Help for beginner with small cabin solar

crisb67 · February 1

I hope I'm putting this question in the right place! As a novice, I need advice from folks who aren't trying to sell me something.
In our cabin, we have a small harbor freight solar system running a few lights/stereo/phone charger with no problem. However, we want to stay up there more regularly so will need more power. We estimate we'll use about 400 amp hours a day. We're located in northern Idaho so temp gets into the teens and lots of gray winter days.
Does anyone have suggestions for what type of battery (acid v lithium), size of charge controller, and how many panel we should get. We're on a budget but don't want something that's not going to last a while. I've done a ton of research but I'm still not sure I'm getting it! 😑
Also, any thoughts on solar generators? Would that be a better way to go?
Thanks for any input!!

wellbuilt · February 1

I started off in a trailer with 2 golf cart battery’s 2 solar panels 500 watts 300 watt inverter 40 amp charge controller .

Battery charger and small genarator .

This system is running after 5 years but the battery need to be replaced .

You really need to figure out what you want to run .

Then how much power it will take .

If you want to run a frig you will need a larger system .

Every one will want to know what equipment you want to run and for how long so they could help .

My panels have been covered in ice for 3 weeks and I have feet of snow in the forecast so I’m on Honda power for the foreseeable future

BB. · February 1

Solar generators--Seems to be a marketing term for the "all in one" units (charger/battery pack/ac inverter+add solar panels). Typically I would tend to avoid a "solar generator"--Or at least do a lot of research first and make sure that it does meet your needs.

Starting with your loads--400 Amp*hours @ 12 volts? You really need to know your loads, and make sure you have done as much conservation as possible... Using as efficient appliances as possible (and use propane for cooking/hot water vs electricity--Although there are some applications that RV'ers are using such as induction cook tops)--It is almost always cheaper to conserve energy vs generating it.

Here is how the math works... I will do a typical relatively" conservative paper setup. 2 days of storage and 50% maximum discharge--Seems to be "optimum" for lead acid battery solar/off grid power systems.

Sizing the battery bank--400 AH @ 12 volts (basically battery bank is ~4x larger than the daily loads):

400 AH * 12 volts = 4,800 Watt*Hours per day (combination of 12 VDC and 120 VAC loads?)

4,800 WH per day is a lot of energy for a "cabin"... I suggest that a very efficient cabin/small home can be a "near normal" electrical existence (LED lightning, energy star refrigerator, RV or small well pump, clothes washer, laptop, cell charger)... But lets go with your numbers (this is your cabin, not mine

)

4,800 WH per day * 1/0.85 inverter eff (drop if all DC) * 2 days storage * 1/0.50 max planned discharge 1/12 volt battery bus = 1,882 AH @ 12 volt battery bank

That is a lot of battery bank @ 12 volts... I would either look more at 3,300 WH per day and a 24 (or 48 volt battery bank) or at a pure 48 volt battery bank and run everything from a 120 VAC inverter:

4,800 WH per day * 1/0.85 inverter eff (drop if all DC) * 2 days storage * 1/0.50 max planned discharge 1/48 volt battery bus = 471 AH @ 48 volt battery bank

I suggest that when you get in the 800 AH or larger battery bank, you look at going up to the next higher bus voltage.

Just to give you an idea of a "less expensive" system... An L16 deep cycle 435 AH battery @ 6 volts (8 in series for 48 volt bus) is pretty close:

https://www.solar-electric.com/trl16vo435ah.html

You can also go with 6 volt @ ~220 AH "golf cart" batteries... 8x 6 volts in series gives 48 volts. 2x 220 AH parallel strings give you 480 AH. Probably about the cheapest and will usually give you around 3-5 years of life (the L16 should last a few years longer)--However, pretty much everyone here has "murdered" their first bank or two with poor maintenance, over discharging, etc....

