Small off grid cabin?

I notice there are a lot of familiar faces from the Outback forum here, but I figured I'd post some of the things I had been discussing over there for some other opinions. My wife and I will be looking for property in Maine very soon where we plan to build a small off grid cabin.

The cabin itself
Basically we want to start with one main cabin with the intention of building two or three smaller cabins later on. The main cabin will have four rooms-three bedrooms and one kitchen/dining/common area. There will be one bathroom with a stall shower and flush toilet with an additional outside shower (like the ones you see at beach/lake houses) for summertime use. The main cabin would be where myself, my wife, and my children would sleep along with maybe one or two additional guests. It would also be where everyone ate meals, watched TV on rainy days, and would generally gather. The additional cabins would be very simple with disposable battery powered lighting (like touch lights) and no running water-those guests would need to come to the main house to use the bathroom.

Total energy usage
Heat - Woodstove in the main house with smaller units in the guest cabins.

Hot water - Propane on demand HWH.

Refrigeration - Propane powered refrigerator.

Cooking - Propane stove with pilot light/no electronics

Electricity - CFL lighting in each room at 15W per light, CFL floods outside at 26W per light, small TV, satellite box, modem, laptop, and of course the biggie-a deep well pump which will gobble up about 2000W.

System
All of this gives me a usage figure of about 3kWh per day. Based on that I was told that 1.5kW of solar panels in northern Maine will provide more than enough power during the summer months and slightly less than what I need during the winter which is where the generator would come in, but I'm getting ahead of myself. I was also told that I would need at least 375Ah of storage in order to be able to have 3 days of 3kWh on hand while only discharging the batteries 50% so as not to draw them down too far. This would all be tied together (including the generator) with an Outback FLEXpower ONE panel which includes the charge controller, "brain", inverter, etc...all in one package. Now to the generator itself-I was told to take my daily load, double it, and add 25%. This was to ensure that I would have the ability to power the cabin, charge the batteries, and still have some wiggle room if need be. It's certainly a situation I could get into especially when going up in November for deer season if we don't have clear skies. So here is the system in short:

1.5kW Solar array (8 220W panels)
4 12V 104Ah Absorbed Glass Mat batteries (48V)
FLEXpower ONE unit
8.5kW Kohler propane standby generator

I'm estimating all of this to come out between $15-16K including shipping costs, wiring, and other associated work (which I'll be doing myself). I'll be honest, its more than I want to spend to be able to run a three room cabin off-grid, but I also don't want to just run a generator constantly because it's noisy, inefficient, and will likely result in a dead generator after a few years. Does the system seem to meet my needs or is it overkill? In what ways could it be scaled down? Would it be possible to shave down my electrical needs even more in order to scale the system down?

Comments

  • icarusicarus Solar Expert Posts: 5,224 ✭✭✭✭
    Re: Small off grid cabin?

    I don't have a lot of time this morning, but one quick thought Since you are going to have a generator anyway, why not consider using the genny to pump water? For example, you could build in a large enough tank structure to last the average day with some reserve. Then run the genny for an hour or so in the morning (or anytime) pumping the water for the day. This would allow you to add some bulk charge to the batteries at the same time (assuming the genny is big enough to run the pump and the charger).

    This strategy would have the net effect of reducing your largest bulk load from your system, as well as reducing your total AH load on your system, thereby reducing your system size.

    Just FYI, our small off grid house, with propane fridge, uses ~.6 kwh/day with the two of us. We resupply the battery with 400 watts of PV, into 450 ah of batteries. Give a choice, I would increase the size of the PV a bit. Also, .6kwh is a pretty small amount of power. (We used to use ~1/2 of that, illustrating the rule that loads will grow with time).

    Good luck, welcome to the forum, there are some pretty sharp, pretty experienced folks here who have forgotten more about PV than most of us will know.

    Tony

    PS I personally would consider adding enough batteries so that the average daily draw down is ~20%. Doubling the battery size would then require a bigger PV, to keep the charging of the Pv between 5-13% of ah capacity.

    There are several links to a couple of very good Battery FAQs linked on this forum, but I am on a different PC so I don't have them bookmarked, and my connection is so slow to go get them. I suggest you find and read them, as it is very good info for the "care and feeding" of batteries.

    T
  • AntronXAntronX Solar Expert Posts: 462 ✭✭
    Re: Small off grid cabin?

