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Off grid cabin without batteries

ms314

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I recently built an off grid 500 square ft cabin in the mountains and I'm looking to install some solar on a relatively tight budget. The cabin is unattended for months at a time and can get down to -20F in the winter, so I would really like to avoid leaving large batteries there (I would also like to avoid the expense). The cabin has a wood stove for heat, so the main electrical loads would be a heat pump (primarily for cooling in the summer), minifridge, 1-2 burner cooktop, and small lights. I also hope to eventually add an EV charging station. It's insulated nearly to passive house standards, so I imagine there would be no problem running cooling systems only when the sun is out. Cooking and EV charging could also be adjusted to accommodate the sun. I figure roughly 3-4kW of solar would allow level 1 EV charging plus other loads when the sun is out.

I'm thinking of using Enphase microinverters to allow for a modular system. With microinverters on each panel, could the panels be wired straight to a breaker box or would I need any other electronics in between?

I realize that power will shut off briefly every time a cloud passes. Are there any problems associated with supplying appliances and EV chargers with intermittent power? I'm worried the contactors on the EV will wear out if they are frequently switching between open and closed. Also not sure how it would affect heat pumps or other appliances.

Will there be a big voltage drop when a load is turned on? Would this cause the load to detect a power supply failure and stop charging? This is what happened when I tried to wire my 12V Dewalt tool battery charger straight to a solar panel, and the charger wouldn't give power to the batteries. I imagine without an intermediary battery the voltage will drop anytime a load is connected. Is this still the case with microinverters on each panel/is there any way around this?

I'm a college student with a few EE classes under my belt but still have a lot to learn. Any advice is much appreciated!

EDIT: Based on the responses, seems like I'll need battery storage at least for an hour or so of use to prevent voltage drop and provide steady output with clouds. I'm hoping to keep the budget under $10k for the system if possible (without including loads/appliances). If I'm using a battery, it seems like microinverters are not the way to go. What are your thoughts on running everything off a single large inverter, vs DC appliances that run directly from the battery with possibly a smaller 2-3kW inverter to power the EV charger? Also, for the size battery that I'm looking at (probably in the 3kWh range) would it be worth it to spend +/- $1000 on a root cellar to keep lithium batteries happy, or should I just go with SLA? Any recommendations for good batteries? Anyone have experience storing lithium batteries underground? Thanks so much for the help!
 
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Far as I know micro inverters do not work without grid. They are grid-tie, unless there's something new.

I'm just going get straight to it, you're asking for a huge headache trying to get these loads to work without a battery.

Do yourself a favor a get enough battery to keep your loads going 30mins - 1hour at least.


EDIT: looks like some Enphase do have off-grid mode. So I stand corrected.
 
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It might be possible but the headaches would likely be endless.

If you were to share your budget I bet some folks would be able to work backwards from that number and rough out standard system that might work for you.
 
“ The cabin has a wood stove for heat, so the main electrical loads would be a heat pump (primarily for cooling in the summer), minifridge, 1-2 burner cooktop, and small lights.”

Couldn’t you just use propane for cooking?

A camper fridge can use propane. And some can also take gas and electricity. Probably can pick up used ones I’d imagine. An example: https://www.amazon.com/Smad-Electri...Fridge/dp/B01M3X2BDJ/?tag=cabinguides-20&th=1
 
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Sounds totally ass-backwards especially since you plan to have pretty big loads like mini-split and cooking stove.
at minimum I would install couple of lead acid batteries or work out solution to keep LiFePo batteries happy at winter(complete disconnect to store them or dig undergound bunker to keep them warm.)
 
It might be possible but the headaches would likely be endless.

If you were to share your budget I bet some folks would be able to work backwards from that number and rough out standard system that might work for you.
Sounds like I'll probably have to get some batteries. I was hoping to keep it under 10k for the system (if that's reasonable). Any recommendations on batteries that last withstand those temps (maybe I'll post in that forum as well). I also have thought about burying a lithium battery in a root cellar which would add 1k ish but might be worth it.
 
It would be nice if you could charge your EV directly from your DC PV/battery system.
Or have you identified unavoidable control issues that makes your EV's inverter necessary?
 
It would be nice if you could charge your EV directly from your DC PV/battery system.
Or have you identified unavoidable control issues that makes your EV's inverter necessary?
There is no current solution allowing this. Most EVs run at 360-400 VDC and require specific control communication from the EVSE/charger. It's not a simple task.
I don't want to say it is impossible, but if you have an example, I'd love to hear it.
I'm currently charging me EV though multiple conversions. PV(DC) > Optimizers > Grid tie inverter (AC) > EVSE > onboard charger (DC)

I'm hoping to keep the budget under $10k for the system if possible (without including loads/appliances).
I think you're in the ball park if you're doing the labor.
If I'm using a battery, it seems like microinverters are not the way to go.
I'd go with charge controllers to the battery. Or a mix of charge controllers and micro inverters, but that means you need to buy an inverter that supports AC coupling.
Probably best to go full DC with charge controller and an inverter.
What are your thoughts on running everything off a single large inverter, vs DC appliances that run directly from the battery with possibly a smaller 2-3kW inverter to power the EV charger?
DC is going to be more efficient as you avoid the DC>AC conversion and related inefficiency. If your buying appliances it might be worth looking at the cost difference for 15% more battery vs DC appliances.
Also, for the size battery that I'm looking at (probably in the 3kWh range) would it be worth it to spend +/- $1000 on a root cellar to keep lithium batteries happy, or should I just go with SLA?
I'd go lithium for the extra life and depth of discharge allowed. But, you hit the nail on the head with the temperature concern. You could go with lead acid to get the temperature tolerance.
Any recommendations for good batteries? Anyone have experience storing lithium batteries underground? Thanks so much for the help!

