like Ontario) noted propane, or thermal mass heaters (such as the rocket heater in the thread).
‘Then the propane could make up the modest difference on long cold cloudy stretches.’
Hypothetically, for the lithium batteries, what if you made a big stainless steel vacuum wall insulated enclosure? Or just a metal box with r-60 value of foam around it?
The amount of heat needing to be generated can then be calculated based on the average temp over a longer period of time, and those batteries I don't imagine have to be "warm" exactly.
Lithium batteries don’t need to be warm for discharge (generally and with an asterisk), it’s charging. Discharge while freezing causes slower chemical reactions (aka lower discharge current), but that is rarely a huge problem compared to the charging problem.
Any charging around or below freezing causes pretty quick destruction of the cells as the chemistry switches to a more favorable destructive one way electroplating process instead of the desirable (and reversible) ion exchange process.
A common engineering solution to this has started to be temperature dependent shunts that route charging energy to heating pads around the battery until the battery temp gets above a certain safe point.
It adds complexity, cost, and adds several undesirable failure mode however, and Lithium battery cost is already a major factor in these systems, so if you’re in a situation where you can’t reliably temperature control your batteries (cold climate, non-conditioned battery space), lead acid is still a very viable and often desirable chemistry instead.
Round trip efficiency isn’t great with them, and you have to way over provision to get good cell lifespan, but it works.
They do not need to stay that cool at all. When used for solar, the C rate for charging usually never exceeds .2C for overpaneled systems. And usually never goes beyond .5C for discharge. So the heat generated is very small. Now when I am pushing 30-100C rates with a lithium polymer, cooling is an issue. this is why nissan leaf cells were recalled. The battery size was too small for the application, and they overheated which caused significant degradation rates. This is not an issue for solar. Just keep them at room temperature and they are fine.
It's good practice to snake a water pipe around every lithium cell anyway in big packs, because otherwise one bad cell can self-heat to the point of catching fire, even while other nearby cells are cold. The flowing water means the bad cell can't get to the point of fire.
Is 5kW not enough to heat the cabin? I guess it must be really cold where you are. Modern heat pumps should unproblematic handle -25°C (air to air), and provide 4-5kWh of heat for each kWh of power used.
None as far as I can tell. Some people I know here in Norway turn off the heat pump when the outside temperature is below -25°C, but those pumps were installed some years ago.
Good point, I did not think about the fact that the effeciency went down. But it seems to me (as a noob) that modern heat pumps specialising in low temperature conditions still can get around 2.5 at -25°, which gives you more than 10kWh of heating at 5kW, which is quite a lot.
Solar panels produce the most power on cold winter days. The 'rated power' is for 20 degrees ambient at full (90 degrees, no cloud) incidence at sea level.
"5kW of panels don't produce 5kW in winter" -> there is no 'time' factor in there.
What you could say is that solar panels in the winter do not produce the same amount of power in an average day as they do in the summer due to the shorter day and the bigger chance of occlusion as well as the longer path the suns rays have to travel through the atmosphere.
That really depends on how far north you live. Where I live a good winter day's production is ~10% of a good summer day's. (Also depends on the orientation, but it's never even close, let alone more)
Yes, in locations like that you will have to use some other solution for those periods. Maybe wind power. Battery storage will not last long enough unless you have an absolutely massive setup that would not be cost effective the rest of the year.
I had 48KWh worth of storage, 2500 W worth of wind and another 5000 W worth of solar. This was in Northern Ontario. Until we added the windmill we fairly regularly needed the generator, after we added the windmill the generator just sat there gathering dust.
Could you say something about the analysis you did to decide that wind was going to be useful for you? I'm interested but I don't really don't have a good sense for how cost effective a wind turbine is or how to evaluate a site.
The Canadian meteorology department puts out pretty useful stats, and I dropped in on a couple of people in the neighborhood that had windmills up to see what kind of windspeeds they were seeing and how long on average their machines stood still.
This was a major factor in designing the machine the way I did, variable pitch is a lot more work but a machine like that will start up far earlier than a fixed pitch machine. It also had its effect on that stator, which I made with the laminates at a slant over 1/3rd magnet width, which reduced cogging to nearly nothing. This is a huge factor in starting up a direct drive machine.
Yeah, some people had problems with all-in-one battery systems in their homes that were AC coupled to their solar here in Australia over some weather disasters where there were prolonged outages for that reason.
