Considerations for Off-Grid PV Systems

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An off-grid home
An off-grid home with a ground-mounted PV array.
Wood heating saves the electricity usage of electric heating.
“Load shifting” means shifting tasks to other energy sources besides electricity, literally taking a load off your PV system. Wood heating saves the electricity usage of electric heating.
Clotheslines save the electricity usage of a dryer.
“Load shifting” means shifting tasks to other energy sources besides electricity, literally taking a load off your PV system. Clotheslines save the electricity usage of a dryer.
Cooking with gas saves electricity usage.
“Load shifting” means shifting tasks to other energy sources besides electricity, literally taking a load off your PV system. Cooking with gas saves electricity usage.
Compact fluorescent light bulb
In an off-grid scenario, every kWh is important. Energy efficiency is the key to keeping system costs down.
SunDanzer Refrigerator
Refrigeration can be the largest load in an off-grid home. Though modern, mainstream refrigerators are more efficient than ever, some off-gridders choose the super-efficiency of units designed specifically for RE systems. Their higher cost is offset by reducing dependence on PV or generator energy.
Sun Frost Refrigerator
Refrigeration can be the largest load in an off-grid home. Though modern, mainstream refrigerators are more efficient than ever, some off-gridders choose the super-efficiency of units designed specifically for RE systems. Their higher cost is offset by reducing dependence on PV or generator energy.
Photovoltaic module
These days, most photovoltaic modules are not made with nominal voltages to match battery voltage. Rather, modern MPPT charge controllers are able to accept a wide PV voltage range and step it down to a lower battery voltage.
Ground-mounted PV Array
Ground and pole mounts allow PV arrays to be located in the sunniest location, even if far from the inverter and/or batteries.
Pole-mounted PV Array
Ground and pole mounts allow seasonal tilt adjustment, optimizing system output for when it’s needed most.
MPPT controller
Modern MPPT controllers maximize PV output and allow a variety of PV configuration voltages independent of battery system voltage.
Off-grid inverters
Most off-grid inverters can be “stacked” for increased power and higher voltage. Here, four inverters are stacked to provide 120/240 VAC and higher power output.
A Battery Bank
Batteries are both an essential element and weakest link of off-grid systems, adding cost, complexity, and maintenance, while having the shortest lifespan of any component.
A backup generator
Whether gasoline, diesel, or propane, a backup generator is instrumental in almost all off-grid power systems. But good system design can minimize run time. This generator has a small solar module to keep the starter battery charged and ready.
System metering
System metering is important for monitoring system status, general condition, and troubleshooting.
An off-grid home
Wood heating saves the electricity usage of electric heating.
Clotheslines save the electricity usage of a dryer.
Cooking with gas saves electricity usage.
Compact fluorescent light bulb
SunDanzer Refrigerator
Sun Frost Refrigerator
Photovoltaic module
Ground-mounted PV Array
Pole-mounted PV Array
MPPT controller
Off-grid inverters
A Battery Bank
A backup generator
System metering

There are many reasons for choosing an off-grid PV system to power a remote home or cabin. Some people want to avoid the high cost of extending a utility line, while others like the independence of homemade energy production, as well as having a silent, emission-free energy source with a 25-year warranty.

Off-grid (or “stand-alone”) PV systems are very different than batteryless grid-tied systems. Without the utility as a supplemental electricity source, a PV system’s sizing is critical. Off-grid systems require their owners’ participation—this means living within the original design’s energy budget, planning for future growth, and having a backup energy source for times of high energy usage or low solar production. All maintenance and equipment servicing is also done on-site, at the homeowner’s expense, and by the homeowner or installer—instead of by a power company.

Loads

Carefully considering the appliances—or loads—in a home is crucial in off-grid system design. The first step is to list the power requirements of every desired appliance and determine the average daily hours each will be used. A load analysis calculates the energy consumed by each appliance, with the ultimate goal of determining the total average daily energy consumed by all loads in the home. This daily consumption value is then used to design a battery bank large enough to store that energy each day and a PV array large enough to produce the energy.

Other considerations include whether the appliances will use alternating current (AC) or direct current (DC). All off-the-shelf appliances that can be plugged into a standard wall outlet are AC. For off-grid homes with full-time occupancy, the benefits of AC appliances typically outweigh the benefits of DC appliances. Conventional appliances are readily available, and they run at higher voltages, so you can use smaller, standard AC wiring in your household. In certain applications, such as a system for a small cabin, an RV, or a boat, the greater efficiency created by eliminating the inverter can justify more expensive, harder-to-source DC appliances. For example, a PV system for a boat may run 12 VDC lights, a radio, a TV, and a refrigerator directly from the battery to avoid the need for an inverter.

Certain loads need special consideration because of their high energy use, including space heaters and coolers, water pumps, refrigerators, water heaters, and cook stoves. For these applications, it is best to first determine if there are non-electric methods of accomplishing the same task, such as drying clothes on a line instead of using an electric clothes dryer. If an electrical appliance is still going to be used, consider ways to reduce the demand for the load, and then buy the most efficient appliance that will serve that need. As an example, correct window placement and properly sized overhangs can help reduce cooling loads, as will high-performance windows and well-insulated walls. After exhausting all non-electrical means of cooling, using evaporative coolers (in arid regions) or low-energy ceiling fans are good options instead of using a compressor-type air conditioner. If an air conditioner is used, consider cooling only a portion of the home.

Off-grid consumers need to be aware of their energy allowance and shop carefully for efficient appliances. Many appliances, having large power draws and standby features, can be large energy users. The U.S. Department of Energy’s Energy Star website (energystar.gov) is a good place to research the most efficient appliances—however, even within Energy Star-rated appliance categories there still are wide variances in energy consumption. For instance, a sample LG Energy Star 42-inch plasma TV energy consumption is estimated at 140 kWh per year, compared to an equivalently sized LCD model which ranges from 83 to 152 kWh per year. Similarly, comparing refrigerators from Whirlpool demonstrates that a side-by-side model uses about 30% more energy than a refrigerator with the freezer on the top.

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Comments (1)

Peter Clark's picture

I've got a Xantrex SW4048 inverter that handles power from my solar powered battery bank and Kohler propane fired generator. I am off the grid. I have not had success with clothes washing machines - have owned a Staber and now a Frigidaire (manufactured by Electrolux). The computer control boards on both (plus on my propane fired dryer) have often needed replacement. I want to buy a new washer and dryer (propane fired) that will work and last. I'd love to hear from people who use the same inverter (Xantrex SW4048) and have had success with their washer and/or dryer.

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