*Assumptions: 3% discount factor & 4.4% annual utility rate increases

*Assumptions: 3% discount factor & 4.4% annual utility rate increases

This econort^Mnalysis can be performed by a spreadsheet like the one that wasMgi to create the "Discount" table. You can download an acti^f^preadsheet to do your own calculations Irom the Web Extri^ section at www.homepower. com. For simplicity, c-hwefgMS'eur discount factor the highest interest rate on any debt you h^ve, including your mortgage. If you have no debt, chocB?ihe highest investment interest rate you can e^with a low-Uk profile (which reflects the investment^risk" in an RE system). A 10-year goVtaindiyaf bond is a good choice; it currently yields almost 3B.

The^Bt column undwrUtilitpH^affitu^rojects the A>g,t of utilita^^'tririty at an annual irl<fl^3^4.*^djacent column is tjilRli9c&unted cJs^rf rfeSmjiyifrom toe utility— which talPp into gfcepnt inflationAtffects oil thaBSng cost Bf electrici^Jliiflnv tfikflalBe of the money you'd spend (^eia 30-year period in Eteagftl-day dollars. As you can see, although you will hava*elte^pjBK28,492 to the utility company, the present value of that Stp ijLcSily $16,942.

The first column undei^,;r\l System" shows what system costs were incurred Hrer?he year^—$27,200 minus incentives, or $10,710 to start. At 2P"^aiWwe added a $3,200 expense to replace the inverter—a OTnterilwjpredicted cost. If you have additional costs, like a maintenaffife^ontract, you would add that cost into that column fAsacfrgear.^ii this case, over a 30-year period, you will have invested® 3,910 in&oiffeystem (in present dollars). The last column slpw^th^Vie^tejji's discounted cost, the present value of your expenditure, factoring in the discount factor of 3%.

The final step is to compare the discounted cost of the system ($12,482) to the discounted cost of electricity from the utility ($16,942). In this example, the present cost of the PV system is $4,460 less than the present cost of utility electricity.

If an RE system is cheaper than buying electricity, it makes economic sense. If it costs more, it doesn't. The greater the difference in the cost of the two systems, the more compelling the economic argument to install.

Do the Math, Make Your Decision

Comparisons based on the costs of energy, ROI, or net present value are vital to making a rational decision about an RE system. Beyond the measure of money, there are a multitude of other logical reasons to invest in a system, such as energy security, a cleaner environment, the satisfaction of producing your energy locally, and adding green energy to the grid. The bottom line is that the decision is yours, no matter what metric you use.


Dan Chiras ([email protected]) is the founder and director of The Evergreen Institute, which offers workshops in residential solar electricity, wind, passive solar design, home energy efficiency, and green building at the Center for Renewable Energy and Green Building in east-central Missouri (

John Richter ([email protected]) is the cofounder of the Institute for Sustainable Energy Education and former president of the Great Lakes RE Association ( He was featured on the PBS special, Michigan at Risk: Michigan's Green Energy Economy.

ty & Firefighting by Matthew Paiss

Courtesy Craig Allyn Rose ty & Firefighting by Matthew Paiss

Courtesy Craig Allyn Rose ire safety is typically the last thing people think of when planning their rooftop solar-electric system, but it quickly becomes a hot topic when a blaze ignites. Here's a look into the potential hazards of PV systems when a fire breaks out—and how to minimize risks to firefighters.

Why do firefighters climb up on the roof of a burning building? In a house fire, superheated smoke and gases (which can exceed 1,200°F) rise to the ceiling and then bank down back to the floor. Just one lungful of this smoke can kill. Cutting a hole in the highest point of the room allows the superheated gases and fire to rise out of the building, rapidly improving visibility as well as the survivability of the structure and those trapped inside. This also allows firefighters on the hose line to advance inside to locate the seat of the fire and any victims. This "vertical ventilation" has saved many lives and valuable property—besides actual rescue, this is one of a firefighter's primary responsibilities.

But the presence of rooftop-mounted PV arrays has made cutting through a roof more challenging. In the past, the fire service had plenty of room to ventilate where it is most effective—directly above the fire. With PV arrays now covering large areas of roofs, firefighters are limited in where they can cut and where they can exit the roof. Since the PV modules cannot be cut through, and moving them is time-consuming and potentially dangerous, rooftop PV systems pose some risks—mainly shock and trip hazards.