Flooded cell lead acid deep cycle batteries are about the cheapest / most forgiving for battery banks. And Lead Acid works pretty well in very cold climates (keep them relatively charged, and they won't freeze). AGM are a form of lead acid--Also nice, no electrolyte to check, clean--But tend to be more expensive and (perhaps) a bit shorter life). LiFePO4 are the typical Li Ion for solar power--Very close to the "ideal" battery--However, you cannot run them (charge/discharge) below ~40F (have to keep them warm when cycling).

For sizing the solar array--We do two calculations. 1) is based on the battery bank capacity and rate of charge; 2) is based on location and amount of sun per day that you get on average.

For charging, typically 5% to 13% rate of charge works well... 5% for "weekend" and summer usage can work. 10%+ for full time off grid can give better battery life and reduce backup genset usage/shutting down loads during bad weather):

471 AH * 58 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 1,774 Watt array minimum
471 AH * 58 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 3,548 Watt array nominal
471 AH * 58 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 4,612 Watt array "typical cost effective" maximum

And there is sizing the array based on the hours of sun per day you get, and when during the year you are using the cabin (sunny weather, 12 months a year, etc.). For a fixed array facing south:

http://www.solarelectricityhandbook.com/solar-irradiance.html

Coeur d'Alene
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 42° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
2.12	3.30	4.37	5.04	5.24	5.53
Jul	Aug	Sep	Oct	Nov	Dec
6.07	5.88	5.11	3.71	2.33	1.88

Solar only works when you have sun... A "break even array" for December and 4,800 WH per day would look like:

4,800 WH per day * 1/0.52 off grid AC system eff * 1/1.88 hours of sun per day = 4,910 Watt array for "December" break even

So--That is the basic way to size the system. If your winter usage is less (refrigerator outside/cold weather uses less power, but you use more lights/hot coffee, etc.).

Normally, I would suggest that you plan your base loads (Lighting, refrigerator, a bit of water pumping) on 50-65% of the "predicted" harvest--And use the other loads (more water pumping, vacuum, washing machine, etc.) during sunny weather... And/or use your backup genset.

Sizing your backup genset for a 471 AH @ 48 volts would typically be 10% to 20% (or 25% max) charging current. I suggest only loading the genset to 80% of its rated output (unless you have a "real commercial" rated genset--Batteries can take "rated" power from a genset for 3-6 hours or so--Running a residential genset at 100% of rated output--They probably will not last long):

471 AH * 58 volts charging * 0.10 rate of charge * 1/0.8 AC charger eff * 1/0.8 genset derating = 4,268 Watt/VA min genset @ 10%
471 AH * 58 volts charging * 0.20 rate of charge * 1/0.8 AC charger eff * 1/0.8 genset derating = 8,537 Watt/VA min genset @ 20%

Just to give you an idea of sizing your AC inverter... If you have minimum AC loads (fridge, LED lighting, small water pump, etc.)--You are probably looking at 1,500 to 3,000 Watt AC inverter... And for a 471 AH @ 48 volt flooded cell battery bank--I would suggest a max inverter around 2,350 to 4,7100 Watt max AC inverter.

The above is a pretty "balanced" system for a Lead Acid battery bank... If you are not there full time, you could probably get away with a smaller array and/or battery bank.

Batteries tend to be expensive and easy to "murder". Installing a larger/oversized solar array will generally help keep the battery bank "happier". Don't want to over discharge/under charge your battery bank--That is really hard on them.

Before we get into details of picking the equipment and battery chemistry/etc.... What do you think of my above guesses at your system sizing (based on 4,800 WH / 400AH @ 12 volt(?) per day) and loads?

None of the above is written in stone... Solar power calculations are within +/-10%. And changes in how you use power (perhaps you work from the cabin and use more power during the day and little at night--Or you can vary your usage based your sun that day--Less sun, less power usage)... That can all affect your choices.

-Bill

mike_s · February 1

> small harbor freight solar system running a few lights/stereo/phone charger with no problem. However, we want to stay up there more regularly so will need more power. We estimate we'll use about 400 amp hours a day.

One of these things is not like the other. How much open space do you have to install solar panels? What's your budget? And, "amp hours" is pretty meaningless unless you also mention a voltage, because Ah doesn't measure energy..

