Taking a Tiny House Off Grid

Intermediate

Inside this Article

PV-powered tiny house
Ben Barthell, with his PV-powered tiny house.
Tiny House interior
Looking through to the wood heater and gas cookstove.
More tiny house interior
Looking threw to the sleeping area.
The sleeping area
The sleeping area of the tiny house.
The tiny house
The solar-powered tiny house set in beautiful surroundings
screen shot of loads calculations
The Barthell system loads spreadsheet.
The PV & SHW collectors
The Barthell tiny house uses solar energy for both electricity and hot water. A 4-by-8-foot flat-plate solar collector provides most of his water heating. A propane-fired 15-gallon water heater provides backup.
SHW mechanical closet
Space heating is accomplished with a small wood heater and some passive solar gain through a south-facing glass door.
The kitchen
Ben already owned a SunDanzer DCRF134 DC refrigerator/freezer, which runs on 12 or 24 VDC, limiting voltage options. We selected a 24 V system, utilizing a step-down charge controller that could accept 90 V PV array input.
Solar Pathfinder
Ben’s home was in a heavily treed area with relatively poor solar exposure among cottonwoods bordering the Rio Grande. I estimated an annual solar access value from shading of roughly 65%.
Tiny house batteries
The Barthell tiny house uses a bank of sealed batteries.
PV system equipment
The balance-of-system components of the solar-electric system.
The owner inside his tiny house
Ben Barthell and friend relaxing in his tiny house.
PV-powered tiny house
Tiny House interior
More tiny house interior
The sleeping area
The tiny house
screen shot of loads calculations
The PV & SHW collectors
SHW mechanical closet
The kitchen
Solar Pathfinder
Tiny house batteries
PV system equipment
The owner inside his tiny house

A battery-based solar-electric system can be the perfect match for a “tiny house,” cabin, or other small home. This guide will help you determine what loads to serve, explain equipment selection and sizing, and present installation and maintenance tips for a small, robust off-grid PV system. We will examine a case study for a real-world tiny house—Ben Barthell’s off-grid, 200-square-foot Steely Cottage in Espanola, New Mexico.

Basic Principles

Off-grid living requires adherence to three basic principles of energy use as the basis for a power system:

•           Shift inappropriate electrical loads (space and water heating, cooking, and clothes drying) to other forms of energy

•           Reduce waste by implementing efficiency measures

•           Use energy in proportion to the amount available.

In an off-grid home, the energy system alone serves the daily household electricity needs, and batteries can store only a few days’ worth of energy. Energy use is typically greatest in winter, when more hours of darkness mean more indoor activities and lighting—and comes at the season with the least solar resource, due to short days and more clouds. An off-grid PV system is usually sized to meet winter demand, which means it is oversized for needs during the rest of the year. In a sunny climate, a well-designed off-grid PV system will typically meet 80% to 90% of the home’s winter load, usually with an engine generator making up the rest.

web extras
For an explanation of these three principles, see “Toast, Pancakes and Waffles” • homepower.com/133.88


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Comments (20)

Frank Heller's picture

Wind turbines and solar arrays and hurricanes. Someone blogged that half the state of Texas is covered with fragments of shattered solar panels; others on how various turbines shut them selves down at a certain wind speed independent of their role in the grid and at a time reducing the output from coastal wind farms by 85%. Then the grid infrastructure connecting the farms to the grid was damaged and in effect removing them as a producer.

Knew about wind turbines kick in and shutdown velocity and the wear & tear on mounts, bearings, etc. from strong sustained winds; but never thought that much about solar arrays, their mounting, and flying debris let along power losses from heavy rainfall and cloud cover.

Any predictability or further evident of them being supplemental as best, and not primary?

Mr. Mase's picture

Hi, The article is quite educative. If I may ask what tool was used for the drawing of the PV Schematic. I love the layout and the useage of icons similar to the real device.
Most often when offgrid solar PV system are designed, the Solar panel is designed to meet only a days battery capacity. What if the system days of autonomy is about 4 days? Does that mean that it will take about 4 days to charge all battery banks the system fully?