Most firefighters have had some education in electrical theory but usually employ the tactic of avoidance when it comes to electrical equipment. However, there are still those who believe that anything is manageable if you can swing an axe hard and fast enough—clearly not the best approach when dealing with electricity. Most firefighters will just ventilate as close as they can to the high point of a room. If an array is in the way, they will move to where they can cut safely and rapidly. One problem is that most roof systems employ lightweight trusses, held together by lightweight metal gusset plates. With small fastening points, they can warp and pull out in fire conditions. These roofs are known as "20-minute roofs," meaning that firefighters have 20 minutes or less to get up, make the necessary cuts, and get down before the roof gives out. So, time is of the essence when navigating a hot roof with a PV system.

Fire Safety Steps

Assessing the Situation. One of the first things firefighters do at the scene is to take a "hot lap"—a quick walk around the building to see all sides and to locate the utility shutoffs. It is usually at this point that a PV system is noted if the array was not visible upon arrival. An inverter—often outside near the meter and service panel—also serves as a signpost. Likewise, if metal conduit is present in an attic, that's a red flag that a PV system may be present.

But there are some cases where obvious indicators of a PV system are not evident—such as in cases where the modules are integrated into the roof or the inverter is located indoors. Besides visual identification, a common way to note a PV system is to look at the labels on the main service panel, typically located on an exterior wall. The labeling may be on the outside or inside of the main panel. There should be a dedicated breaker for the inverter, labeled "solar inverter" or some variation. This breaker also may be in a subpanel inside the structure, but a label on the main service panel should always state that there is a second generating source onsite as well as identify the dedicated breaker for the inverter. New guidelines from the California Office of the State Fire Marshal advocate labeling along the PV array's DC conduit run as well.

Shutting Down the System. With any structure fire, shutting off all the circuit breakers at the main distribution panel, closing any gas mains, and notifying all on the scene that the utilities are secure is standard operating procedure. Shutting down a PV system is not as simple or straightforward.

With grid-direct systems, the first step is to disconnect the inverter, which happens automatically when the utility

Taking Solar-Electric Systems Seriously

California's Office of the State Fire Marshal has put together a PV task force to respond to safety concerns of the fire services—as well as concerns from the solar industry over limits and inconsistent regulations being imposed on installations. Last year, a final draft of installation guidelines was created. These guidelines address:

Marking: High-contrast, reflective, and consistent wording must be used on conduit, electrical panels, and disconnects.

Access, pathways, and smoke-ventilation space:

Providing a 3-foot setback from the edges of the roofline from gutter to ridge will ensure that firefighters can get onto and off the roof rapidly, if necessary. There is also a recommendation to provide a 3-foot setback along the ridgeline for ventilation. This is a highly contentious area, given the value of that space to both the fire service and solar installers. Alternative means of compliance are being considered.

Conduit runs: Considering the common use of chain saws for vertical ventilation, conduit runs should be kept 10 inches below roof decking (when run in attic spaces) to minimize the chance of being cut into.

For additional information on the draft guidelines, visit solarphotovoltaicguideline.pdf i Key Safety Points for PV & ^ Firefighting

• Daytime = Danger—the PV system will be energized whenever sunlight hits the modules.

• Nighttime = No hazard from the PV system, but be aware of potential battery bank dangers elsewhere on the premises.

• Ensure that all breakers at the main electrical service panel are shut off.

• When a PV system is identified, it is imperative that everyone on the fire ground be notified. Securing the main electrical panel does not shut down the PV modules. Voltage can continue in the wiring between the modules and the inverter in the daytime.

• Firefighter-supplied lighting, like high-lumen emergency lighting—called "scene lighting" in the fire service—will not produce enough light to generate dangerous voltage from the arrays.

• Sunlight can be blocked only by opaque material. Remember: All of the modules need to be covered for the array to be rendered inoperable.

• Stay away from the arrays and conduit. Don't break, remove, or walk on the modules.

power is shut off. Inverters also are designed with very good ground-fault interruption (GFI). If an inverter detects voltage between the ground and any of the metal conduit, the modules themselves, or the mounting racks, the inverter trips the GFI and opens the circuit. The circuit is also opened when the inverter is shut down manually by tripping the main circuit breakers either for just the inverter or for the whole house.