I’d quite like a battery system, I think I’d DIY (well, design the system but here you need a licensed electrician to wire up the AC/grid connect side) using Victron equipment. I have a 5 kW solar system already with a AC inverter which I can keep, so I would probably use a 5000KVA inverter-charger (Multiplus-II), but then I’d add another 1.5kW or so of solar that is directly DC coupled to the batteries and the DC side of the inverter-charger through a separate DC MPPT tracker.
So the DC coupled solar would be enough to kick-start the system to get the AC side back on after a bit if the grid was out and the batteries were depleted overnight.
What I did in Canada was to dig a bunker into the side of a hill near the house. That solved both the frost and any fluid issues in one go. It was a bit of work though to get that all done.
The problem with holes in the ground for storage of things like batteries is that... they're holes in the ground. The ground, as a general guideline, actively dislikes holes in it and strives to fill them in whatever way possible. Liquid rolling in, soil creep, debris blowing past and building up, it doesn't matter - given a long enough time, a hole in the ground won't exist anymore.
You can build a hole that will hold up to this sort of stuff for a while, but that tends to be expensive - underground concrete work is neither cheap nor easy, and given the number of people injured in trench collapses who do it for a living, a serious underground hole isn't a good DIY project either.
That just leaves the maintenance, which also sucks for a hole in the ground. Anything you have to do on the batteries is easier with them on or above the ground, instead of underground in a hole. For lithium, it's fairly low maintenance but you may have to replace some BMS boards every now and then, manually check balance, etc, and for lead it's water every few months, unless you go with sealed, which are "maintenance free," but also "maintenance and status check impossible," so you really just set them and run them to failure - you can't monitor condition like you can with flooded.
It's the sort of solution that, technically, would work - it just ends up being radically more expensive and a hassle than it's remotely worth. Think "Spending $5000 to save $20/yr" sort of thing.
You see the same trend with things like the complex solar trackers that were common in the 80s and early 90s. With the cost of panels, optimizing every bit of power from them was critical, so spending an awful lot of money on support infrastructure to optimize $10/W panel production made quite a bit of practical and financial sense.
Today, with panels at $0.50/W, very few people bother with trackers anymore (for off grid installs). Most things you could improve with a tracker, you can improve with less effort and less money by just hanging more panels. Bonus: With more panels, you get more production on cloudy days when panel orientation doesn't matter (because the sky is more or less evenly lit).
I've got south facing, east facing, and west facing panels on my office, of two different brands, all fed into a single MPPT charge controller input (I do have some blocking diodes to prevent backfeeding a shaded string, though). It's not nearly as "efficient" as some other options, but it's a whole lot cheaper, and gives me more power on the low cloud days that are the rough ones. The rest of the year, I have so much power out there that I'm demand limited, not supply limited (actually, inverter limited - if I added some 48VDC loads, I could use another 5kWh/day easily).
I think you are way overselling the difficulty of digging a hole in the ground. Go to any grave yard- full of sealed coffins, all below the frost line.
Get a plastic storage box, drill holes for cables and seal them with caulk. Put inside a treated lumber box for extra crushing resistance. Run cables through metal conduits to avoid mice and moles chewing on them.
Rent a backhoe, and you are done in half an afternoon.
All of that said, I dont know that I would actually go that far- you avoid the battery freezing, but then you get to worry about it cooking. For an off-grid cabin that isn't constantly heated in winter, it isn't the worst idea.
I dug a hole in the ground to act as a makeshift cellar. About 1 meter cubed, dug it with a shovel, poured gravel as a floor with a French drain sunk in it, lined the walls with breezeblock, made a lid of plywood and foam insulation board. Erected a kind of tent on top to keep the rain out.
No issues so far, two years in. It’s not terribly cool in the hottest days of summer and does require a small heater if I want to keep it above freezing through winter, but in terms of being difficult or expensive to build, not at all.
> actually, inverter limited - if I added some 48VDC loads
I know it’s only lighting as a load, but I’ve heard standard LED light bulbs will happily run as low as 36VDC, as they chop the voltage down to that level anyway.
It makes me wonder how many “120VAC” devices with switch mode power supplies could be plugged into 48VDC without any thought at all. And not just electronics like a laptop or non-plasma flatscreen, but everything is moving in this direction:
Like… what’s my Variable Frequency Drive “inverter” refrigerator motor voltage really running at? In theory it should happily run off 120VDC at least. Don’t do that, but somewhere under 110VDC should work fine. And it’s inherently soft start and easier on one’s low voltage wiring than an on/off duty cycle fridge.
Most modern switch mode power supplies will run on DC, but you really, really want to jack the input DC up high unless you're a fan of replacing bridge rectifier diodes and other input wiring.
