Unless you have a good hydro or dependable wind source, or an excessively large PV system, sometimes the only way to keep your off-grid power system operating is by using an engine generator. But many residential generators are poorly matched to the job, since the selection and purchase of a generator is often an afterthought. The result is the generator becoming the weak link in the system, with inefficient, unreliable performance, rather than being an integral component of a well-functioning system. Here’s how to analyze your generator needs and choose the right unit for the job.
Generators serve three main roles in off-grid home systems:
“You get what you pay for” might as well have been coined by a generator owner. Even in a well-designed off-grid power system, the generator is likely to be the single greatest source of maintenance and aggravation. It’s a mechanical device, rather than electrical, so it requires regular maintenance and will eventually break down—usually when it’s most needed.
Ask yourself: Is a breakdown just an annoyance or will it completely disable my home’s power system? The more serious the consequences of the inevitable breakdown, the greater the appeal of a top-quality commercial/industrial generator. Consider also that unless you are prepared to maintain and repair the machine yourself, you will need reliable local support when the eventual failure occurs. An investment in quality quickly pays off.
Generators for home use may be purchased for as little as $300 or as much as $12,000. The least-expensive units are designed for occasional homeowner use with power tools or cement mixers, for instance. “Contractor’s specials,” common at home-improvement centers, are just that—with their low initial cost, they are designed to provide temporary power until utilities are installed or to build a remote cabin. They’re designed to be worked hard and then written off as a business expense when they die.
For off-grid backup, your choice will constitute a compromise between application, cost, availability, and support. But a higher-quality unit will generally have the following features:
Generators are fueled by gasoline, propane (or its cousin, natural gas), diesel, or biodiesel. The least expensive generators and nearly all portable units use gasoline, which offers some advantages—mainly that gasoline is familiar and manageable by the unskilled user. However, it’s highly flammable, and fuel storage will need to be a consideration. Allowed to sit in a generator’s fuel tank, gasoline will eventually deteriorate, leading to clogged orifices and preventing the generator from starting when needed. Cold-starting also requires engaging a choke. Most auto-start generators are not gasoline-fueled for these reasons. A gasoline generator may be best suited to applications where it is only used occasionally and the gasoline is drained (or run out) after each use.
LP gas (propane) is commonly used in higher-quality stationary generators. Many off-grid homes already have an LP tank and lines for cooking and heating, so the fuel is convenient and relatively safe. Propane has an almost unlimited storage lifespan. LP generators will generally start reliably down to about 10°F. Natural gas and propane burn cleaner than gasoline and diesel, and wear on internal engine parts is reduced.
With some notable exceptions, diesel generators are typically long-lived and high-quality—and relatively expensive. However, the fuel is best suited to warmer climates, as it gels below freezing temperatures. The drawback to diesel fuel is that, compared to natural gas or propane, it’s a “dirtier” fuel, producing more pollution. Fuel storage requires extra care against spills and leaks, and may even be regulated. For the eco-tinkerer, diesel units may also be run on biodiesel or either virgin or waste vegetable oil, taking the appeal of “living on renewable energy” to a greater level. Vegetable-based fuels don’t generally need additives to prevent gelling; the fuel is heated instead.
As a general rule, a generator should be able to supply the full charging capacity of your inverter, plus loads that typically run while charging and provide some reserve capacity. This both minimizes runtime and maximizes C-rate—the charge rate necessary for good battery care. Generators used in home systems will vary between about 2,500 watts and 20 kilowatts of continuous rated power.
Often, a generator is purchased before the power system is designed, as an early source of power for construction and water supply. The result is that the RE system designer must take the existing generator and adapt the system design to best use it—a backward approach to system design. A better plan is to consider a generator as part of the overall power system design. The preferred approach to sizing a generator starts with the battery bank.
Most PV arrays for off-grid homes will be sized to meet or exceed design loads for part of the year. This seasonal duration may cover all except the heart of winter in the sunny Southwest, or may only cover half of the year in northern climes. Battery banks are typically sized to provide two to four days of autonomy (time the batteries can supply energy without further charging), depending on the seasonal climate and other factors.
Because arrays and batteries are sized to different purposes, the array is seldom large enough to meet even the minimum C-rate needed to routinely fully charge and regularly overcharge (equalize) the battery bank. Hence the need for the generator, which can add the necessary charge current to get this done, and may be even the only contributing charging source during winter—so it should be sized to handle the job.
Typical utility power is split-phase 120/240 VAC. If you look inside a home’s electrical distribution panel, you will see three non-ground wires coming in: two “hot” conductors and one “neutral” conductor. The voltage between each hot and the neutral conductor is 120 V, and the voltage between the two hot legs is 240 V. Most off-grid homes have only one hot conductor, and operate at 120 V.
An off-grid system’s generator must be matched to the voltage needs of the inverter/charger. Generators provide AC output in one of four ways:
Most inverters manufactured since the late 1980s can only receive 120 V input from a generator. For most off-grid homes, this is not a problem, since 240 V loads are typically large bulk loads from water heaters, clothes dryers or electric ranges that are inappropriate in an off-grid home. With the exception of AC well pumps, off-grid homes are unlikely to have 240 V loads.