However, even with the inverter off, there's no easy way to shut off the high-voltage DC electricity flowing through the array and the DC wiring. In daylight when there is an open circuit, the modules are still putting out full voltage. There's no current flowing—that is, unless it finds a path to ground, like through a firefighter or an axe breaking through walls or ceilings.

The use of a rooftop disconnect at the array can lead to a false sense of security, since that merely opens one side of the circuit. Complicating matters is that many PV systems have more than one subarray—which can be located on another section of roof. These subarrays could backfeed power through the inverter or combiner into the conduit that was supposed to be de-energized by the rooftop disconnect. The concept of having both the line and load sides of a disconnect energized is a potentially dangerous situation in this application. For

Bright, clearly labeled disconnects are important to firefighters on the scene.

Bright, clearly labeled disconnects are important to firefighters on the scene.

Don't Forget!

Even though PV systems can produce high voltages, the AC line voltages that come into all structures can be much more dangerous. On a recent fire, I noticed that the utility service drop was within easy contact and the insulation on the wires was missing in some spots. Since this is on the utility side of the meter, the current would be unregulated up to the limits of the wire, which could be close to 1,000 amps—a serious electrocution hazard.

this reason, the California Office of the State Fire Marshal frowns upon the installation of rooftop disconnects.

During daylight, there can be enough voltage and current to injure or even kill a firefighter who comes in contact with the energized conductors. A hanging conduit with wires sticking out of it is nothing out of the ordinary on the scene of a fire, so firefighters must exercise extreme caution when navigating a fire. It is best to always assume that a PV system is energized and steer clear of the modules and conduit. Here's an example: If a firefighter accidentally or deliberately axes through a string of twelve 44 VDC modules, he or she will experience a potentially deadly surge of 528 volts.

Off-grid and grid-connected systems with battery backup have a few more circuits to consider. First, PV array disconnects should be shut off. Second, the battery banks in these systems are another power source that need to be shut down via a main DC battery disconnect to fully de-energize the inverter and any AC circuit fed by the inverter, as well as any DC circuit that might be present. Like the PV array, battery banks are live even after they have been disconnected from the rest of the system. And even though most battery banks are wired to low DC voltages (commonly 24 V to 48 V), which pose less of a shock hazard than the high-voltage DC circuits discussed earlier, their low voltage and high current nature can cause fires from overheated connections. Batteries are also full of sulfuric acid and emit hydrogen gas that is highly flammable.

Battery banks are usually contained in a plastic, metal, or wooden storage box located near the system control panel—typically in the garage, basement, or shed. Battery banks also may be placed somewhere on the exterior of the house. If the heat and flames of the fire are near the battery bank, firefighters should use dry chemical or CO2 extinguishers instead of water to avoid the potential for shock hazard of spraying a live inverter and to minimize damage to the batteries. That said, salvaging a battery pack is always secondary to firefighter safety and saving (continued on page 92)

Courtesy Matthew Paiss

Firefighters cover a PV array with opaque tarps to cut off array power production.

Firefighting & Off-Grid PV Systems

Wildfire is a constant danger in many rural and remote areas, where off-grid PV systems are most common. Rural fire departments respond to both wildland and structure fires, ever mindful that each type of fire can quickly turn into the other. Federal, state, county, and other local wildland fire agencies often respond to all fires in their area as "mutual aid," depending on whose land is or might become involved. Unlike their counterparts in the suburbs and cities, wildland firefighters do not carry equipment to ventilate roofs or enter burning buildings, and usually do not have enough water or apparatus available to extinguish a burning building—their job is to prevent your home from igniting in the first place.

Prevention & Preparation

The more remote your area, the more responsibility you must take for fire prevention and preparation at your home. The time to prepare, though, is well before fire season—not when the sheriff drops by and tells you to leave within 15 minutes.

Keep your system clean and clear of flammable debris.

Regularly remove any leaves, pine needles, or twigs caught between your PV modules and the roof. Firebrands can be carried by wind for long distances and can easily ignite such litter. If your PV array is on a ground-mounted rack, keep grass and brush trimmed underneath and around it.

Label all racks, combiner boxes, and conduit. Should burning debris land on your roof, firefighters may need to climb up there to extinguish it, and may not understand the electrical hazards from PV modules. Simple warning labels—such as "Caution: High-Voltage Solar-Electric System"—could help prevent personal injury as well as avoid damage to your system and home.