Photowhit · February 1

crisb67 said:

I hope I'm putting this question in the right place! As a novice, I need advice from folks who aren't trying to sell me something.
In our cabin, we have a small harbor freight solar system running a few lights/stereo/phone charger with no problem. However, we want to stay up there more regularly so will need more power. We estimate we'll use about 400 amp hours a day. We're located in northern Idaho so temp gets into the teens and lots of gray winter days.
Does anyone have suggestions for what type of battery (acid v lithium), size of charge controller, and how many panel we should get. We're on a budget but don't want something that's not going to last a while. I've done a ton of research but I'm still not sure I'm getting it! 😑

Great that you are asking questions now, before buying a bunch of stuff...

I worry about your expected energy use, 500 amp hours, I'll assume at 12 volts, is only 6000 Watts (watts is a measure of power). That is a large load for a solar electric system during winter. See Bill's calculations above.

I live on about 3-4000 watts in the winter months and it's difficult with long periods of overcast weather to maintain battery health without a generator. So let us look at your loads and see if we can suggest alternative energy or systems to cut those down before suggesting a large system with a generator.

crisb67 · February 1

Maybe I'm not saying it right. We'd only be adding a small fridge and small chest freezer. I talked at great length to a fella at battle born batteries and that's where I came up with that number. I researched what we'd need and thought getting a bigger inverter, couple more panels and 1 more lead acid battery would do the trick but after used it for about a week the alarm on the charge controler went off and our batteries were quite depleted. And that was without the freezer. The man said it was because of the size of the inverter I got. Which I thought I needed to handle the surge.
I appreciate all y'all's knowledge...it is impressive!

crisb67 · February 1

The battle born rep said we would ideally need 3 - 100 ah batteries ($900 each) a larger charge controller ($750), and 3-4 new 300+ watt solar panels (which they don't sell but run just under $200 each). It's a lot $ so I'm just trying to make sure it's what we really need.
Thanks for your help!!

Photowhit · February 1

Weather can require a generator this time of year unless you have planned for a considerable large battery bank to make it through the long periods with little charging.

crisb67 said:

"...thought getting a bigger inverter, couple more panels and 1 more lead acid battery would do the trick but after used it for about a week the alarm on the charge controler went off and our batteries were quite depleted. ..."

I'm not sure what you are saying here?

So you added a battery and a couple more panels?

Did you also add a charge controller? If so what did you add?

Do you know what the energy star rating is for your fridge?

Most fridges will use 1000 watt hours or more each day, even small ones. If you look at your regional daily solar insolation (Bill looked this up for you) You are only averaging 2 hours of direct sunlight a day. So if you wanted to run an efficient fridge you would want enough array(solar panels) to produce 1000 watts of energy each day to break even. I could do the math, but losses for energy use from stored lead acid batteries in a solar electric system is roughly 50% efficient, losses include the inverter conversion, the battery efficiency and the solar panels derating. So figure 2000 watt hours needed from the 2 hours of direct sunlight, so about 1000 watts of array.

Now understand that the sun doesn't shine everyday! So you need storage to get you through the overcast days. So a 2000 watt load converts too about 2000watthrs/12volts=166 amphours of storage for each overcast day you need to supply energy from your battery bank. A pair of golf cart batteries store about 210 amp hours of energy at 12 volts.

A solar electric system MUST be inefficient! You need to produce enough wattage so that the batteries are fully charged a couple times a week.
=================================================================================================
Now things that may be a problem, inexpensive inverters are typically modified sine wave and will use more wattage with motors like a compressor. 12 volt inverter with higher wattage tend to need large cables to prevent voltage drop. They may also use more energy idling than quality inverters.

You don't want to add new batteries to old, and want batteries to be the same size so they share the load and charging equally.

The nice thing about charge controllers, is they are amperage based, so a 60 amp charge controller can handle roughly a 1000-1200 watt array at 12 volts, and 2000-2400 watt array at 24 volts...

