Off Grid Cabin in Ohio

Toonces

Hi Everyone,
My last post was two years ago, inquiring about a small solar powered system for my cabin. I am finally in a position where I would like to start getting the components together for my system, but just want to go over the design with you all again before proceeding. Last time I was trying to do the math myself and probably made it more confusing than if I would have stayed out of the way, and let the experts just handle it. So this time I will just provide the facts and let you all decide what I should do. I thank you all in advance for any advice you may have toward starting me down the correct path! Here are the facts..


Cabin Decription: SE Ohio, 200sq/ft, used 3-4 days at a time every 3 to 4 weeks. Used in most seasons. No possible grid power.
Solar Insolation : 3.5hrs
Solar Array Distance from Cabin: 140' one way (poles in the ground already)

Current Power Sources: (2) Trojan T-105 6V 225Ah batteries in series (charged at home).
(1) cheap B&D 400W inverter.
(1) 3500W Yamaha gen (for power tools, vac, heater,window ac unit).
(1) 20lb propane tank for propane heater
Note: The current batts and inverter to be replaced with whatever we determine is best.

My Current Loads:
AC Loads: (5) 13W CFL bulbs used roughly 4hrs per day.
DC Loads: (1) Car stereo w/(2) 50W outdoor speakers used roughly 8hrs per day.


My Future Additional Loads:
DC Loads: (1) Shurflo 2088 demand pump 84W to be used maybe 1hr per day for shower and washing dishes.
DC Loads: (2) more 50W speakers for inside the cabin.


Additional information:
I want to do things right, so lets pretend money is no object.
I am fairly certain I will be using a MPPT CC to maximize pv power.
Last time we discussed using a 24 volt system because of the distance from the array to the CC.
I was never really able to determine the power consumption of the radio because I kept forgetting to test it..stupid. Can we ball-park this?
I might have to scrap the car radio and pump, and switch these over to 120V units instead of running a 12V system.

Now the big question... How would you design this system from the ground up?

Any solid advice would be much appreciated!

I

BB.

Welcome back Toonces

Toonces wrote: »

Hi Everyone,
My last post was two years ago, inquiring about a small solar powered system for my cabin. I am finally in a position where I would like to start getting the components together for my system, but just want to go over the design with you all again before proceeding. Last time I was trying to do the math myself and probably made it more confusing than if I would have stayed out of the way, and let the experts just handle it. So this time I will just provide the facts and let you all decide what I should do. I thank you all in advance for any advice you may have toward starting me down the correct path! Here are the facts..

I will do the basic math based on my assumptions. Please feel free to ask questions/correct me if I get anything wrong.

Cabin Description: SE Ohio, 200sq/ft, used 3-4 days at a time every 3 to 4 weeks. Used in most seasons. No possible grid power.
Solar Insolation : 3.5hrs
Solar Array Distance from Cabin: 140' one way (poles in the ground already)

Using PV Watts for Columbus, OH, fixed array, tilted to 40 degrees from horizontal:

Month
Solar Radiation
(kWh/m²/day)


1
2.91


2
3.24


3
4.03


4
5.07


5
5.07


6
5.47


7
5.26


8
5.56


9
4.96


10
4.51


11
2.77


12
2.20


Year
4.26


3.5 hours of sun will cover about 8 months of the year... Solar panels are historically "cheap" right now. So, if you need more power, doing it all at once is pretty nice.

Reverse the questions--Start with loads first:

My Current Loads:
AC Loads: (5) 13W CFL bulbs used roughly 4hrs per day.
DC Loads: (1) Car stereo w/(2) 50W outdoor speakers used roughly 8hrs per day.

My Future Additional Loads:
DC Loads: (1) Shurflo 2088 demand pump 84W to be used maybe 1hr per day for shower and washing dishes.
DC Loads: (2) more 50W speakers for inside the cabin.

Check the Car Stereo--Many of them are not very energy efficient. You need to measure their power usage (DC current clamp meter, DC WH/AH meter, etc.).