Active power factor correction circuitry will have a fit, but... eh, whatever. It's DC now, nobody cares!
A 120VRMS AC signal peaks around 170V, and most (not all, but most...) switch mode power supplies are auto ranging, so they'll tolerate from about 100VRMS to about 250VRMS - which is near 400V peak. If you can get 300+VDC into them, they'll be quite happy, but the boost converters to do that are a pain to find, and by the time you build one, you may as well just get a 120VAC inverter and call it good.
The main thing to be concerned about, and the reason to jack the input voltages about as high as you can, is that almost every switch mode power supply has a full bridge rectifier on the input side. From an AC input source, each diode has a 50% duty cycle. Put DC on it, now two diodes are at 100% duty cycle, two are at 0% duty cycle. How much overhead was there in the system? Well... you'll find out! However, a system capable of running and not overloading the diodes at 50% duty cycle at ~100VAC should be totally fine in terms of diode heating at 300VDC - just, perhaps not at 100VDC.
I never considered the diode duty cycles. If nameplate was 120-240VAC (my TV says 120 but the p/s PCB inside says 100-240…) I’d like to think each would have the overhead to handle under 100VDC, but maybe I’m underestimating the impact of higher current. Edit: you’re right that the “hot” ones will run twice as hot with their 0.6V or whatever drop.
All I could think about was the (potential) improvements on capacitor lifespan since they won’t have to smoothen, but wasn’t sure if their electrolyte could still heat/dry if it was actually the connection to power that makes them hot.
Thinking further, an internal fuse might blow with the (expectedly) larger current draw at 48V.
And yeah on input wiring, you’re pulling more current at 48V.
Welcome to the world of powering AC equipment on DC. It can be done, surprisingly often.
But as soon as you've got some stuff that requires an inverter, you may as well just light the inverter up for everything. I've got the equipment to do a separate, lithium-backed "DC rail" in my office that would be around 40V, and then buck it down for various devices, run my routers direct on it, and... I've never gotten around to doing it, because until literally everything is on that, I'd still have to run the inverter for things like system sleep (I sleep most of my computers overnight). I'm fiddling watts around and it's just not worth it unless I can actually shut the inverter down entirely. Unfortunately, boosting up to 300VDC isn't cheap or easy.
I've wondered if you could run some of that stuff straight off solar during good sun, but I've never really wanted to subject my computers to that sort of abuse.
> I'm fiddling watts around and it's just not worth it unless I can actually shut the inverter down entirely.
That’s where I’m coming from: trying to convince a family member to setup a DC system so the inverter can stay in sleep mode during evening/overnight, because the efficiency is horrendous at small loads. it’s really academic in kWh savings per day, but it saves you the most when sunlight’s the least and reduces battery cycling. But if you’re running an office, the inverter is pretty much running a good load or off, I think.
Could you insulate the batteries to reduce the heating demand? Instead of heating the entire cabin just heat the battery compartment, and only to the minimum necessary to keep them from being damaged (plus a margin for safety).
Maybe even just toss them in an old discarded deep freeze? You could even have one battery in there that is disconnected from the rest of the system so it can be used to jump start the system if it ever gets fully discharged.
An old deep freeze is probably not sufficient, but packing them into a thick layer of insulation should be fine for the winter. The insulation would have to completely enclose them though, including the bottom.
Maybe best to have them sit on a thick layer of insulation on the bottom, and a box that can be removed during the summer months so the batteries don't overheat.
Another a bit weird idea would be to somehow put them next to a compost heap, which produces low temperature heat for a long time (years) including during the winter.
You can insulate them to reduce heating demand, and it will help to an extent (undo that before summer to avoid cooking them), but for an unattended system, it's never going to work out very well. If the system is a year round cabin, the solution is easy enough - keep them in the heated space (which you'll have propane/kerosene/biomass heaters for), and it's not a real problem. But unattended stuff... you really just need to plan for something that can tolerate bitter cold without damage. Or to seriously overpanel the system, 10x+ what you need for summer use, and hope they produce something.
I think most people underestimate just how much delta in production there can be on solar. My house array is grid tied, 16kW, a bit weird looking (mostly east-west panels, designed for post net metering). On a really good, cool, clear, windy spring day, I can produce about 105kWh out of the system from sunup to sundown.
On a really bad, low, grey, winter day, that same system produces about 2kWh. It's literally a factor of 50 difference. And we can get several days of that in a row. I have a backup generator that I use for my office on weeks like that (separate, off-grid system).