In an Emergency

Your ever-reliable off-grid system can help keep firefighters safer while they are protecting your off-grid home and give them a critical edge in saving your home from an approaching wildfire. The key: Leave your PV system on when you evacuate!

In remote areas with no fire hydrants, shuttling water in fire engines is a difficult problem. Prepare a sign in advance with details about your water—where it is, how much there is, and how to access it. Post the sign on your front door before you evacuate, and leave the porch light on to illuminate the sign and make your home visible to firefighters at night. Make sure all of your doors are unlocked—to your home, outbuildings, and water pumping control point.

Since your PV system has been left on, your domestic water pressure pump will be working. Connect garden hoses to outdoor faucets before you leave. If your well pump requires a generator to fill the cistern, then keep the generator gassed up, with instructions posted on how to pump from your well.

Also, keep in mind that "scene lighting" for a typical wildland fire crew consists of helmet headlamps and fire engine headlights. Your outdoor lighting could be very helpful for defending your home.

Courtesy Matthew Paiss

The author and his ground-mounted PV array.

Courtesy Matthew Paiss

The author and his ground-mounted PV array.

the structure. However, when dealing with a minor fire or an overheated battery connection, firefighters should act prudently and do their best to avoid damaging thousands of dollars' worth of equipment.

Eliminating the Source. One option for shutting down a PV system is to cover the arrays with opaque material, such as heavy canvas tarps or black plastic. Most fire response vehicles carry some type of salvage covers or tarps that are commonly used to protect belongings from water damage during firefighting. These same tarps can also be used to prevent light from reaching the PV cells, shutting off the flow of electricity to the inverter. However, high winds, tarp sizes, structural conditions, and the size and shape of the array may prevent using this option. Some departments do not carry suitable tarps, and common blue poly tarps will not work because they let too much light through.

Dealing with Conduit. If the array cannot be tarped, it is important for the crews inside to be careful when opening holes in the ceiling, as they may contact the conduit from the array with their tools. Since plastic insulated wire (Romex) is all that's typically required for home wiring, metal conduit is rarely used in an attic spaces.

Firefighters must verify whether a metal conduit run is intact. If so, it is grounded from the array to the inverter, so any wires that may be shorted to it from the high heat of a fire will carry any voltage/current to ground rather than to the firefighter who contacts it. In other words, it is safe to touch. But if portions of the roof have collapsed, it should be assumed that the conduit is no longer grounded and therefore dangerous to touch. Most fire departments carry noncontact voltage detectors that can be used to find hot AC lines. Unfortunately, DC noncontact voltage detectors, which would alert firefighters to the presence of PV-generated electricity, are unavailable.

In a nighttime fire where the attic space was exposed to severe heat damage, the conduit and wires inside may have become compromised. Some arcing could begin as the rising sun energizes the modules the following morning—a potential for starting a new fire. A qualified solar contractor should be called in to disconnect the arrays. Unfortunately, most PV companies do not have an on-call technician available, so the disconnect usually must wait until the next day—not always the safest measure. In this case, most fire departments will post a "fire watch" until a qualified contractor can ensure the array is disconnected. Local utility companies are not responsible for the customer side (the house side of the meter) appliances and will not respond just to secure PV systems. This is where cooperation with the solar industry comes into play. The fire service recommends that all PV installers have an after-hours response contact for such emergency situations.

Keeping Safety in Mind

Identification is the key to understanding these systems and avoiding injury. Taking precautions with a PV array is one way to ensure the safety of firefighters and reduce the risks to your system. Beyond siting your system for optimal safety in the event of a fire, homeowners should consider installing interior fire sprinklers, which could be the critical difference between saving or salvaging your home and system. Invite your local fire department to tour your PV system if it's a rarity in your locale and provide them with a schematic of the system for their records. Further information about installing safe PV systems can be found in the California Office of the State Fire Marshal's draft guidelines.


Fire engineer Matthew Paiss ([email protected]) has been a member of the San Jose (California) Fire Department for 13 years. He teaches classes in PV safety, and has degrees in both fire science and solar technology. He has PV and solar thermal systems on his home.

Dan Fink ([email protected]) has been a firefighter with the Rist Canyon (Colorado) Volunteer Fire Department for 10 years, and lives in a remote, completely off-grid corner of their response area. He is a writer and photographer for, with 16 years of experience in renewable energy system design, installation, and consulting.


by Allan Sindelar


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.