More later, feel free to ad info on system and loads...

crisb67 · February 1

Gosh thank photowit! We only have 200 watts of solar panels so I know that's part of our issue. I bought a Jupiter 2000 watt inverter which is the modified sine wave. We added a new lead acid battery (I know now that was a mistake) We have not bought a new charge controller yet. I got the info off the back of the fridge for the fellow at battle born. It said 110 w power input rated current 1.4 amp voltage 115 v ac...if that makes sense?
I'm sorry I dont understand this better. I am trying and we'd like to stop paying rent by moving out to our cabin.

BB. · February 1

Just to try and make things a little easier to understand:

Watts is a "rate" like miles per hour... 100 Watts ~ 100 mph.

Watt*Hours is an "amount" like miles driven:

100 watts * 5 hours = 500 Watt*Hours
100 mph * 5 hours = 500 miles driven

Amps and Amp*Hours is like Watts and Watt*Hours... The difference is we are "missing" the voltage for Amps and Amp*Hours to know about your loads/etc...

100 amps is a rate--But it could be at 12 Volts DC (like your battery bank) or at 120 VAC (like the output of your AC inverter/utility power)
Watts = Volts * Amps
Watt*Hours = Volts * Amps * Hours (time)

For example, 1.4 amps for your refrigerator (I know it is 120 VAC--just an example of the confusion just working with Amps and multiple voltages in an off grid Battery+AC Inverter system):

12 VDC * 1.4 Amps = 16.8 Watts
120 VAC * 1.4 Amps = 168 Watts

And there gets to be more confusion in AC power (Watts vs VA, power factor, cosine, inverter losses, etc.)... But we can skip most of that for now.

More or less--You are probably paying somewhere around $0.10 to $0.40 per kWH for your utility power ($0.40 if you live in Hawaii or California)... And off grid solar power costs around $1-$2 per kWH (very roughly)... And somewhere around $1.00-$2.00 per kWH for fuel costs running a genset (very rough numbers)...

As you can see, off grid power is something like 5-10x (again very rough numbers) the cost of buying power from your utility... So knowing/measuring your daily loads and getting the smallest/most efficient loads (such as LED Lighting, Energy Star Fridge, etc.) can save you a lot of money (if you have not done "conservation" before--It is not that difficult to cut your electrical usage by 1/2).

For example, your refrigerator... 110 Watts @ 1.4 amps @ 115 VAC... Most refrigerators/freezers run somewhere around a 25 to 50% duty cycle--I.e., in hot weather, your fridge may run for 30 minutes out of every 60 minutes. With the above numbers:

110 Watts * 0.50 duty cycle * 24 hours per day = 1,320 Watt*Hours per day (assuming warm weather, average usage--A realistic number for a relatively efficient refrigerator/freezer)

And the battery bank:

1,350 WH per day * 1/0.85 inverter eff * 1/12 volts * 2 days storage * 1/0.50 max discharge = 529 AH @ 12 volt FLA battery bank
"golf cart" batteries: 2x 6 volt golf cart batteries in series (12 volts) with 3x 200 AH parallel strings = 2*3= 6x GC batteries (6 volts @ 200 AH)--Just to run the fridge.

To run that refrigerator, you probably need a 1,200 to 1,500 Watt AC inverter minimum (standard refrigerator compressors take upwards of 600-1,000 Watts to start for a second or two). To run just the fridge in December from solar:

1,320 WH per day * 1/0.52 off grid solar eff for AC power * 1/1.88 hours of sun per average December day = 1,350 Watt array "break even"

And, to account for some bad weather, you should only plan on using 50-65% of your predicted harvest for your "base loads" (i.e., refrigerator--You cannot turn it off for a few days of bad weather).

1,350 Watt array (Dec Break Even) / 0.65 bad weather "fudge factor" = 2,077 Watt array to run fridge "full time" in December and some other loads when it is sunny (washer, vacuum, TV, laptop, etc.).