When a typical 120/240 V generator that is not capable of full output at 120 V is paired with a 120 V inverter, two problems occur. First, a 120/240 VAC generator is only capable of half of its rated output when connected to the input of a 120 V-only inverter. Second, the load on the generator is unbalanced, with one 120 V leg fully loaded and the other unloaded. This imbalance can lead to premature generator failure.
This means that the less-expensive generators are least able to efficiently back up the most common inverters. For example, a low-cost generator may be specified at 5,500 W maximum output. But the usable “rated” output is not “maximum” output. Similar to the way an inverter’s continuous-duty capacity is compared to its capacity to meet larger surges, “rated” output is the level of power a generator can deliver continuously. Usually, it’s about 85% of the “maximum” power, although some lower-quality generators are not even capable of their “rated” capacity. How much power is really available? First, check the rating carefully: the 5,500 W rating is likely to be around 4,700 W in continuous duty, such as battery charging through an inverter. Second, the continuous rated power is available only at 240 V, so a single 120 V inverter can only access half of this output capacity, or 2,350 W.
One solution is to use a balancing autotransformer to step down the 240 volts to 120 V before sending it to the inverter. This allows the full power and both 120 V legs to work in balance. However, a typical autotransformer for this purpose retails for more than $500, making the “low-priced” generator much more expensive.
Another solution, usually practical only in larger systems, is to use a stacked pair of inverters to both receive and produce 120/240 V power. This is a common approach with OutBack inverters. A few modern off-grid inverters, including Magnum’s AE series, Xantrex’s XW, and Apollo’s TrueSineWave are configured as 120/240 V split-phase units. These units are able to use both 120 V legs of a generator to provide balanced charging from both hot legs. The new Silent Power SP4024 inverter can use either 120 or 240 V input. No step-down transformer is needed with any of these inverters.
Another option is to select an inverter-generator, a relatively recent development. The inverter (which cannot substitute for the RE system’s inverter) is an integral part of the generator, electronically synthesizing the voltage, current, and waveform, while maintaining full-peak voltage up through their maximum load. As the 60-hertz AC waveform is not dependent on engine speed, the engine speed varies according to the load. This allows an accurate pure-sine waveform, while reducing fuel consumption and noise.
Generator models are rapidly evolving, with new models entering the market and old favorites being discontinued. Most of the reliable, small generators of years past were made obsolete by California’s emissions standards and changing markets. Newer models have been developed primarily for the on-grid home standby market, rather than for stand-alone use. Standby models, intended for backup power during utility outages, are marketed as low-maintenance home appliances. They will commonly require utility power to maintain controls and charge the starting battery, and even for running block and intake heaters in cold weather—all unacceptable loads in off-grid use. In addition, fierce competition in this market has encouraged an emphasis on low price over high quality. There is a clear and unmet need for simple, reliable, commercial-grade generators not designed for the residential standby market.
The safest bet is to buy a well-respected brand, or at least a generator that comes with a brand-name engine—it will be much easier to find parts and service for a well-established brand than for some no-name model. Finding a high-quality used machine with low hours and evidence of proper care may also be a possibility. Consider also an inverter–generator, as these are gaining wide acceptance, with sizes available up to 6.5 kW. Honda, Yamaha, Robin, and Cummins make high-quality inverter-generators. All run on gasoline, but some of these brands may be converted to LP gas without voiding manufacturers’ warranties.
Among conventional portable units 6 kW or less, Honda’s EM or EB, and Yamaha’s EF and YG series have been well regarded for years. Multi-Quip and Robin—made by Fuji Heavy Industries and marketed under the Subaru, Makita, and Baldor names—also have solid reputations.
In higher-quality commercial units, Kohler’s 10ERG is a well-regarded model developed for the RV market and adaptable for off-grid use. Onan makes several adaptable units, including the RV QG 6500LP, RS 12000, and RS 20000 LP, the latter two suited to larger systems. There are some nice Onan diesel inverter/generators (the HQD series) up to 18 kW.
Notable negative comments have generally been directed at the cheapest generators, with poor waveforms, inflated output claims, and lack of long-term reliability. Also, avoid knockoffs of well-known models, especially low-cost inverter-generators now on the market. Reliability is unknown, and repair parts may be unavailable.
A PV system installer since 1988, Allan Sindelar founded Positive Energy Inc. of Santa Fe, New Mexico, in 1997. He is a licensed commercial journeyman electrician and a NABCEP-certified PV installer.
This article was prepared with the assistance of Roy Butler of Four Winds Renewable Energy, Arkport, NY; Tad Jennings of Power Generation Services, Albuquerque, NM; John Klesk of Cummins Power Generation, Minneapolis, MN; Terry Graybeal of Backwoods Solar, Sandpoint, ID; and Tom Bishop of Sunelco, Hamilton, MT.