Allan Sindelar's picture

Mr. Mase,
The drawing was prepared by Home Power using their in-house graphics software, so I can't address that question.
I have never designed an off grid system in which the PV array is designed to only meet a day's battery capacity, so I can't directly answer your second question either. Nowadays I typically design to 2-3 days' predicted total load, adding more array rather than more battery capacity. Many pro off grid installers have shifted in their basic approach, toward smaller battery banks and larger arrays, as a result of increased battery costs and greatly reduced PV module costs. An array that meets 100% or more of expected daily average winter load will fill batteries faster after cloudy weather, will often keep up during times of partial overcast, and will allow batteries to remain full most of the year, leading to longer battery life at lower overall cost.

Thermaheal's picture

Very instructive article and much appreciated for the overview. What would be your recommendations or alterations to the suggested system, if this mobile tiny home were to relocate to the Canadian West Coast recognizing there might be more DOA required during winter months when the usage is greatest? Could a sufficient PV array be placed on a south oriented roof of a 8 X 33 foot home and how many batteries would be required if this were to be used as a full time residence with standard friidge, cook top range, toaster oven, washer/dryer?

Fred Golden's picture

If I were to design a power system with your parameters, I might suggest a small wind generator, such as Air 400 watt machine, along with 2,000 to 4,000 watts of solar panels. I happen to like Outback inverters. 48 volt system.

I tend to agree smaller battery bank, more solar. Due to your long sunless winter, a wind turbine should provide enough power during winter,

I lived in a 30' motorhome from 2006 to 2013. I have a 415 watt solar system and 1,500 watt inverter. Heating and refrigerator are propane. 18 gallon tank would last 8 weeks in summer, 3 weeks on cool winter (25 to 45F overnights). My battery was 400 AH at 12 volts. For your home, the largest battery I would recommend is 400 AH at 48 volts, using 200 AH Trojan T-105 golf cart batteries that ate a "lightweight" 67 pounds each. Yes they seem light next to a 8D battery or 85 pound 5SP 12 volt 185 AH battery.

200 AH at 48 volts is nearly 10 KW, or 5KW without depleting the battery bank to far. Still it would take 16 X 6 volt 67 pound batteries, or about 1,000 pounds of batteries. One string of 6 volt batteries would work fine if you are using a propane refrigerator, or can have it on a timer to run only in daylight hours.

Michael Welch's picture
Of the thousands of tiny wind turbines like the Air that have been sold, I do not know of a single one that is making acceptable amounts of energy. (That does not mean there aren't any, just that people buy them for locations that won't support them and owners won't do the installation properly, which includes using a tall tower.)

Better advice may be to spend your time and money on more PV modules and energy efficient appliances. And yes, find a reputable off-grid designer/installer to make sure you get the right system for your highly individual needs.
Allan Sindelar's picture

Thermaheal,
You are asking questions specific to your geographic area and climate, and with an entire set of unknowns. I'm not willing to make wild guesses when I have so little specific information with which to work. I would suggest that you find a reputable off grid designer/installer in your region, who knows the specifics and can guide you in appliance selection and system sizing specific to your application and energy needs.

Thermaheal's picture

Can you suggest how you would go about finding a reputable off grid designer/installer in Eastern or Western Canada?

Allan Sindelar's picture

Off grid has become a specialty in most areas, as mainstream grid-tied PV has come to so dominate. I would try these resources:
1) Word of mouth among other off gridders - which may mean stopping in at strangers' homes with an array and no utility connection;
2) Solarreviews.com is a sort of "Angie's List" for the solar world. Customers can review the company's work on the basis of sales process, installation quality, on schedule, etc.
3) Seek out NABCEP Certified installers through nabcep.org.
4) Both Home Power and Solar Professional maintain searchable professionals databases.

None of these, however, focus on those with off grid experience, so you will have to press each person or company to gauge their experience with this specialized knowledge.

chrischolette@gmail.com's picture

I wonder why the author didn't just choose an Outback FX-series Inverter rather than a separate Inverter and Battery Charger. Also a 45 AMP Morningstar MPPT charge controller for 3 panels seems a bit much.