Lets just guess that the stereo averages 8 amps @ 12 volts:

• 8 amps * 12 volts * 8 hours stereo = 768 Watt*Hours per day
• 5x 13 Watts * 4 hours per day lights = 260 WH per day
• 84 Watts * 1 hour water pump = 84 WH per day
• ============================================
• 1,112 Watt*Hours per day

I would suggest that you look very closely at the stereo power usage--If my guess is close, you can probably do better (find an old battery powered boom box or something?).

Battery sizing--Assuming 2 days of storage, 50% maximum discharge, 12 volt battery bank (1-3 days recommended range):

• Battery Bank Sizing: 1,112 WH per day * 1/0.85 inverter eff * 1/12 volts * 2 days storage * 1/0.50 max discharge = 436 AH @ 12 volts
• Solar Array sizing 5%-13% battery bank rate of charge: 436 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 821 Watt array "nominal"
• Solar Array Sizing (based on loads+hours of sun): 1,112 WH per day * 1/0.52 system efficiency * 1/3.5 hours of sun per day = 611 Watt array minimum

Not saying the above numbers are the right amount--Just a starting point based on nominal power assumptions and system design (10% rate of charge, 3.5 hours of sun, etc.). You could cut the above by 1/2 if more cost efficient (i.e., your 2x 220 AH @ 6 volt batteries)--But then you would only have 1 day of storage, or use only ~500 Watt*Hours per day.

Lots to discuss.

Current Power Sources: (2) Trojan T-105 6V 225Ah batteries in series (charged at home).
(1) cheap B&D 400W inverter.
(1) 3500W Yamaha gen (for power tools, vac, heater,window ac unit).
(1) 20lb propane tank for propane heater
Note: The current batts and inverter to be replaced with whatever we determine is best.

For weekend/seasonal/RV use, yes, you could get away with just 2x T-105 batteries. But perhaps an AC battery charger (with genset) if you use as much power as I have guessed (deeper cycled batteries will not last as long, but for weekend/seasonal usage--The two batteries should last you several years. And more genset run time depending on your actual loads.

Are you planning on using any solar panels at this moment?

140 Foot one way cable run for the solar array is pretty far. Say you get 3x 300 Watt panels (Vmp~30 volts and Imp~10 amps). Get an 80 amp MPPT charge controller (Midnite, Outback, etc.), put the three panels in series for Vmp-array~90 volts, Imp-array~10 amps, Voltage drop ~2.7 volts max (1-3% typical voltage drop). Use a generic voltage drop calculator:

http://www.calculator.net/voltage-drop-calculator.html

140' one way run, Vdrop~2.7 volts max, Iwire~10 amps:

8 AWG wire:
Voltage drop: 1.76
Voltage drop percentage: 1.96%
Voltage at the end: 88.24

The 3.5 kWatt genset may use a fair amount of gasoline (perhaps around 0.25 to 0.5 gallons per hour). Its efficient output power range will be around 50% to 80%+ of rated output (1.75 kWatt output). As the output load falls below 50%, the fuel flow will remain about the same. If your average loads (when genset is running) is less, then you may wish to look at a smaller (1.6 kW or so) genset and use 1/2 the mount of fuel.

Additional information:
I want to do things right, so lets pretend money is no object.
I am fairly certain I will be using a MPPT CC to maximize pv power.
Last time we discussed using a 24 volt system because of the distance from the array to the CC.
I was never really able to determine the power consumption of the radio because I kept forgetting to test it..stupid. Can we ball-park this?
I might have to scrap the car radio and pump, and switch these over to 120V units instead of running a 12V system.

Now the big question... How would you design this system from the ground up?

Any solid advice would be much appreciated!

Energy usage is a highly personal set of choices... I would suggest looking real closely at your radio/music needs.

A 1 kWH per day system is usually about the right size for a small cabin to power lights, radio, laptop, cell charger. The automotive radio is probably not a great choice.

If you go with 120 VAC devices, a nice TSW inverter like this one from MorningStar 300 Watt @ 12 volts would be a good choice (also has remote on/off, low power "search mode").