    30A charging current into 100Ah battery is too much. Your system will miss out on a lot of solar energy due to charge controller being in absorption mode. Ideally you would want C/20 charge/discharge rate for your Lead-Acid batteries. That means 600Ah 48V bank, but I think C/10 rate or 300Ah would be minimum. Contrary to what some here may say, you can parallel more batteries later - your newer batteries will just work harder than older ones and wear unevenly until they catch up to older batteries.

    One idea is to get cheap 3000W Modified-Square wave inverter for water pump. Make sure it has stand-by power saving mode, where inverter shuts down when no load present, and automatically wakes up when load appears. Only use it for water pump or resistive heater element loads. You can also get one with built in charger. For electronics and smaller loads, get 600W - 1000W Pure-Sine wave inverter and aim your maximum average load for it at about half capacity, or 300 - 500W.

    Is it windy in November at your location? If so, maybe 500 - 1000W wind turbine and energy conservation can replace Kohler generator?
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: Small off grid cabin?

    it looks ok except for your battery system. to keep within the parameters of feeding that much power it will be necessary for the 48v battery bank and is why i assume you picked out 4 12v batteries. that battery bank is at 104ah and only the voltage is adding when in series. it will be tough to utilize a bank this low in ah as only 52ah is usable. if you want 375ah you may want to go with 2 series banks of 4 12v 210ah batteries for a total of 8 batteries. now 3kwh at 48v is 62.5ah and to prevent going below 50% is 125ah per day, hence 3 x 125ah is 375ah so that's where you got that, but it's at 48v and not at 12v.
    don't forget for 12v cb draws power too.
  • System2System2 Posts: 6,290 admin
    Re: Small off grid cabin?
    icarus wrote: »
    I don't have a lot of time this morning, but one quick thought Since you are going to have a generator anyway, why not consider using the genny to pump water? For example, you could build in a large enough tank structure to last the average day with some reserve. Then run the genny for an hour or so in the morning (or anytime) pumping the water for the day. This would allow you to add some bulk charge to the batteries at the same time (assuming the genny is big enough to run the pump and the charger).

    This strategy would have the net effect of reducing your largest bulk load from your system, as well as reducing your total AH load on your system, thereby reducing your system size.

    Hmmm...We would need about 150 gallons of water per day (5 people, 30 gallons per person)-a vertical 330 gallon tank is about 4' in diameter and less than 5' high. I suppose I could build it its own little "room" and then pipe the well pump to it. I would just need a small pressure pump to pressurize the system. The other advantage would be that I could maybe even incorporate some sort of passive solar heating in order to lighten the load on the water heater. I'll need to recalculate, but I suppose it would make a big difference!
  • BB.BB. Super Moderators, Administrators Posts: 29,482 admin
    Re: Small off grid cabin?

    The Battery FAQs' that Tony referenced:

    Deep Cycle Battery FAQ
    www.batteryfaq.org

    Given that you are planning on using the cabin maybe 3-5 months of the year--Probably better to install the minimum amount of solar panels to run during the summer (lots of sun) and bring a small genset (Honda eu2000i or eu1000i + a 40 or 20 amp AC charge controller--sorry, those are typical ratings for a 12 volt battery charger--yours is a 48 volt system).

    The 8.5 kW Propane Kolher is a nice genset--but probably 5+ times larger than you can ever use (unless you are installing A/C)...

    I fear that you will be burning 5x the amount of fuel because you have no place (useful) for that electricity to go.

    Basically, the Kolher running at 50% electrical load (the typical plateau below which less loads does not drop fuel flow much at all):
    • 8,500 watts * 0.50 loading * 1/58 volts charging * 0.80 charge eff = 59 amps @ 48 volts
    And the typical recommended range of charging current for a battery bank is around 5% - 13% of rated AH capacity (yes, many AGMs can charge at much higher rates--but lets stay withing a practical range).
    • 104 AH * 0.05 = 5.2 amps
    • 5.2 amps * 58 volts charging * 1/0.80 eff * 1/0.50 gen loading = 754 watts minimum recommend generator rating
    • 104 AH * 0.13 =12.5 amps
    • 12.5 amps * 58 volts charging * 1/0.80 eff * 1/0.50 gen loading = 1,813 watts sort of maximum generator rating
    So, unless you have a lot higher external loads for the Kohler (running washer/drier/Air Conditioning/deep well pump + Battery Bank Charging)--you would be much more fuel efficient if you got a (roughly) Honda eu2000i (1,600 watt continuous rating -fixed).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • icarusicarus Solar Expert Posts: 5,224 ✭✭✭✭
    Re: Small off grid cabin?

