How to Design an Off-Grid House

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How to Design an Off-Grid House

Before you go off-grid, think about a few important design issues

Posted on Jun 2 2017 by Martin Holladay
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A very small percentage of U.S. homes are off the electricity grid — far fewer, for example, than in Africa. That said, North American designers of off-grid homes often end up posting questions on GBAGreenBuildingAdvisor.com.

To help this subset of builders avoid common design errors, I’ll share what I’ve learned from living in an off-grid house for 42 years.

Off-grid homeowners use less electricity

Off-grid homeowners have a much stricter energy budget than owners of grid-connected homes. This is due to simple economics: off-grid electricity is expensive — on the order of $0.50 to $1.00 per kWh.

Sure, photovoltaic(PV) Generation of electricity directly from sunlight. A photovoltaic cell has no moving parts; electrons are energized by sunlight and result in current flow. (PVPhotovoltaics. Generation of electricity directly from sunlight. A photovoltaic (PV) cell has no moving parts; electrons are energized by sunlight and result in current flow.) modules are relatively cheap. But batteries and gas-powered generators (or propane-fired generators) are expensive, and those of us who live off the grid need batteries and generators — because the sun doesn’t always shine.

During certain seasons, off-grid homeowners are likely to have an energy surplus. In
Vermont, for example, a 1-kW PV system that produces 48 kWh of electricity in the month of December will produce three times as much electricity (145 kWh) in the month of May. If weather forecasters predict three days of sunny weather in May or June, you can plug in extra hairdryers, do several loads of laundry, and vacuum the house. But during a snowstorm in the middle of December, you’ve got to be careful about energy use. That’s when you will be using your broom instead of your vacuum cleaner.

House design issues

You can power your off-grid house with a micro-hydro system or a wind turbine if you want, but by far the most common way to generate off-grid electricity is with a PV system.

If you have a large lot, you probably want to install a ground-mounted PV array. If you live in a snowy region, ground-mounted arrays make snow removal easier than it would be for a roof-mounted array. (Off-grid homeowners usually pay more attention to the chore of removing snow from a PV array than grid-connected homeowners, because off-grid homeowners need every possible kWh they can produce during the dark days of winter.)

If you plan to install your PV modules on your roof, make sure that your south-facing roof:

  • is unshaded;
  • is free of valleys, dormers, skylights, chimneys, and plumbing vents;
  • has the right slope (with an angle close to your latitude);
  • is large enough for your planned array;
  • is equipped with durable roofing.

If you live in a cold climate, you probably don’t want to use a pier foundation. (With a house on piers, it's difficult to keep the plumbing pipes from freezing without electric-resistance heat tape. If you live off-grid, you probably won’t have enough electricity for electric-resistance heat tape.) Either a crawl space foundation or a basement foundation makes more sense than a pier foundation.

Where do I put the batteries?

Batteries need to be warm in winter, so they need to be located inside your home’s thermal envelope. There’s a potential problem, however: when lead-acid batteries are being charged, they can produce hydrogen gas, which is explosive.

My batteries have never exploded, but a friend of mine experienced a small explosion when he created a spark near his charging off-grid batteries.

I advise off-grid homeowners to build a battery closet inside their home’s thermal envelope. The closet should have a tiny exhaust fan that operates when the battery voltage is high.

There’s a lot more to say on this issue, of course. My advice: research the topic thoroughly before you decide where to put your batteries.

Space heating

The two most common ways to heat an off-grid house are: (1) propane space heaters with through-the-wall venting; and (2) a wood stove.

Remember, you want to choose space heaters (or a wood stove) that does not require electricity.

Conventional space heating equipment — boilers with circulators or furnaces with fans — will quickly drain your batteries during the darkest days of the year. Outdoor wood-fired boilers are electricity hogs, and are therefore best avoided.

Cooling

Off-grid homes west of the Rocky Mountains are usually cooled with an evaporative cooler. Homes east of the Rocky Mountains usually use ceiling fans.

What about air conditioning? The traditional advice to off-grid homeowners was, “Get used to living without air conditioning.” These days, however, PV systems are getting bigger. Air conditioning loads tend to be greatest when sun is abundant, so a very large PV system may be able to keep a small air conditioner humming. Such an air conditioner will still be a luxury item, however, and will require thousands of dollars in extra PV modules and batteries to operate.