You might want to stay at 12 volts (if MorningStar inverter) and DC pump--There are some 24 VDC pumps available too. 12/24 VDC pumps are probably fine for what you need.

I have been using little rechargeable Bluetooth speakers (something like 5-10 hours on a USB charger).

Anyway--We can talk more details. Just a "nominal" stab at some numbers.

-Bill

Toonces

Wow, thank you so much for the incredible reply Bill!! I really appreciate the time you spent on the calcs, and having them all in one place makes things much easier.
The components you are talking about seem right on par with what I was hoping for. I do have a couple questions, just to make sure I don't paint myself into a corner.


The battery bank sizing you calculated is 436 AH @ 12V. I could get (4) new Trojan T-105's (6V 225AH), put them in two strings for a total of 12V 450AH.
1. Should I just forego the 6V batts and step up to a 12V batt in a single string to get better/more even charging?

It seems that my power usage is somewhat on the edge of needing (3) 300W GT panels vs just using (2) panels. I believe the usage stats I gave are worst case scenario.
2. Could I get away with using just (2) 300W GT panels for my current usage until I get into my future usage needs? And would I want to down-size the batt bank too?

3. Would there be any advantage to installing a 6 gauge wire instead of the 8 gauge you spec'd? The price difference is marginal, but it seems like the answer would be "no" since we are already maximizing the CC input. Just a thought in case I wanted to expand the system someday (beyond my future usage stated).
Note: I have 4" conduit in the ground already so I can run one or more sets of wires.

Thanks again for all the great help!
Doug

BB.

Toonces wrote: »

The battery bank sizing you calculated is 436 AH @ 12V. I could get (4) new Trojan T-105's (6V 225AH), put them in two strings for a total of 12V 450AH.
1. Should I just forego the 6V batts and step up to a 12V batt in a single string to get better/more even charging?

6 volt golf cart type batteries are usually a good deal. Cheap and relatively rugged. If you mess up (forget to turn off the inverter, guests kill the battery bank, etc.), they are not too expensive to replace.

After a few years of operation--You can up size (or down size) the system without having to worry about an expensive 8-10 year battery bank.

Also, I like 6 volt batteries in series for a 12 volt battery bank. Make it really easy to use a volt meter to measure the voltage on each 6 volt battery and make sure everything is balanced. With 12 volt batteries in parallel on a 12 volt bank, you cannot do a quick volt meter check to find a weak battery without disconnecting the battery from the rest of the set.

In any case, get a good hydrometer and a basic DC Current Clamp DMM so you can double check battery charging/discharging.

It seems that my power usage is somewhat on the edge of needing (3) 300W GT panels vs just using (2) panels. I believe the usage stats I gave are worst case scenario.
2. Could I get away with using just (2) 300W GT panels for my current usage until I get into my future usage needs? And would I want to down-size the batt bank too?

That is a tough call... 1,000 Watt system is a fair amount of power for a weekend cabin. 500 Watts--Doable, but if you want to bring a laptop up for work/keep the kids occupied, pretty easy to blow through 500 WH in a day:

30 watt computer * 10 hours per day = 300 WH

Certainly, you could try 2x golf cart batteries + 2x 300 Watt panels and see how things go. Adding an extra parallel string of batteries and a 3rd panel is not a huge amount of work.

• 3. Would there be any advantage to installing a 6 gauge wire instead of the 8 gauge you spec'd? The price difference is marginal, but it seems like the answer would be "no" since we are already maximizing the CC input. Just a thought in case I wanted to expand the system someday (beyond my future usage stated).
• Note: I have 4" conduit in the ground already so I can run one or more sets of wires.

Won't hurt anything at all... Would make running 2x 300 watt panels a bit more efficient--And give you some head room if you decide to add more power later.

12 volt vs 24 volts... With 900 watt array and 440 AH battery bank, you are getting close to the "practical" (cost) limit of a 12 volt system.

24 volts would allow you to have a 2x larger array and battery bank with the same size MPPT charge controller and wiring (you might want to go to ~4 AWG for solar array wiring or two parallel 6 AWG runs (the solar array is the only thing operating near is "optimum" limit with 3x 300 watt panels in series).