    That's the link, thanks Bill.
    I might also add, as I think about this, there are some other things I might do.

    I might size the Pv/batteries for the expected lighting/tv/computer loads, but use the genny for the large loads like water pumping, shop tools, (even microwaves/toasters etc)

    Living off grid, with a simple solar system comes with some considerable trade offs between price and capacity. We, for example have no resistance electic heating devices. We make toast with a propane toaster, our oven is of course propane. No hair dryer, curling irons etc. The coffee maker is a Coleman Mr Coffee that sits on the gas stove.

    We run the genny for most power tools, but we are evolving with a number of battery tools like small saws and drills. We also have a gasoline powered washing machine, but for the trouble, I would use a generator to run a washing machine.

    I concur with Bill's suggestion about the Honda vs the Kohler. The Eu series are great, quiet gennies and you can paralell two together for large loads. We use a single Eu 1000 for nearly 90% of our other loads.

    As for the water, depending on toilets (use and volumes) and showers, 30 gallons per person per day is probably a failry big estimate. We do pump with the solar, but we are only two, and we have no flush toilets. The real reason for the solar water pumping is it allows us to have water over the winter because of the way I designed the system to drain down. A large storage tank, with a small booster pump to feed a pressure tank might be a fine idea. The idea of pumping from a deep well into a no pressure (cistern) type tank would probably use less power net/net than pumping a similar volume into a 30-50 pressure tank, (Or even a 20-40 for that matter). (Someone wisely pointed out here on this forum, that the energy required to pump into 50 psi was considerably more than pumping into 40, but the liveability of 40 psi was no worse than 50 for example)

    Even at 150 gpd is not too hard. At 5 gpm it is only 30 minutes of pump. A submersible Shurflo 9300 will pump ~3 gpm @12vdc into 60 psi (~10 amps) so you could provide that water with ~200 watt/hours/day, certainly in the realm of reason for a small solar system.

    To some extent it seems your choices will depend on what you have installed already in the well.

    Good luck, keep in touch,

    Tony
  • System2System2 Posts: 6,290 admin
    Re: Small off grid cabin?

    This has been eye opening-I'm glad I decided to post here!

    With a little more research I found the following:

    - Bosch makes a tankless HWH that uses the energy from gas rushing through the pipe to create an ignition spark.

    - Peerless makes a gas range/oven that will operate without being plugged in (you have to light it manually, but I can live with that)

    - There's really no point in having satellite TV or internet at the cabin because all will require yearly contracts. Does anyone know what antenna reception is like in Maine?

    After that I re-thought my usage a bit:

    - Cell phones (if they even work up there) can be charged from our vehicles.

    - We don't need as many lights.

    - As long as we have a stove, oven, source of fresh water, and a grill we can cook pretty much anything.

    - As we discussed, I use the generator to fill a cistern inside the house.

    Given all of that I came up with a new load of 1.2 kWh per day. Could I get away with a .5 kW array with proper batteries and a generator?
  • BB.BB. Super Moderators, Administrators Posts: 29,482 admin
    Re: Small off grid cabin?

    Regarding Bosch pilot-less heaters--I think I remember them being a bit of a maintenance issue. You can always fire up the pilot for use or use one of those D Cell powered heater.

    You can still charge your cell phones and laptops with a small 300 Watt TSW inverter like the 12 volt SureSine. It also has a low power "search mode" which can be used to give you 120 VAC lighting on a wall switch without wasting too much additional power.

    Using PV Watts Website, use 1 kW of panels as a round number, 0.52 derating fixed array, near Caribou Maine:
    "Station Identification"
    "City:","Caribou"
    "State:","Maine"
    "Lat (deg N):", 46.87
    "Long (deg W):", 68.02
    "Elev (m): ", 190
    "PV System Specifications"
    "DC Rating:"," 1.0 kW"
    "DC to AC Derate Factor:"," 0.520"
    "AC Rating:"," 0.5 kW"
    "Array Type: Fixed Tilt"
    "Array Tilt:"," 46.9"
    "Array Azimuth:","180.0"

    "Energy Specifications"
    "Cost of Electricity:","12.2 cents/kWh"

    "Results"
    "Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
    1, 3.36, 58, 7.08
    2, 4.34, 67, 8.17
    3, 5.23, 86, 10.49
    4, 5.75, 89, 10.86
    5, 4.99, 74, 9.03
    6, 5.09, 71, 8.66
    7, 5.15, 73, 8.91
    8, 4.99, 72, 8.78
    9, 4.26, 61, 7.44
    10, 3.45, 53, 6.47
    11, 2.38, 35, 4.27
    12, 2.73, 46, 5.61
    "Year", 4.31, 784, 95.65
    So, for 1,000 watts of solar panels, for summer months you would get at least 70 kWH per month or:
    • 70,000 WH per month per 1,000 watts / 30 days per month = 2,300 Watt*Hours per day (average)
    For November--35 kWH per month or around 1,000 WH per day...

