When choosing PV modules for your off-grid system, it is important to look at the price, the technology, how they attach to the roof or rack, the voltage and current specifications, the UL listing, and the warranty. The most typical type of module (240 W, aluminum-framed, with quick-connect positive and negative wires) has an output voltage that integrates easily with arrays configured for grid-tied inverters. The off-grid market can also take advantage of these common modules by using a maximum power point tracking (MPPT) charge controller that can step down array voltage to the lower voltage of a battery bank. There are still some modules on the market that are a nominal 12 V (or 24 V), made to directly charge a 12, 24, or 48 V battery bank through a non step-down charge controller, but they are becoming harder to find and are typically more expensive.
Proper array sizing is crucial in off-grid design. It ensures that the loads you need to run will have energy and that the battery can be fully recharged after a period of no sun. To size an array, you’ll need to know the modules’ STC watts, the average daily peak sun-hours in the worst month, and the amount of energy the loads consume. An array needs to produce as much as the average daily loads consume (plus efficiency losses) and be able to recharge the batteries and “catch up” after cloudy periods. Whenever possible, oversize the array to account for inclement weather (see “More PV, Fewer Batteries” sidebar). Most designers also specify a generator to accommodate for long stretches of low (or no) sun, which then removes the need to oversize the array further.
Because off-grid home sites often have more room than city lots, there are usually more locations for array placement beyond a home or garage roof. Ground mounts, pole mounts, shed or barn roofs, and solar trackers are options. A pole- or ground-mount system allows the array to be adjusted seasonally, and this additional energy production can reduce a backup generator’s run time (reducing fuel use and maintenance) during winter. For example, adjusting a 3,000 W array in Pueblo, Colorado, from a fixed latitude tilt to a steeper tilt will gain 0.5 daily peak sun-hours during the winter. That additional energy calculates to be about 1,500 Wh per day (3,000 W x 0.5 hours/day).
For the greatest energy harvest from an array, a solar tracking mount can be used, so long as a clear solar “window” is present (dawn-to-dusk solar access is ideal). With decreasing module prices, however, the additional cost of the tracker plus the introduction of moving parts to an otherwise non-mechanical system makes this option harder to justify. Often, it’s a better investment to increase the array size to increase the system’s year-round output.
A charge controller’s primary function is to prevent the batteries from overcharging. Charge controllers monitor the battery voltage—when the batteries are fully charged, they disconnect the charging source (in this case, the PV array) from the battery until it is next needed. Some smaller controllers also have an additional feature that prevents overdischarging from DC loads.
When choosing a residential-sized charge controller, first evaluate whether MPPT, which helps maximize the energy harvest from the array, is needed. MPPT controllers continually track array output—during shifting temperatures and irradiance levels—to optimize the amount of energy sent to the battery. The additional cost is justifiable in nearly all larger systems, since it yields between 10% and 25% more energy. MPPT charge controllers also have a “voltage step-down,” so they can convert high array voltages (up to 600 VDC) to lower battery voltages (typically 24 or 48 VDC). This allows more modules to be wired in series and the use of smaller-gauge (and less expensive) wire from the modules to the controller. Having a large difference in voltage between the array and the battery decreases a charge controller’s efficiency, but the benefits of being able to place the array farther away from the battery bank, reducing the wire size, and having smaller overcurrent protection devices, can be worth it. Non-MPPT controllers still hold a large market share, but generally make sense only in smaller system applications, such as for boats, lighting, RVs, and small cabins. Additional information needs to be considered in choosing a controller, including monitoring requirements, temperature compensation, voltage and current specifications, and the size of the array-to-battery voltage step-down window.