"Winter, electric heat on solar" is a fairly good filter for those who have read about solar or pondered it, and those who have a few years of it under their belt. The second group are the ones who've tried it, discovered just how horribly it doesn't work, and are trying to convince the first group that, no, really, insert well worn idea here really doesn't fix the problems with it.
I guarantee if it were easy, people would be doing it. That you won't find any serious off grid system doing electric battery heating is because it doesn't work.
Adding on to this the energy usage of my multi-head heat pump system has been quite surprising when it drops below freezing. I have a Mitsubishi unit which is rated down to something like -15F however below 32F it uses enough energy to get pretty close with fossil fuels on price. It looks like I will have to add on to my 11.85kW solar system to be able to get the projected energy use throughout the year. Winter time will have a significant deficit and summer overproduction will make some of it back but not all. I am also investing in air sealing and additional insulation.
Have you priced LTO? They're rather staggeringly expensive for their capacity. The cycle life is excellent, but they excel in very high C-rate applications (10C-20C), which is the opposite of what you need for off-grid systems. If an off grid system is getting anywhere near C/10 discharge rates, you're going to be in the dark awfully soon.
It's old tech, it's boring, but it works fine... flooded lead acid. Don't fully discharge it in the winter, and they won't freeze. They'll cycle just fine in the cold. Well tested, proven, and unless you're full time living and heating an interior space that can be lithium-friendly, they're the right option. Disconnect all the loads and run them on float in the winter with a high, vertical, south facing panel (obviously swap that if you're in the southern hemisphere), and they'll do just fine.
Yeah, lead wins hands down. This is a few years out of date now, but when speccing out my system I did a bit of analysis of battery types. We went with OPzS, and so far, so good.
Recycled Lithium Ion is much cheaper still, but more risky (and less maintenance). You'll have to do some pretty fanatic inbound QA to determine whether all your cells are safe to use. but if you do that and fuse your cells individually you should be able to make that work.
>> 1) These Controller+Inverter combo units require a battery to turn on
Which is why you want to install disconnects between your battery bank and the electronics. In a worst-case scenario of total discharge you can then bypass the dead batteries, hook the control electronics up to car batteries if necessary (be careful about voltages) and perform some diagnostics or at least read the logs of what went wrong. But the controller+inverter should be running if supplied with voltage from solar/wind/generator regardless of how the batteries are doing.
> But the controller+inverter should be running if supplied with voltage from solar/wind/generator regardless of how the batteries are doing.
Most of them don't operate that way. They need the battery bank connected first, and run off the battery bank side, while controlling the panel side.
Some may tolerate that sort of recovery from deep discharge, but it's not a safe bet, and some controllers are pretty clear that if you connect the input side before the batteries, you'll damage the controller.
A dumb PWM controller stands a better chance of doing this, but something like an MPPT controller, unless it's explicitly designed to fail into a PWM configuration at low battery voltage, won't - they need power to drive the buck converter circuitry (the controlling ICs and oscillators and such) in order to get power out of the panels.
Have you considered a small backup generator? If it exists I would go with one that is propane powered. Propane goes to -50° before freezing so unless you live in Alaska, you have quite some margin here.
You need to manually set the low voltage disconnect so that your batteries bms does not go into safety mode. then you won't have the first problem you stated.
What you need is more insulation. I've tested that battery and the internal heaters work great, but you should put it somewhere where the heat that is created stays in the battery.
You can dig a 2 meter deep hole and store the batteries there. It won't freeze during the winder if left turned off. If you plan to turn it on on the holes, you might need to plan for heat dissipation though.
LiFePO4 batteries can be stored and used below freezing, they just shouldn't be charged. All you have to do is turn the system off when you're not there.
I used the Solar Controller+Inverter Combos recommended on this site (Growatt) but ultimately switched to ones by Renogy. The Growatt one I purchased has a wifi adapter and app - wifi requirement was a deal breaker when remote. The renogy one has a Bluetooth adapter and app, plus feels better designed (though actually I think the internals are all pretty similar).
There are a couple surprise gotchas I didn’t know:
1) These Controller+Inverter combo units require a battery to turn on. This means: if your battery runs completely out of charge, you have to get electricity from somewhere to charge your battery a bit before the Controller+Inverter will turn on and charge your battery again. I was pretty annoyed at how not “off grid” this felt.
2) Temperatures below freezing are bad for batteries. I purchased a Renogy 48v smart battery with self heating. Unfortunately less sun in winter means less energy, and heat production is energy intensive. I’m not at the cabin 100% of the time so I have to take the battery with me when I leave to avoid spoiling/freezing it. Still not sure how I’ll ultimately get around this.