Only 30 years ago an off-grid homestead’s photovoltaic (PV) system was likely to consist of two 35-watt modules and one or two car batteries. Such a system had no inverter, as reliable inverters were still years away. The typical 12-volt system powered little more than a few low-voltage lights, a car stereo, and a few 12-volt products developed for early RVs. But for power pioneers, this kind of system was heaven-sent — a wonderful alternative to kerosene lamps and engine-powered generators.
The batteries used in early PV systems weren’t designed for the deep discharges common in home use, so they often failed to hold charge after only a year or two. As a result, early deep-cycle batteries for homestead systems were adapted from other uses. The most common batteries were originally developed for golf carts, supermarket floor scrubbers, and mine cars. Only a few people could afford to order industrial-grade batteries directly from manufacturers.
Renewable energy systems should be sized so that the total amount of daily charging energy from all sources — PV, wind, hydroelectric, or generator — exceeds a home’s total average daily load. The role of the battery bank is to provide stored energy between charging periods — nothing more. When carefully planned and executed, the batteries in a home power system will run everything in your home — but “everything” will include far less than in a typical mainstream home.
Living well on a small and finite amount of electricity starts with careful adherence to three basic principles: Shift inappropriate loads to other forms of energy, reduce waste through efficiency, and use power in proportion to the amount available.
Efficient Use of Electricity
All forms of energy are not created equal. Electricity is a specialized, high-quality form that’s best suited to specific applications: lights, electronics, and motors, plus a few other specialized uses. By matching the best form of energy to its application, total electricity consumption can be greatly reduced while enhancing comfort and convenience. Five common uses of electricity in conventional grid-tied homes won’t typically show up on an off-grid homestead: space heating, water heating, cooking, clothes drying, and air conditioning. Each of these consumes too much energy to be appropriate when the supply is finite and the task is best-served by other forms of energy.
In the early years, PV modules were much more expensive than they are today, and batteries were less expensive. Early practice was to size for a week of storage capacity, but that led to small arrays and large battery banks, resulting in chronically inadequate charging. Modern systems are often sized for only two to three days of storage capacity, as long as the system includes a backup generator. As budgets allow, users add an additional array rather than increasing battery capacity.
Battery-based systems are generally wired to 12-volt, 24-volt, or 48-volt standards. Many folks still assume that a homestead system will be 12-volt because that’s all they’ve seen. Twelve volts is a holdover from the days before reliable inverters were available. Automotive and RV products — including lights, TVs, and unreliable blenders — were all available in 12-volt capacities to serve that industry. Good inverters have been available since 1987, and systems have progressively moved toward inverter-based, alternating current (AC) loads in the years since. With AC-based systems, 12-volt advantages have largely disappeared, and the strong disadvantages of high-current and large-wire sizes discourage the use of 12-volt batteries for all but the smallest cabin homes.
Flooded Lead-Acid Batteries
The most common batteries in off-grid residential systems are flooded lead-acid. Among flooded lead-acid batteries, there are three common battery types from which to choose; each has its niche. Golf cart batteries are mass-produced by the millions and remain the best low-cost choice for small systems. Seldom do sets last more than seven years, and typically they last four to five years. In return, they stand up to remarkable abuse, including chronic undercharging and lack of equalization, and still provide adequate service. For some systems, frequent recycling and replacement of a set is a reasonable, low-cost approach. However, golf cart batteries only work in smaller systems.
The second commonly used flooded lead-acid battery type is the L16. These were originally designed for use in supermarket floor scrubbers — they were used all night and charged from the grid the next day. Like golf cart batteries, L16s have been adapted from another application for their manageable size, deep-cycle performance, and relatively low cost. In the years since, their size has become a standard, and battery manufacturers build and market them for PV use. L16s are more expensive per amp-hour than golf cart batteries are, and they don’t supply substantially more cycle life — about five to six years is typical. They’re well-sized for small-to-medium systems, and at 120 pounds, they can be moved by two people.