Domestic hot water

Off-grid homeowners generally choose from among the following three ways to heat domestic hot water:

  • A propane water heater — either a conventional tank-type heater or a tankless heater. Choose a model that doesn’t require electricity.
  • A solar thermal system.
  • A stainless-steel heat exchangerDevice that transfers heat from one material or medium to another. An air-to-air heat exchanger, or heat-recovery ventilator, transfers heat from one airstream to another. A copper-pipe heat exchanger in a solar water-heater tank transfers heat from the heat-transfer fluid circulating through a solar collector to the potable water in the storage tank. in the firebox of a wood stove, connected to an insulated storage tank above the wood stove. With this type of system, the hot water thermosyphons between the heat exchanger and the storage tank. No electricity is required.

Don’t even think about using electricity to heat domestic hot water.

Water supply and septic systems

If you are off the grid, you’ll need to develop your own water system and your own septic system.

Most off-grid homes get their water from a drilled well. If you hire a local well driller to install your submersible pump, you’ll probably be sold an oversized pump. In most cases, all you need is a 1/3-hp submersible pump; that’s what I chose to install in my 200-foot-deep well. Unless your well is extraordinarily deep, you won’t need a 1/2-hp or 3/4-hp pump.

A 1/3-hp pump may be a special-order item, but it’s worth seeking out. Otherwise, your oversized pump will be putting a strain on your inverterDevice for converting direct-current (DC) electricity into the alternating-current (AC) form required for most home uses; necessary if home-generated electricity is to be fed into the electric grid through net-metering arrangements. and batteries for the next 30 years.

It should go without saying that you want a conventional septic system that operates by gravity. You certainly don’t want a system that needs a sewage pump.

Lighting

Finding energy-efficient light bulbs has gotten a lot easier recently. Your home should be incandescent-free: Choose LED or compact fluorescent bulbs, and turn lights off when you leave the room.

Mechanical ventilation systems

An HRV(HRV). Balanced ventilation system in which most of the heat from outgoing exhaust air is transferred to incoming fresh air via an air-to-air heat exchanger; a similar device, an energy-recovery ventilator, also transfers water vapor. HRVs recover 50% to 80% of the heat in exhausted air. In hot climates, the function is reversed so that the cooler inside air reduces the temperature of the incoming hot air. or an ERV(ERV). The part of a balanced ventilation system that captures water vapor and heat from one airstream to condition another. In cold climates, water vapor captured from the outgoing airstream by ERVs can humidify incoming air. In hot-humid climates, ERVs can help maintain (but not reduce) the interior relative humidity as outside air is conditioned by the ERV. can use between 300 and 1,300 kWh per year. That’s a lot of electricity — more than most off-grid systems can handle. To ventilate your off-grid home, you have three options:

  • Open a window when the indoor air feels stuffy. This is the method I use.
  • Install a Panasonic WhisperGreen exhaust fan — for example, the FV-05VK3 (rated at 4.3 watts) — and connect it to a very well-designed duct system. Operate it as little as possible.
  • Install a pair of Lunos fans, each rated at 0.14 watts per cfm. One reason that this approach may be appealing to off-grid homeowners: these fans use 12-volt DC motors, obviating the need for an inverter. As with the Panasonic exhaust fan option, I advise off-grid homeowners with Lunos fans to operate the fans as little as possible.

Appliances

Here are some tips on appliance selection:

  • The general principle to keep in mind is the off-grid homeowners are willing to pay a much higher premium for energy efficiency than grid-connected homeowners, because an expensive energy-efficient appliance is still cheaper than a less efficient appliance plus the PV equipment needed to keep the appliance functioning.
  • Most off-grid homes have a propane-fueled kitchen range. Make sure to buy a model with electronic ignition or an old-fashioned pilot light, not a high-wattage glow bar, for the oven.
  • Some newer washing machines have electronic control boards that don’t work well with some inverters. It’s always safer to buy a washing machine with an old-fashioned control dial rather than electronic controls.
  • If you decide to buy a clothes dryer, you’ll want a propane model, not an electric model.
  • Select a refrigerator that uses 350 kWh per year or less. If you want to pare this load to a minimum, buy a Sunfrost RF-12 refrigerator/freezer; that model uses only 205 kWh per year. (For more on this issue, see Choosing an Energy-Efficient Refrigerator.)

Learning to live without a few things

If you’ve read this far, you realize that off-grid homeowners have to be more aware of energy use than grid-connected homeowners. Traditional advice to off-grid homeowners included a list of forbidden appliances; for years, this list included air conditioners, dishwashers, hair dryers, and toasters. My family still lives without these appliances. That said, the dropping price of PV has allowed some of these appliances to sneak into a few off-grid homes — as long as these appliances are used judiciously.

The first generation of off-grid homeowners bought their first PV modules in the early 1980s. At that point, we were all making the transition from kerosene lights to simple systems that powered a radio and a single fluorescent light, and we were all delighted by these new luxuries.