If you're really looking at EV charging, it probably need some time and calculations spent on charging speed and mileage requirements. A standard level 1 (120 vac) charge puts 1000-1400 watts (~4 miles) into the EV per hour.
 
Sounds like I'll probably have to get some batteries. I was hoping to keep it under 10k for the system (if that's reasonable). Any recommendations on batteries that last withstand those temps (maybe I'll post in that forum as well). I also have thought about burying a lithium battery in a root cellar which would add 1k ish but might be worth it.

A battery to consider for cold temperatures and less usage/cycling:

 
There is no current solution allowing this. Most EVs run at 360-400 VDC and require specific control communication from the EVSE/charger. It's not a simple task.
I don't want to say it is impossible, but if you have an example, I'd love to hear it.

Inverting the power just to charge a DC battery seemed wasteful to me as well. I watched this video a couple days ago where someone got their leaf to charge directly from solar with a DC-DC boost and a circuit board that controlled the contactors.


It's a bit beyond my technical ability, but seems like another reason to start with all DC appliances, so if some type of low power DC charger becomes available in the future I could add that in (I'm not looking to add an EV charger for at least a couple years).
 
I suppose that I should have said there isn't an off the shelf solution to DC charge with solar through the vehicle's DC charge port.

Most people aren't up for the task of modifying the high voltage contactor controls. It's quite possible to run 10x ~40 volt VMP panels on series and put them right into a custom box on a modified car, no need for a DCDC, the panels would be close enough to pack voltage already.

We're quite far off topic for the OP...
 
Wow, that's new to me.
Looks like a $30,000 (minimum) closed ecosystem where you need their optimizers, inverter, charger, and possibly batteries.

Also, I don't think any vehicles (save for maybe the F150) currently support bi-directional through the charge port.
 
might be possible but the headaches would likely be endless.
I would disagree with that statement empirically.

In four years my only headaches have been too many cloudy days and water ingress in MC4s- neither of which have anything to do with the batteries.
 
The biggest challenge here is the level 1 EV charging, which is 12A at 120VAC. Everything else listed is quite doable with a smaller scale system. I'd say go with a system based at 48V.

First the batteries. Assuming you can live with minimal nighttime resources, you could make do with lead-acid golf-cart batteries. That's what I started out with. They work in the cold with no special pampering. Go to Costco and get eight of their 99$ golf-carts. That will be 210Ah at 48V. Even draining only 50% max, that's still 5kWh of nighttime power.

To keep them happy, charge at 1/8th of C. That works out to be (210Ah/8) X 50Vcharging X 1.175fudgefactor = 1542W of panels. I'm seeing REC260W panels on Craigslist right now for 65$. They are quality panels and I bought some already. Six would be <400$.

1500W though most likely will NOT support 1440W going into an EV. I use a 2X rule for everything I design. If you need 1440W of power, you need at least 2880W of panels. Here's the problem. You need 2880W of panels to RELIABLY make 1440W of power, but that's too much for a little battery like the golf-cart. What you might want to do is place your panels on rotating mounts like I use, and keep one pointed SE, and the other pointed SW unless you are EV charging. Reducing the noontime current will keep you from melting your small batteries.

I like Schneider inverters, and I could recommend the 48V version of what I already have, the Conext 4048. I'm happy with mine, and it will put out both split-phase 120/240V AC, and also has generator input charging. You'll need at least a 60A charge controller to go with that. I use Midnight.
 
I would disagree with that statement empirically.

In four years my only headaches have been too many cloudy days and water ingress in MC4s- neither of which have anything to do with the batteries.
That's a big word thus it's not in my vocabulary. :)

OP doesn't want batteries. It's pretty clear that I was warning the OP that going off grid without batteries is a non-typical application and it's likely going to be more trouble than its worth. Now, almost a week later, you're telling me (why not the OP??) that batteries aren't a problem. Um... so I agree with you "empirically". Did I use that word right? Please take your grievances up with the OP since we're on the same side.
 
IME, the only thing I would run off PV directly are things designed to, like a PV water pump. I can charge my cell phone off a 100 watt panel and a buck converter to stabilize the voltage, but a half amp at 24 volts charging is not a lot.


Bigger than tiny wattages becomes an issue as power starts to cycle on and off for clouds and surges. Comes down to, is the equipment able to stand this or not? I would not want to hook something to this.

There is not a single commercial off the shelf, off grid application for a solar generator without a battery. Not one. If you dig deep enough on YouTube you will find one, but they are not longer in business.

There’s been a lot of brainpower put into these projects by DIY communities, but never has it been successfully done, at least to a standard of living in a house like a running AC. There is Will’s video where he is able to run an AC off solar directly, but this is not left on all day long, only shows it running for a minute.

I have a solar 300 watt generator with a small battery pack, started with around 600 wh of batteries, with these batteries acting as a buffer for these cloudy events. You can do a lot with a “reduced” battery pack with less capacity operating towards peak output, if you can go without power or have backup for several days but run large loads during the sunshiny days.
 
MPP Solar LVX6048 Hybrid inverter which can run off-grid and batteryless? It's not modular but at least matches the batteryless requirement plus EV charging presuming there are enough panels and sun.
 
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