Function & Purpose

Generators serve three main roles in off-grid home systems:

• The first is backup charging, where the generator makes up for any deficit in energy from the solar array or wind turbine, since the generator will work in any weather.

• The second function is battery equalizing. All flooded batteries need to be overcharged several times each year for best service life and performance. Equalizing is the deliberate overcharge of an already-full battery—raising the battery voltage to a higher-than-normal voltage (as specified by the battery manufacturer) and keeping it there for 2 to 3 hours. A minimum charge rate of about C/20 is necessary to overcome internal resistance to achieving such a high voltage (see "C-Rating" sidebar). Most off-grid PV arrays aren't powerful enough to accomplish this, especially during the winter, so a generator becomes an essential tool to achieve this.

• The third role is to run specific loads that exceed the capacity of the inverter(s), such as 240-volt deep-well pumps, stationary shop tools, and even air conditioners.

by Allan Sindelar

The Basics

"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

The rated output of this generator is the combined wattage of both 120 V legs. At most, half of that power is available at 120 V—full power is only available at 240 V.

The rated output of this generator is the combined wattage of both 120 V legs. At most, half of that power is available at 120 V—full power is only available at 240 V.

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:

The term C-rate refers to the rate of charge or discharge of any battery, expressed in relation to the battery's capacity. A C/10 rate on a 220 amp-hour battery string would be 22 amps (220 - 10); a C/20 rate would be 11 A (220 - 20). For long service, batteries must be charged at a C-rate sufficient to overcome internal resistance under varied conditions and age. As a general rule, flooded lead-acid batteries need at least a C/20 rate to charge and equalize properly. A C/7 charge rate is about the highest that these batteries can tolerate.

• Electric start with two-wire remote start capability: "Two-wire" means all starting control functions (such as cranking and crank duration) are handled within the generator, rather than externally. Remote start capability means that the start and stop signals can be sent from an inverter or other controller. (See "Automatic Generator Start" sidebar.)

• No or minimal standby load: Some generators, designed primarily for on-grid standby use, draw energy—even when not running—for control functions, charging the starter battery, and even for block-heating in cold climates. This may be acceptable for use with utility power, but not for off-grid systems. Look for a generator that has no parasitic draw when off, or expect to add a separate small PV module and charge controller to keep the starting battery charged.

• Engine speed: In general, quality conventional generators run at lower speeds—typically 1,800 rpm versus 3,600 rpm—which directly translates into longer life. An 1,800 rpm unit will also operate more quietly, with less vibration and lower fuel usage.

it. Portable generators are seldom properly grounded, so manufacturers ground the neutral output conductor to the chassis. When connected into a grounded power system, however, this presents a safety hazard, as the neutral is now bonded to the ground at two places—one in the power system AC (such as in the main AC service panel) and one in the generator—and the safety ground wire becomes current-carrying. Some units also include AC ground-fault protection, which is incompatible with connection to a grounded power system. There is no simple, code-compliant solution to this. The safest noncompliant approach is to bundle an insulated, green ground conductor with the power conductors between the generator and the main ground bus, which serves to ground the chassis and minimize shock potential.

• A well-earned reputation for after-sale support by the local dealer. Even the best generator will eventually need service and repair, often on-site.

Onboard starting battery charging when the engine • A warranty is running. Generators designed for on-grid "residential applications.

standby" operation sometimes require an external battery charger.

of at least two years, valid in off-grid

Liquid cooling, rather than air cooling. Liquid-cooled machines will run quieter and, being thermostatically controlled, will run at a more even temperature year-round, resulting in longer life.

Sound-attenuated housing and a residential muffler for quiet operation.

Field-configurable voltage output, unless the supply voltage matches your needs.

Floating or unbonded neutralto-chassis connection. Quality generators allow the neutral conductor to either connect to the generator chassis (for prime power application) or remain separate from

Look for a generator that includes electric start and a panel switch that internally reconfigures the generation coils to provide full output at either 240 or 120 V— common on higher-quality portable generators.

Look for a generator that includes electric start and a panel switch that internally reconfigures the generation coils to provide full output at either 240 or 120 V— common on higher-quality portable generators.

Selecting Your Fuel

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.

This Onan RS20000, 20 kW LP generator is a perfect match for four OutBack VFX3648 inverters (see photo at right).

Sizing a Generator

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.

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