Or--You can go with a smaller solar array and battery bank, and run a (for example) Honda eu2200i "inverter-generator"---It will take about 2 gallons of gasoline per day to run your fridge and other (relatively light) loads (something like a 300-400 Watt average load, upwards of 1,800 Watts at full power when needed (microwave, etc.). And pay something like $2-$4 per gallon for gasoline/diesel/propane/etc.

Cold weather and fuel storage also are issues that you will have to look at too... Propane may not vaporize well in cold weather. Diesel can be hard to start. And even gasoline engines like some "pre-heat" to make starting easier in deep cold climates.

You are (in the above example), of sizing a larger battery bank + solar array to run (mostly) from solar even deep into winter--Or designing a system for 9 months of solar and 3 months of solar+genset. (if you have trees, in mountain valley, etc.--The solar harvest can even be worse--No sun/shading on array--No solar harvest).

The name plates on appliances are difficult to use in planning your loads... They are not usually accurate and/or do not supply Watts/WH on how the appliance is used.

Getting a Kill-a-Watt meter (or similar) for 120 VAC plug in appliances is a great start. You measure the loads for the appliances in your home, put it into a spread sheet (daily loads) and then "do the math".

https://www.amazon.com/s?k=kill-a-watt

I understand the draw to your cabin--And looking at reducing your costs (no rent, no utility bills, lower taxes for the cabin--I hope)--But look at your power costs/energy usage for the cabin--And taking into account you are your own "utility company" now. You have to plan/buy/maintain the equipment (batteries typically last 3-7 years, electronics start failing at 10+ years).

There are things that can be done to help reduce costs (whether at home or at the cabin)--Using propane for heat/cooking/hot water... And for cool weather, using mini-split heat pump type units (they can provide heat more efficiently down to when there is some snow on the ground vs electric heat)...

Energy usage is a highly personal set of choices that you make... What I suggest may or may not work out for you and your family.

Ask lots of questions--Think about seriously "minimizing" your energy foot print (if you are trying to conserve money).

My suggestion is to aim for 3.3 kWH per day (3,300 WH per day) or ~100 WH per month. The average North American home uses around 500 to 1,000 kWH per month. And getting to under 200 kWH per month took some work on my part (and we have natural gas for cooking/heating/hot water--And I bet my usage has ratcheted up a bit over the decades of family ).

Thinks like using a laptop computer at 30 Watts vs a desktop computer at 300 Watts:

30 Watts * 10 hours per day = 300 WH per day
300 Watts * 10 hours per day = 3,000 WH per day

Smaller loads that run for many hours per day can really hit your solar usage... A microwave running 15 minutes per day at 1,400 Watts:

1,400 Watts * 1/4 hour = 350 WH per day

Or a small laptop and microwave can use the same amount of energy per day... And a desktop could use 2x more than your refrigerator.

Do not start buying any hardware until you have nailed down (or at least have a better estimate) of your energy usage.

-Bill

Photowhit · February 1

crisb67 said:

I am trying and we'd like to stop paying rent by moving out to our cabin.

That's the reason many of us start with the idea, generally off grid solar is MORE expensive than the grid. Is the grid available?

I suspect you are heating with wood or gas, as well as water heating/cooking. I recently proposed a bare minimum system for a fridge, laptops and lights for a someone with a better year round solar insolation, roughly 3 hours a day average in winter.

They wanted to start as inexpensively as possible and grow a system, so It would sort of work for you. Remember that growing a system will be somewhat more expensive than starting with one to size, but an idea of the rough costs now would likely help.

Samlex makes a nice 1200 watt inverter/charger in 24 volt. This would allow you to charge the battery bank with a generator through the inverter/charger. It runs $600, Northern Arizona Wind and Sun doesn't carry it, but might price match if you talk with them (They sponsor this forum)

https://www.donrowe.com/samlex-evo-1212f-hw-pure-sine-inverter-charger-p/evo-1212f-hw.htm