The second generation of off-grid homeowners — those who are now building retirement homes in rural areas — often assume that they can bring every modern appliance with them into the woods. If their budget is big enough, they probably can. But after these homeowners spend their first dark and cloudy winter in their new off-grid home, they may realize that they can live without a few luxuries. It’s doesn’t take long for a cash-strapped rural resident to get tired of filling the fuel tank of a gas-powered generator.

Climate matters

At the beginning of this article, I noted that a PV system in Vermont only produces one third as much electricity in December as it does in May. It’s worth noting, however, that not every climate experiences such a severe winter penalty. For example, the May-to-December ratio in Phoenix, Arizona, is only 1.7 to 1 instead of Vermont's 3 to 1 ratio.

Here’s another way to explain how geography matters: during the month of December, a PV system in Phoenix produces more than twice as much electricity as a system of the same size in Vermont.

The bottom line is that off-grid homeowners who live in sunnier regions of the U.S. can enjoy the use of a few energy-hungry appliances that Vermonters must learn to live without.

More resources

An excellent online resource for off-grid homeowners is maintained by an Idaho distributor of off-grid equipment called Backwoods Solar Electric. Here is the link: Backwoods Solar Learning Center.

Martin Holladay’s previous blog: “Night Sky Radiation.”

Click here to follow Martin Holladay on Twitter.


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Image Credits:

  1. Image #1: Fine Homebuilding
  2. Image #2: Martin Holladay

1.
Jun 4, 2017 11:54 AM ET

DHW
by John Spier

Martin,
Didn't you write recently that PV prices had come down so much that solar thermal DHW systems were obsolete?
John


2.
Jun 4, 2017 1:14 PM ET

Edited Jun 4, 2017 1:15 PM ET.

Response to John Spier
by Martin Holladay

John,
I think you are referring to my article titled Solar Thermal is Dead. In that article, I wrote, "Solar thermal systems still make sense for off-grid homes."

So I haven't contradicted myself.

As well, it's worth noting that I explained in the article on this page ("How to Design an Off-Grid House") that "off-grid homeowners are willing to pay a much higher premium for energy efficiency than grid-connected homeowners, because an expensive energy-efficient appliance is still cheaper than a less efficient appliance plus the PV equipment needed to keep the appliance functioning."

The bottom line: If you are paying between 8 cents and 24 cents per kWh for your electricity (as is the case for most grid-connected homeowners), a solar thermal system is a poor investment. But if you are paying between 50 cents and $1 per kWh for your electricity (as is the case for most off-grid homeowners), a solar thermal system makes sense.


3.
Jun 7, 2017 11:34 AM ET

An important topic for concerned ratepayers
by Brian Bailey

Thanks for covering this, Martin.

The importance of this topic extends well beyond the Northeast Kingdom and remote home sites. For those of us in states where governments and/or utilities are fighting distributed renewable energy tooth and nail, some homeowners (well, at least one) consider "grid defection" a means of protest.

A relatively small number of grid-connected homeowners choosing to forego electrical grid services will certainly send chills down the spines of utility company execs, whose business models depend on full ratepayer participation. To borrow a term from some other (baser) political discourse in recent memory, grid defection on a significant scale could cause a "death spiral" of higher costs to remaining ratepayers, which increases grid defection, rinse/repeat. Utility companies are acutely aware of this possibility.

While grid defection may not make financial sense (yet) for those with access to cheap fossil-fueled electricity, a grassroots movement in that direction would be a very powerful incentive for power companies to take a more cooperative approach to distributed generation. For some ratepayers, the potential to play a role in bringing electric utilities kicking and screaming into the 21st century may be well worth a relatively modest energy premium.


4.
Jun 7, 2017 11:48 AM ET

Edited Jun 7, 2017 11:53 AM ET.

Response to Brian Bailey
by Martin Holladay

Brian,
I've written several blogs that touch on the topic of grid defection, which in my view is a long way off for most Americans. For example, see these two articles:

Batteries for Off-Grid Homes

Green Building in the Cheap Energy Era

While grid defection is both more expensive and much more of a hassle than the average dreamy-eyed green builder realizes, we can state with certainty that any utility that thinks it can avoid the "death spiral" problem by raising rates will eventually face the fact that two lines will cross: the increasing cost of grid electricity and the decreasing cost of off-grid electricity.

When those lines cross -- the exact time will depend on local conditions -- the death spiral will be in its final stage.

Nimbler utilities (like Green Mountain Power in Vermont) will enthusiastically jump on the photovoltaic bandwagon, and will develop a variety of services for PV-equipped homes. Utilities that embrace rather than reject the new "distributed generation" future are the ones that are most likely to survive.


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