Allan Sindelar's picture

Chris,
There were several reasons why I chose what I did. Ben knew he did not need a larger inverter and was budget conscious. The smallest Outback FX lists for $2,369, plus $295 for the Mate, while the Exeltech inverter and Iota charger together cost Ben $1,575. Needing at least a 175A DC inverter breaker, the Outback inverter would have also required a higher capacity and thus more expensive E-Panel.
Also, among us graybeard Wrenches, the Outback FX series is known for not having tight regulation of generator amp draw. If the inverter is charging and an inductive load comes on, the inverter can allow the surge load to add to the charging load, potentially tripping the generator's output breaker. I have especially seen this with Honda's 3,000W inverter-generator, and as Ben told me that he already had a 1,000W inverter-generator, and I wanted to avoid the possibility of a similar issue. The Iota is more tightly regulated by its fundamental structure.
Regarding the Morningstar TriStar MPPT, the three PV modules were together rated at 750W, which after MPPT conversion could total over 30A at lower battery voltages. This is of greater concern given our high-desert winter sun, which regularly exceeds 1,000 W/meter squared. The 45A model lists for only $80 more than the 30A model, and puts this valid concern over capacity to rest. Plus, at that time I wasn't even aware that Morningstar had developed a 30A version.

chrischolette@gmail.com's picture

Thanks Allan for the clarification.

I've experienced the generator amp draw issue myself, so I understand that. The Outback street price is a lot lower than that generally.

Otherwise I like my Outback a lot and would likely use it as my first choice for something like this.

I appreciate the response and the detail on your reasoning.

Frank Heller's picture

Hi...in Maine we have an abundance of water; so the ideal small house would have a hybrid energy system using solar for heating and micro-hydro for electricity. Depending on flow and other factors a small house with a similar battery storage system can be powered with even the smallest micro-hydro turbine--->300 watts over 24x7 w/a load controller to gen. hot water. The cost is usually under $1,000, depending on he intake, and generator house. Batteries not included.

Small houses are traditional and the Maine 'add-on' concept has been upgraded with modular designs featuring one unit with the kitchen as the activity center, and a very comfy combo of shower, hot tub, toliet and storage; and then another for sleeping, another for an office, another for recreation...arranged in a U shape.

The arrangement is solar sited and features a lift ceiling so you get the impression of being outside. The interior is usually a garden and the exterior is a deck or overlook if on a hill.

Allan Sindelar's picture

Frank,
Microhydro has long been considered the holy grail of RE sources, and you're fortunate to live where water is plentiful, the land is hilly and small homes are a cultural tradition. Living in the high desert Southwest, in 25 years of PV work I have yet to have an opportunity to work with microhydro. Around these parts, as Mark Twain famously said, "Whiskey's for drinkin' and water's for fightin' over", and sites with the necessary head and flow are rare. On the other hand, we can get through the entire winter just on the energy of the sun.

Also, this particular tiny house was designed and built to be portable: while it's not a travel trailer, it's small enough to be moved occasionally behind a standard pickup. The PV array as set up can fold down and move with the house. Microhydro would have to stay with the home's surroundings, as streams don't relocate well.
Thanks for writing!

Frank Heller's picture

hmmm. no rain in April, May has been bone dry.

Both Maine and N. Carolina have plenty of 'small house' sites next to small streams w/constant flow and sufficient head ---100' +, to generate enough power to run even a large house. Several clients are storing ground seepage in 'farm' ponds and use it when they need power.

But in the West, water is carefully measured and allocated, esp. in New Mexico; while our lakes are overflowing.

Andy Fidandis's picture

Can you get fed tax credit for an off grid house?

Michael Welch's picture
Yes, off grid homes qualify for the federal tax credit.
Wa6lqb's picture

If freezing is an issue, provision should be made for protective heating load. Generator autostart should be considered.

Allan Sindelar's picture

New Mexico is a freezing climate, but generally a combination of tight construction, some passive gain, and owner involvement pretty much precludes the need for electric freeze protection. In Ben's case, if he's away in winter the water system can be simply drained; nothing else is at risk of freeze damage.

I have long advised against generator autostart provisions, for several reasons. Autostart usually requires a stationary generator with remote start capabilities; it can lead to excessive unintended runtime if something fails, like a water pump; off grid homes with sufficient thermal mass to remain above freezing temperatures typically have very little energy consumption when left unoccupied; and depending on a generator and electric heat is a sure recipe for eventual failure, as that's a complex system to leave unattended. Better to have a tight home, a good system monitor and an attentive user. Thanks for writing!

Frank Heller's picture

In my corner of Maine, it can be anywhere from (-) 15 to (+) 95 outside; but damned if it isn't always 54 degrees in the basement. It's common for a food business to locate compressors for freezers/reach in reefers,Geothermal is popular around here, although fracking and electricity use for extracting heat is a concern. Thinking of some kind of 'air conditioner' to distribute cold air through house in summer.

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