In addition to the PV modules, you must purchase balance-of-system (BOS) equipment. This includes battery charge controllers, batteries, inverters (for loads requiring alternating current), wires, conduit, a grounding circuit, fuses, safety disconnects, outlets, metal structures for supporting the modules, and any additional components that are part of the PV system. Below, we’ll discuss PV and BOS configurations first for loads requiring direct current, then for loads needing alternating current.
Note that, in many systems, the cost of BOS equipment can equal or exceed the cost of the PV modules. When examining the costs of PV modules, remember that these costs do not include the cost of BOS equipment. Ask your PV dealer about the BOS equipment required by your system.
Charge Controller. The charge controller regulates the flow of electricity from the PV modules to the battery and the load. The controller keeps the battery fully charged without overcharging it. When the load is drawing power, the controller allows charge to flow from the modules into the battery, the load, or both. When the controller senses that the battery is fully charged, it stops the flow of charge from the modules. Many controllers will also sense when loads have taken too much electricity from batteries and will stop the flow until sufficient charge is restored to the batteries. This last feature can greatly extend the battery’s lifetime.
Controllers generally cost between $20 and $400, depending on the ampere capacity at which your PV system will operate and the monitoring features you want. When selecting a controller, make sure it has the features you need; cost should be a secondary consideration.
Battery. The battery stores electricity for use at night or for meeting loads during the day when the modules are not generating sufficient power to meet load requirements. To provide electricity over long periods, PV systems require deepcycle batteries. These batteries, usually lead-acid, are designed to gradually discharge and recharge 80% of their capacity hundreds of times. Automotive batteries are shallow-cycle batteries and should not be used in PV systems because they are designed to discharge only about 20% of their capacity. If drawn much below 20% capacity more than a few dozen times, the battery will be damaged and will no longer be able to take a charge.
Deep-cycle batteries cost from about $65 up to $3,000. The cost depends on the type, capacity (ampere-hours), the climatic conditions in which it will operate, how frequently it will receive maintenance, and the types of chemicals it uses to store and release electricity. APV system may have to be sized to store a sufficient amount of power in the batteries to meet power demand during several days of cloudy weather. This is known as “days of autonomy.” Consult with your PV dealer before selecting batteries for your system.
Most types of batteries contain toxic materials that may pose serious health and safety problems. The National Electric Code (NEC), battery companies, and PV system designers recommend that leadacid and wet cell batteries, which give off explosive hydrogen gas when recharging, be located in a well-ventilated space isolated from the other electrical components of the system and away from living spaces. Allow enough space for easy access during maintenance, repair, and replacement. Most important, maintain the battery according to the manufacturer’s instructions, and recycle the batteries properly when they wear out.
Inverter. AC systems also require an inverter, which changes the DC electricity produced by PV modules and stored in batteries into AC electricity. Different types of inverters produce a different “quality” of electricity. For example, lights, televisions, and power tools can operate on lowerquality electricity, but computers, laser printers, and other sophisticated electronic equipment require the highest-quality electricity. So, you must match the power quality required by your loads with the power quality produced by the inverter.
Inverters for most stand-alone applications (i.e., those systems not connected to the utility grid) cost less than $1 per rated output watt. The cost is affected by several factors, including the quality of the electricity it needs to produce; whether the incoming DC voltage is 12, 24, 36, or 48 volts; the number of AC watts your loads require when they are operating normally; the amount of extra surge power your AC loads need for short periods; and whether the inverter has any additional features such as meters and indicator lights.
Tell your PV dealer if you plan to add additional AC loads in the future. If you are considering building another room onto your house or adding electrical loads, consider purchasing an inverter with a larger input and output rating than you currently need. This may be less costly than replacing it with a larger one later.