Off Grid Cabin in Ohio

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 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