Considerations for Off-Grid PV Systems

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.