True industrial batteries are the final type of flooded lead-acid batteries. They no longer fit a dimensional standard because they’re not adapted from another industry. Rather, the size and capacity of each cell is determined by the number of plates of active material. Industrial cells are more expensive up front, but their substantially greater cycle life — 15 to 20 years of good performance is typical — has proven to be the best long-term value.
For new off-grid homesteaders, a set of L16s or even golf cart batteries will work as a great training set. Some homesteaders are simply better than others at maintenance duties, and a ruined bank of industrial cells is a bitter pill to swallow. An inexpensive first set is a smaller investment and allows for several years of adjustment to a healthy off-grid lifestyle.
The Future of Off-Grid Batteries
Battery options have been slow to evolve because of a quandary unique to batteries: Given that deep-cycle batteries can last 15 to 20 years, learning by experience can take decades. Few off-grid installers have been selecting, installing, and maintaining batteries long enough to learn from entire battery life cycles. Without much long-term data, we tend to use what has worked previously, rather than trying new and possibly expensive approaches.
While tremendous advances are taking place in battery development, most are based around increasing a battery’s performance and energy density per pound — that is, lightweight, high-capacity batteries for electric vehicles and portable applications. In homestead systems, weight isn’t a key factor; a more relevant figure has been energy density per dollar. For most homesteaders, conventional flooded lead-acid batteries still fit this bill best.
A significant change is occurring in the selection of batteries for off-grid applications. This is the increasing shift to sealed, absorbed glass mat (AGM) batteries from the more traditional flooded lead-acid. The transition is still under the radar in much of the industry. Sealed batteries are capable of supplying acceptable performance and cycle life in the deep-cycle demands of many off-grid applications. This is a relatively new shift without years of experience from which to draw conclusions and predict performance. The expectations of some experienced installers are that high-quality, maintenance-free AGM batteries may be expected to last for eight or more years of full-time, off-grid use with good care.
Sealed batteries offer some substantial benefits over flooded batteries. They require no maintenance beyond proper charging. Because the electrolyte is either gelled or absorbed, these batteries don’t release gas during normal charging, and they don’t need a sealed enclosure with controlled ventilation to the outdoors. They can tolerate smaller arrays and lower charge rates as long as they’re fully recharged regularly. They don’t leak and won’t contaminate battery storage areas or attract corrosion on terminals. AGM batteries can be shipped via common ground freight with no hazardous-material costs. Because access to the cell tops to add water isn’t necessary, they can be mounted in any orientation without harm. They may occupy less floor space when stacked than flooded batteries do.
Sealed batteries aren’t without their drawbacks. They’re substantially more expensive and more susceptible to damage from overcharging. They’re well-suited to homeowners who don’t want to perform their own battery maintenance, as the charge for professional service several times each year adds up. This group might include many newcomers to off-grid living, who value the benefits but don’t desire the DIY involvement of earlier generations. They want (and can afford) a professionally designed and installed system and can live well within its limitations, but they prefer not to be involved with maintenance. Sealed batteries are also well-suited to homes used seasonally and maintained more infrequently, and for weekend cabins in which small arrays and larger banks provide energy for weekend-only use.
My advice on current battery technology for homestead use is to stick with tried-and-true lead-acid batteries for now, whether flooded or sealed. For most homesteaders, I advise against using any of the new technologies at this time. Tremendous advances in battery technology are happening now, and battery choices will look very different in five to 10 years. Homesteaders may want to consider waiting through one more set of batteries before trying lithium or other emerging technologies. Superior technologies are coming, and prices will drop as PV module prices have dropped in recent years, but we’re not there quite yet.
Allan Sindelar has been designing, installing, servicing, and teaching about off-grid PV systems since 1988. He’s a licensed electrician with dual NABCEP certifications. He’s the retired founder of Positive Energy Solar and has lived off-grid for most of the last 25 years.