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.
Sizing the Battery Bank
In the example above, we are looking for at least two days of autonomy, with a total energy consumption of 2.628 kWh—but that doesn’t mean a 2.628 kWh battery bank will be enough. Efficiency losses of about 15% from our inverter and wire resistance bring that two-day need to 3.09 kWh (2.628 kWh ÷ 0.85). And, lead-acid batteries of any formulation—by far the most common and affordable choice for most off-grid applications (see below)—should rarely be discharged below 50%. That means only 50% of the battery bank capacity is actually usable without drastically reducing battery lifespan. A 6.18 kWh battery bank is required (3.09 kWh ÷ 0.5).
Off-grid batteries are rated and sold by amp-hours (Ah) of capacity, but fortunately the conversion to kWh is easy: Just multiply the Ah rating by the voltage of each battery and add them up. For example, a typical L-16 battery provides 350 Ah of storage at 6 V, and 6 times 350 equals 2,100 watt-hours (2.10 kWh). Our required capacity for two days of autonomy was 6.18 kWh, and 6.18 divided by 2.10 equals 2.94 of these batteries. At 6 V each, they must be used in pairs for a 12-volt system; groups of four for a 24 V system; and groups of eight for a 48 V system. Four would be the minimum batteries required here, offering 8.40 kWh of storage which actually increases autonomy time and also reduces average long-term depth of discharge for longer battery bank life.
Sizing the PV Array to the Battery Bank
Here’s where the occupancy patterns of a remote, off-grid cabin can significantly affect the system design and the budget. For a full-time off-grid residence, with such low prices per watt on PV modules, conventional wisdom now dictates designing a PV array to bring the battery bank from 50% SOC to 100% SOC over the course of one sunny “average” fall or spring day—likely less than a day of charging time during the summer months, and likely more than a day during the dead of winter. If the system has more days to charge the batteries to 100% before the next occupied period, the PV array could conceivably be sized smaller, with the option of expanding it in the future if usage patterns change.
Take a typical PV array in Colorado for the example above, sized for the loads, autonomy time, and battery bank for full-time occupation, with 100% of loads powered by PV during spring and fall, an energy surplus during summer and some backup generator run time required in winter. Bringing the four L-16 batteries (a total of 8.4 kWh of energy storage) from 50% to 100% SOC during spring and fall (including a derate of 30% for battery charging system inefficiency and PV output loss from higher-than-STC cell temperatures) would require 6.0 kWh (8.4 x 0.5 ÷ 0.7). With the site’s insolation of 5.5 peak sun-hours per day during that season, about 1,091 W of PV would be required. With four or five days (in an unoccupied situation) to charge the system, it would be no problem to halve that amount of PV—perhaps two modules instead of four, and still have excess charging capacity.
System Equipment Phantom Loads
Inverters are phantom loads—they still use power even if no loads in the home are running. Most off-grid inverters can be set for “search mode,” which drastically reduces their no-load power use. Typical search-mode power drain is only 5 to 10 W. The inverter remains asleep, but will wake up quickly if a load that draws enough power is turned on. The watts required to bring it out of search mode are adjustable. This can be very convenient upon arrival back at the cabin at night for a vacation weekend, with no need for flashlights to find and turn on the inverter again. But too many phantom loads can keep the inverter on and out of search mode, so it’s best to test carefully before buttoning down the system prior to an extended departure.
Set the inverter for search mode, and turn off or unplug all your loads. After a few seconds, the inverter should drop into search mode, usually indicated by a blinking indicator light or a message on the remote. If it doesn’t do this, increase the search mode watts setting on the inverter. Now, turn on the first light you’ll need when you arrive. If it doesn’t work or it flashes, decrease the search mode W setting until the light operates properly.
Keep in mind that if you have equipment such as a Wi-Fi router and security system that must remain running when you are away, search mode won’t be an option, since the inverter will remain on. Inverter power draw when on (but with no loads operating) varies by manufacturer, but 20 to 40 W is typical. And remember that even if your loads use less than that, the inverter’s no-load draw will still be the minimum. You can find the no-load draw on the inverter specifications sheet.