You will likely want a 2000 watt array or larger to minimize winter generator run time, but if you can charge with the generator you can look for deal on solar panels in your area or online. Shipping is often very expensive for large solar panels. I would suggest a minimum array of 600watts so you could just run the generator for a couple hours in the morning and the solar would likely top off on sunny days. BUT I would go ahead and get a larger charge controller so you wouldn't need to replace it for a larger array. The FM80 has a reasonable price now at under $500 and should be able to handle an array of 2000 watts. ;

Outback FM80 Charge Controller | Northern Arizona Wind & Sun (solar-electric.com)

I'm a fan of Midnite's Solar classic 150, which Would be fine for a bit larger array but runs a tad over $600 for the solar only version;

MidNite Solar Classic 150-SL Controller | Northern Arizona Wind & Sun (solar-electric.com)

Since you are in northern Idaho, I seriously suggest stopping in at Backwoods solar. They are located in Northern Idaho at;

1589 Rapid Lightning Rd, Sandpoint, ID 83864. They won't have great deals other than their bargain basement, but are local and will have knowledge in your area, which will make a big difference in suggesting systems for your environment. Ask for their catalog/learning manual! I has helped a lot of people figure out things, I think I got one 25 years ago, maybe longer!!!

Off-Grid Solar Electric Systems | Solar Panels | Solar Energy Shop (backwoodssolar.com)

So with a inverter/charger and a controller and a small array, you could work on your battery bank. You could likely get by with 4 golf cart batteries for the next 4-5 years... remember they age together, so you won't want to upgrade until they are near dead.(To be cost effective) You can add solar panels to your array too reduce run time on your generator...

Sam's club and Costco have golf cart batteries for around $90 each. They are very generous on cores! I've used lawn tractor batteries as exchange/cores!

Here is my current build, costs from about 2012, I got good deals and costs will be roughly the same other than solar panels.

It's going to be close to cost effective! or Off grid at 26 cents a KWh! — northernarizona-windandsun (solar-electric.com)

The usual suspects for cheap solar panels are;

SunElectronics

Sun Electronics – World's Lowest Solar Panel Prices

and Santan Solar

Wholesale Solar Panels in Phoenix, Arizona (santansolar.com)

Also there is a 26% federal tax credit (against any tax liability) for the next 3 years. But read carefully when looking at these solar discounters. used panels should not qualify (I've never heard that said directly, but IRS frowns on double dipping)

wellbuilt · February 2

Well here is the Reality at my Mountain cabin.
I’ve been scraping on these panels for a hour or so ☹️ Yesterday
If it warmed up to 20o today I may of cleared 3 or 6 panels and had some charging , but I got 12” of snow today ☹️ Chances are I won’t see these panels till sometime in March .

Soooooooo

I have a flex power one system from outback
Since I have Limited electrical experience This was the easiest route I bought the system from AWS
my panels 15 295 watt panels and mounting iron ridge 1000 came from my local supply house .

I bought 16 golf cart battery’s from sams club for 89$ each plus core charge .

I bought good 2g wires with lugs on line pre made , then built a battery box with vent in the garage .

The system is 48volt and the battery’s are 2 banks in series parallel . For 430 ah
The 3600 watt inverter draws a lot of power just being on but I can charge thru it at 17/2000 watts .

I have a2800 watt Honda that charges my battery bank and runs the house I use about 5 gallons of fuel a week with no sun .

My place runs on 300watts day or night .

I have a fridge and small freezer going and can run the house for 2/3 days if I use search mode on the inverter over night .

The cabin is heated with a wood stove .

I have 1300 sq feet of hot water radiant heat in a concrete floor but it’s not hooked up yet .

I have a 735, well but no pump yet
so I bring water up in a 275 gallon tote or collect it off my deck roof .

The system just works .

I water the battery’s 4 times a year , it takes about a hour .

I can run from aprial to December with out running a genarator .

February and March has plenty of sun here if there is no snow on them .

This is very doable and you could get a smaller system to save some money but I think you are better off with extra power .

I have 10k in my system with a extra 12 panels and fm80 charge controller

Help for beginner with small cabin solar

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Coeur d'AleneAverage Solar Insolation figures

Coeur d'Alene
Average Solar Insolation figures