Power Systems for Off-Grid Vacation Cabins

Intermediate

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In an off-grid, part-time cabin, battery bank sizing and care are often more important than PV array size.
This off-grid cabin has solar water heating collectors and solar-electric modules.
This tiny house, occupied part-time, requires only a couple of PV modules to meet its electricity needs. A solar water collector provides domestic water heating.
This compact, modified sine wave, off-grid-only inverter is offered in 600 W and 1,500 W versions. It has a built-in 120 VAC charger for charging the battery bank from any AC source, such as a generator.
This multimode inverter can receive grid power and send out PV power, and can also operate in off-grid mode.
Nickel-iron batteries are an “old” technology that offers superior longevity, but also has efficiency and financial costs.
High-performance lithium-ion batteries are light and small, but require sophisticated charge management and are expensive.
Saltwater batteries are quite new to the scene but the industry is hopeful about their efficacy.
AGS units usually work fine—it’s other, less reliable parts of the system (like the generator and fuel supply) that cause many pros to discourage using them.
Though often less than desirable because of the noise and pollution it generates, a backup generator for battery charging during times of little sun can be a necessary addition.
The author’s family cabin, at 8,200 feet of elevation in the northern Colorado mountains, uses a small PV array to keep full-time loads running and batteries well-charged, even when it’s vacant.
Most inverter manufacturers and third-party companies offer remote system monitoring, but you’ll need always-on internet access at your cabin.

Owning a cabin in the woods to escape during those all-too-rare vacations is a dream come true for many people. And where better to go for peace and quiet than a remote location, far from the nearest power line?

Off-grid solar-electric systems with battery storage are becoming more affordable every year as equipment prices drop. But cabins that are occupied only seasonally or on weekends present unique challenges for system design, operations, and maintenance compared to full-time off-grid residences, where the owners are home regularly to keep an eye on things.

The first thing to consider when designing a power system for a vacation property is the expected pattern of use. For how many weeks or months at a time will the dwelling be unoccupied, and does that use pattern change depending on the season? For example, in snowy climates the cabin might be occupied every weekend during summer with an occasional stay of a week or two, weekend stays during spring and fall, and only a rare weekend during winter for some cross-country skiing or snowmobiling. In warmer climes, the use patterns may not vary so much seasonally.

These use patterns can even change the basic system design parameters of battery bank capacity versus PV array size. With an off-grid home that’s occupied year-round, it’s common to size the PV array to bring the batteries from 60% state of charge (SOC) to 100% over the course of a single sunny day. But if a cabin is only occupied on weekends, the PV array could be smaller—it has all week to get the batteries back up to 100%, and it is possible to add more PV modules in the future if use patterns change. Battery bank autonomy time (how long the battery bank can run the cabin with no energy coming in from any source) is also important, and should be factored in when designing the system for both occupied or unoccupied periods. However, a backup generator, which will bring the batteries to 100% SOC in a short time, is highly recommended for most off-grid installations.

Occupied vs. Unoccupied Loads

It is essential to look at what loads need to be running while the cabin is unoccupied. Ideally, these will be minimal or non-existent. It is certainly possible to keep everything running as usual, but the system will have to be larger and more expensive, with more regular maintenance required. Possible loads during these periods might include wireless internet for remote system monitoring, circulation pumps, fans and controls if solar thermal heating is also installed, or a remote security system. Larger loads, such as refrigerators and freezers, are best left emptied and unplugged if possible. Phantom loads can be eliminated by using switchable power strips, by unplugging the load, or turning off their circuit breakers before leaving.

Consider a cabin occupied only on weekends with some typical occupied loads (see table). When cloudy weather strikes, your battery bank may become quickly depleted unless you can also employ “load shifting”—running larger electrical loads only when the battery bank is fully charged and there is extra power coming in. This will bolster autonomy time. Simple energy conservation measures, like forgoing that DVD movie for a game of cribbage during a snowstorm, can make a big difference in autonomy time, too.

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