Operations & Maintenance When Vacant
PV systems require very little regular operations and maintenance care, with one exception: the battery bank. Lead-acid batteries—no matter what type—will be permanently damaged by sulfation (deposits of sulfur on the lead plates, which partially block electrolyte contact with the plates) if left at a low SOC for an extended period of time. Simply shutting down the entire system including PV modules, inverter, and loads doesn’t help this situation, as all lead-acid batteries “self-discharge” when sitting unused, noticeably lowering their SOC as time goes by. Therefore, it’s essential to keep the PV to the battery-charging side of the system operational during absences.
Remember also that PV charge controllers themselves are phantom loads that use a small amount of power all the time, typically 1 to 4 W. That can be problematic in cold, remote locations where the PV array can be covered with snow for weeks or months at a time with nobody around to clear it. One solution sometimes used by wily remote system designers is simply a single PV module mounted vertically on a south-facing exterior wall. It doesn’t have to live there all summer long, and can be quickly deployed just before vacating for the winter. That single module can provide enough charge to make up for battery bank self-discharge, plus charge controller and (possibly) inverter phantom loads, over an entire winter. (Note: This strategy assumes the module, charge controller, and battery bank voltages are carefully selected to allow the single module to charge the battery bank.)
All lead-acid batteries emit gas when charging, and some require the electrolyte be topped off regularly with distilled water. If this maintenance isn’t performed and the electrolyte level drops below the tops of the internal plates, the batteries will be permanently damaged. Different strategies to solve this problem depend on exactly what type of battery is selected for the installation, and how often the batteries need to be topped off.
Temperature also plays a role in battery selection for an unattended system. Consistently high temperatures (greater than 80°F) cause all battery types to age prematurely, which can be a problem in desert and tropical locations. Cold temperatures won’t damage most fully charged batteries. Common lead-acid batteries are good down to -50°F (and lower)—but only if fully charged. If deeply discharged, their electrolyte is mostly water with the sulfuric acid essentially absorbed into the lead plates; they can freeze and even burst at temperatures near 0°F. Never try to charge a frozen battery; it must be removed from the battery bank, allowed to thaw, and then charged very carefully to see if it is salvageable. Most likely, it will need to be sent for recycling and replaced. I highly recommend a sealed, vented, and insulated battery bank enclosure for all battery types. This tempers both heating and cooling of the batteries.
Battery Types & Charging Strategies for Unattended Locations
Using the proper battery type is crucial when the system won’t be receiving regular maintenance for an extended period of time. Ambient temperature is an important consideration, as is the specific charging regime programmed into the charge controller.
Flooded lead-acid batteries are a common and cost-effective choice for many remote installations. They handle low ambient temperatures gracefully and without damage, as long as they are kept at full (or nearly full) by the PV array. Their biggest disadvantage is that they require regular watering—at least four times per year, and sometimes more frequently. Automated watering systems with a central reservoir and valves in each cap are available, but will not function if the battery enclosure can reach temperatures below freezing. Catalytic re-combiner caps are also available, but do not entirely eliminate the need for regular watering—they simply reduce the frequency.
Flooded lead-calcium batteries are another option, though they are more expensive and difficult to source. They use calcium instead of antimony in the plates, resulting in less water use and a slightly lower voltage versus SOC curve.
Clever PV installers have found that it is possible to change the charging regime to reduce gassing in flooded lead-acid and lead-calcium batteries by changing the charge controller settings. Setting the absorb voltage high (for example, 15.0 V for a 12 V system), absorb time to only an hour or two, and the float voltage quite low (around 13.0 V for a 12 V system), minimizes water loss. This runs the risk of not charging the batteries to 100% SOC, but during an extended absence of a month or more, they will likely fill with no trouble if there are no loads during this time.
Absorbed glass mat (AGM) batteries are the star when it comes to remote systems that receive little or no maintenance, but their benefits come at a price—they are two to three times the cost of flooded cells for a battery of the same capacity. AGMs use lead-calcium chemistry and contain integral catalytic recombiners to prevent gassing under normal charging conditions. They are sealed so they can’t spill, and are almost immune to freeze damage no matter what their SOC. But because they are sealed, there is no way to top off the electrolyte if too much gassing occurs, so following the battery manufacturer’s charging instructions precisely is essential.