    Anyway, with 500 Watts of solar panels, you would get about 1,170 WH per day--Very close to your requirements.

    What you may choose to do, oversize the battery bank (rough rule of thumb, 3 days no sun, 50% maximum discharge, 85% efficient inverter, example of 12 volt battery bank):
    • 1,200 WH per day * 1/12 volt battery * 1/0.85 invrt eff * 3 days no sun * 1/0.50 batt maximum discharge = 706 AH @ 12 volts
    The rough maximum cost effective panels for a 706 AH 12 volt battery bank assuming a maximum charge rate of 13% (again, battery charging rule of thumb of 5%-13%):
    • 706 AH * 14.4 volts * 0.13 rate of charge * 1/0.77 panel-controller losses-derating = 1,700 watts cost effective maximum of solar panels
    Again, these are nice round numbers... You can cut from 3 day to 1 day of no sun--you can charge less than 5% and use the genset more. You can have a larger battery bank and use minimum panels+genset--find out your real loads--then add panels when needed to reduce generator use.

    And don't really bother to try for 100% solar in November--That would oversize your system for the rest of the year.

    Most people grow their loads over time (get your self one of those new smart phones with video/sound out and a small LCD/LED projector and you can have nice Internet/movie night (assuming there is a carrier that works in your neck of the woods).

    -Bill

    PS: Get a Kill-a-Watt meter to measure your AC loads (watts, and Watt*Hours). Also, if you will have smaller DC loads, you can look at a DC Amp*Hour/Watt*Hour meter too.

    Especially with AGM/Sealed batteries where you cannot measure specific gravity--A Battery Monitor is a really nice thing to have too.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • icarusicarus Solar Expert Posts: 5,224 ✭✭✭✭
    Re: Small off grid cabin?

    Bosch does still make standing pilot water heater, as well as water activated pilot and battery pilot heater. Personally, I have had a mixed luck with the Bosches over the years, but my neighbour swears by them. I prefer(ed) the Paloma PH6-12-24 versions, but sadly they are no longer made, leaving the Bosch as the only mainstream units that don't require outside power. The real maintenence issue is keeping your water debris free, as the water course can get clogged.

    As for charging other items, you can wire your house with paralell wiring, a 12 vdc system as well as a 120vac system. (assuming you are going to keep your battery bank at 12vdc). That way you can charge cell phones, and use other automotive devices such as Rv lights when you don't need the inverter on.

    There are several "rules" of thumb regarding small off grid systems. The first is, most people over estimate thier loads, and at the same time over estimate the amount of solar they actually get. The next is, loads ALWAYS grow with time. You may not want TV and internet now,, but 3 years from now, you spend more time, your feelings may change. Having 24/7 power allows one to use power 24/7 in ways you don't anticipate. My suggestion is do some real world estimate calculations to find a realistic base line of load, then ad ~50% fudge factor, and then add a bit more!

    System can grow with time, but certain things don't grow well. Charge controllers often start out too small, but they can be paralelled for example. Batteries don't grow well, either in size or age. Inverters that are too large initially are no very efficient, but can be out grown quite easily. I suspect you will find, that everything you can do now to educate yourself, will lead you to avoid the worst of the, ready, fire, aim syndrome that effects many newbies. (You will have surplus hardware at some point as the system needs change)

    Once again, good luck,

    Tony
  • dwhdwh Solar Expert Posts: 1,341 ✭✭✭
    Re: Small off grid cabin?
    BB. wrote: »
    So, unless you have a lot higher external loads for the Kohler (running washer/drier/Air Conditioning/deep well pump + Battery Bank Charging)--you would be much more fuel efficient if you got a (roughly) Honda eu2000i (1,800 watt continuous rating).

    -Bill

    Um...pretty sure that's 1600w continuous rating on the eu2000, not 1800w. (See, I actually DO read this stuff! Well...some of it anyway. :D )
  • BB.BB. Super Moderators, Administrators Posts: 29,482 admin
    Re: Small off grid cabin?

    Ooops--you are right--and I knew better... 1600 watts max continuous.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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