24 volt is sort of a no brainer--Other than the MorningStar 300 Watt TSW inverter is a really nice 12 volt unit. There are few small AC inverters that have the features/efficiency of this guy. If you don't need a larger inverter/more power--That would be enough to keep me at the 12 volt system / battery bus.

In the end, do several paper designs (smaller 12 volt and larger 24 volt) and see how they pencil out.

Have fun!
-Bill

Photowhit

Not sure what your 'radio' requirements are, if you like loud music, or just normal radio back ground. I've been happy with an efficient and high quality (if you believe the audiophiles) 10 watt 'T' amp. Plenty of volume for me inside and outside gardening. For me, it's enough for background music with company, but I don't like music loud enough to have to raise my voice much to talk over. It will run off 12 volt, I use it with a MP3 player but you could use an MP3 player with a radio or a tuner which should be a minimal wattage item. Note that the radio represents more than half of your power consumption.

unseenone

Do plan on running a fridge on this setup?

Toonces

Thanks again Bill. I may have more questions, but for now I think you got me heading in the right direction!


Photowhit, I searched briefly for the 10w T amp and came up a little short. Does Pyle make that?
My wife and I are using (2) 50 watt outdoor speakers under our pavilion. We listen to classical music quietly in the morning, but in the evening we tend to listen to rock/alternative pretty loudly. The coyotes never complain though. The only problem with our system is that we don't have speakers in the cabin. I'm curious though about the 10w T amp..


Unseenone, that is also a good question. We tend to use coolers, and occasionally have to 4x4 into town to get ice which kinda sucks. My brother did give me a mini fridge from his pop-up camper that I have yet to try out. Its propane OR 110V electric I believe. Has anyone had good/bad experiences with these?

Do you have a fridge that you would recommend for a setup like mine?

westbranch

The power consumption of a little fridge is not much less than a new E-efficient full sized fridge. look here http://www.energystar.gov/index.cfm?c=most_efficient.me_medium_fridges_freezers and here http://library.cee1.org/content/qualifying-product-lists-residential-refrigerators

Photowhit

Toonces wrote: »

Photowhit, I searched briefly for the 10w T amp and came up a little short. Does Pyle make that?
My wife and I are using (2) 50 watt outdoor speakers under our pavilion. We listen to classical music quietly in the morning, but in the evening we tend to listen to rock/alternative pretty loudly. The coyotes never complain though. The only problem with our system is that we don't have speakers in the cabin. I'm curious though about the 10w T amp..

I think sonic made the cheapy I use most often, I have also bought a couple boards to play with off Ebay, around $10. The Chip is Tripath they come in assorted versions, 2020 is the one usually used in the 10 watt amps.

Sonic use to make this design now Dayton? I believe my Sonic was 10 watts per channel, this is reportedly 15, I think they have very good/clean output up to 75-80%. I don't have the ears to tell the difference anymore. But there are some great (and some not so great) reviews out there often comparing it to much higher dollar systems. and people customize them to improve the quality.

Here they compare a higher dollar version to some very high end equipment.

For under $30 shipped you can get a more advanced setup, with treble and base and power adapter for those not using 12 volts...

cupcake

Toonces wrote: »

Do you have a fridge that you would recommend for a setup like mine?

Try the Kenmore 40-watt refridgerator -- that's what i use... and it works just fine...


--cakeie

Toonces

Thank you to everyone for the great responses! I Will do some more research on radios and fridges. I would venture to guess that adding a small fridge to my solar setup would probably double the size of the whole thing?

Photowhit

Toonces wrote: »

I would venture to guess that adding a small fridge to my solar setup would probably double the size of the whole thing?

About, we're working on providing 1.1 KWH modern fridges in un-airconditioned space would draw 1- 1.5 Kwhs a day, might consider a freezer conversion that would be turned off when not there. I've hear .15 - .4 Kwh's a day, less effected by hotter temps dues to large amounts of insulation. Might even add one (3-5 sqf with a drain) to system and use when sunny or bring ice if cloudy. Would likely work with planned system, particularly if you can reduce the stereo energy use.