Nickel-iron (NiFe) battery technology goes back more than 100 years, and has recently been experiencing a resurgence in popularity for off-grid systems. NiFe cells have an extremely long lifespan—at least 25 years compared to the four to 10 years expected from lead-acid chemistry—but are considerably more expensive. They can be left idle in storage, not charging or discharging, for long periods of time over a range of temperatures (-40°F to 140°F) without damage, and are not harmed by a low SOC. However, during daily cycle use, they consume a lot of distilled water. NiFe cells would be best-suited for a remote vacation cabin in which the entire system—including PV input, all controllers, and inverters—is shut down for long periods of time.
Lithium batteries (there are many different specific chemistries available) are the relative new kid on the block, and certainly show promise. (See “Gear” in this issue for a few Li-ion options.) They are expensive (but coming down in price), and require a sophisticated battery management system to keep them healthy during cycling. Advantages include long cycle lifespan; efficient charging; very low self-discharge rates when left idle; and small size and weight. Be sure to check their specified operating temperature range, both high and low—charging at low temperatures can permanently damage them.
Silicon salt batteries are a promising newcomer to the world of remote off-grid system design. They claim to handle a high rate of charge and discharge, are nontoxic and maintenance-free, and are rated to perform in a very wide temperature range (-40°F to 158°F). They are more expensive than AGMs, and have not been around long enough to know if the claims of a 15-year lifespan are accurate.
Saltwater batteries are another very new addition to energy storage technology for off-grid systems. They are expensive and can’t provide much surge current (for example, to start an off-grid well pump) without paralleling additional stacks, which can create a battery bank capacity that is larger than necessary. While they can survive temperatures as low as 15°F, their capacity will be permanently lowered. On the positive side, they are maintenance-free, nontoxic, nonflammable, and can be left at a partial state of charge and even discharged to 0% SOC without damage.
Automatic Generator Start (AGS) Systems
For larger remote cabins in which owners don’t want to shut off all the loads and drain the pipes during extended absences, most off-grid inverter/chargers can be programmed to automatically start a backup battery-charging generator if battery voltage (or even battery SOC) gets low (i.e. reaches a certain threshold setpoint). Gasoline, propane, and even diesel generators are possible for AGS with the addition of some simple circuitry from the inverter/charger manufacturer or third-party companies. Complicated startup routines for diesel generators are no problem—AGS circuitry can preheat the glow plugs, attempt to start the generator, sense if it actually starts or not, and, if unsuccessful, retry the whole procedure after a waiting period.
Unfortunately, there are a lot of “ifs” in the process, especially when no humans are available to intervene when (not if) things go wrong. My informal survey of remote, off-grid system installers here in the remote Colorado mountains match my own experiences: All of the various AGS systems from different inverter/charger manufacturers and third parties work equally well—it’s the generators themselves that are cantankerous. Dead starting batteries due to multiple failed starts can be caused by a generator that’s low on fuel, or low on coolant, or has low or dirty crankcase oil; by an owner who accidentally messes up the inverter AGS settings while trying to reprogram; if AGS settings are lost when the owner shuts down system for regular maintenance; a generator buried in snow, that overheats due to lack of ventilation; a wasp nest in the generator electronics box that shorts AGS circuitry...the list goes on. I prefer to keep things simple by reducing cabin loads to zero (or as close to zero as possible) when the property is vacant—and avoid AGS systems entirely.
Remote System Monitoring
If your vacation cabin location has internet service, installing remote monitoring equipment is often simple and inexpensive, requiring only a router that uses only a few watts and, possibly, an outdoor antenna. Most inverter and charge-controller manufacturers include data ports on their equipment that can talk directly to the internet via Wi-Fi, and third-party companies also provide this service. In most cases, you don’t need your own website to view your system data live from anywhere in the world—it appears on the company website after you log in. These services usually are free initially; after a year or two, a small yearly fee is charged.
If your only internet connection is broadband satellite, it gets more complicated, as the satellite dish and modem can draw from 50 to 100 W when turned on. For most systems, that draw is usually too large to leave on unattended for weeks or months at a time. Ham radio can also be used to remotely send data about the status of your home temperatures, power system, and battery bank SOC while using very little energy, as can satellite short-burst data transmitters. The sky’s the limit on remote system monitoring, if it fits your budget.
Plan Carefully, Then Go Remote!
Keeping a vacation cabin power system alive and well during extended absences is not difficult—it just requires pre-planning, careful component selection and using a thorough checklist before you button up. One last piece of advice: don’t forget to turn off the lights before you leave!