Toonces

Thanks again guys for all the great ideas and suggestions. i really appreciate it!

petertearai

Hi . Having started on a 12 volt system, I would now definitely go 24 volts from the start, leaves more room for expansion . Also consider an energy efficient fridge freezer much simpler and in my opinion safer than lpg fridges . Also cheaper ( in my country ) than the lpg ones. In fact i would venture that even with extra pv needed it would still be more cost efficient . Regards Peter

Toonces

I have a new question regarding my setup that we have been discussing. I am now considering replacing the 12v water pump with a 1/2 hp 115 v submersible cistern pump. Can anyone tell me how this would change the size of my solar setup?
Thanks !

westbranch

you willneed the equivalent of at least a 2000w generator to run the pump, or a generator

BB.

Some quick math... Assume that your pump will draw 5x running power at starting... You are looking at a minimum of 1,500 Watt AC inverter.

Assuming a 12 volt battery bank (getting close to where a 24 volt battery bank would be a very good idea--Some people are pretty happy with a good AC inverter running 2,000 Watts on a 12 volt battery bank).
Nominally, I would suggest that you do not want to draw more than C₈ (8 hour discharge rate) from your battery bank... Assume the 1/2 horse induction motor pump draws near 750 Watts

• 750 Watts * 1/0.85 AC inverter eff * 1/12 volt battery bank * 8 hour discharge rate = 588 AH @ 12 volt battery bank

If you run the pump only minutes or 10's of minutes at a time & usually have a near fully charged battery bank, you could possibly go with C₅ discharge rate:

• 750 Watts * 1/0.85 AC inverter eff * 1/12 volt battery bank * 5 hour discharge rate = 368 AH @ 12 volt battery bank

And then there is the starting starting surge--Assuming C_2.5 maximum discharge rate (seconds worth of surge current):

• 5 time surge current * 0.5 HP * 746 Watts per HP * 1/0.85 inverter eff * 1/12 volt battery bank * 2.5 hour discharge rate = 457 AH @ 12 volt battery bank minimum to start pump (reliably)

Anyway--That is how I would guess the math... If you decide to go 24 volt battery bank, all the abave AH numbers for the 12 volt bank would divided by 2 for a 24 volt battery bank.

If somebody else has experience with working numbers--Listen to them.. The above is just my approach using typical numbers and math.

-Bill

Toonces

Thanks guys, I was figuring the wattage of the1/2hp submersible pump at 373 watts. I figured it would draw 4.4 times more than the 12v, 84w demand pump. Am i not figuring this correctly? Wish they made a smaller submersible pump. I'm trying to avoid having to constantly drain a 12v pump for fear of it freezing in cold weather. My future cistern will probably be a 500 gal tank underground.

Doug

westbranch

the kicker is the surge demand and it is hard to estimate what it is . You would need a scope at least or access to a bunch of generators to see what will run it. Same applies to fridges, the surge supply is the king maker. My 1/2 HP AC submersible runs on 2 legs of 4.5 A, its 120V cousin at 9A.... There are submersible DC pumps.:cool:

Photowhit

Toonces wrote: »

Thanks guys, I was figuring the wattage of the1/2hp submersible pump at 373 watts. I figured it would draw 4.4 times more than the 12v, 84w demand pump. Am i not figuring this correctly? Wish they made a smaller submersible pump. I'm trying to avoid having to constantly drain a 12v pump for fear of it freezing in cold weather. My future cistern will probably be a 500 gal tank underground.

You likely can find the actual energy use of the pump you have spec'd, Add another 8-15% for inverter use, also if this will require the inverter to be on demand, if you don't need it on for other items, check its idle use as well.

mike95490

Toonces wrote: »

Thanks guys, I was figuring the wattage of the1/2hp submersible pump at 373 watts.....

My 1/2 hp submersible @ 240VAC, is right at 1,000w as reported by the inverter. So you may want to dig deeper into the tech specs for your pump/motor before you spend much more $