In addition to wind turbines, PV modules, or a microhydropower generator, you must purchase BOS equipment, when generating your own power. This may include battery charge controllers, batteries, inverters, wires, conduit, a grounding circuit, fuses, safety disconnects, outlets, metal structures for supporting the PV modules, and any additional components that are part of the system.
In very small systems, DC appliances operate directly off the batteries. If you want to use standard appliances that require conventional household alternating current (AC), however, you must install an inverter to convert DC electricity to AC. Although the inverter slightly lowers the overall efficiency of the system, it allows the home to be wired for AC, a definite plus with lenders, electrical code officials, and future home buyers. We’ll discuss BOS configurations first for loads requiring direct current, then for loads needing alternating current.
In grid-connected systems, the only additional equipment needed is an inverter that makes the turbine output electrically compatible with the utility grid. No batteries are needed. Work with the manufacturer and your local utility on this process.
When examining the costs of wind turbines, PV modules, or microhydropower generators, remember that these costs do not include the cost of BOS equipment.
Battery. In off-grid systems, the battery stores electricity for use at night or for meeting loads during the day when the generation source (wind turbines, PV, or microhydropower) is not generating sufficient power to meet load requirements. To provide electricity over long periods, renewable systems require deep-cycle 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 renewable 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.
The cost of deep-cycle batteries depends on the type, capacity (amperehours), 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. An off-grid PV or wind 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 or low winds. This is known as “days of autonomy.” Consult your 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, battery companies, and renewable system designers recommend that lead acid 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. Also, allow enough space for easy access during maintenance, repair, and replacement. Most importantly, maintain the battery according to the manufacturer’s instructions, and recycle the batteries properly when they wear out.
Charge Controller. The charge controller regulates the flow of electricity from the generation source 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 the charge to flow from the generation source into the battery, the load, or both. When the controller senses that the battery is fully charged, it stops the flow of the charge from the generation source. 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.
The cost of controllers generally depends on the ampere capacity at which your renewable system will operate and the monitoring features you want.
Inverter. Alternating-current (AC) systems also require an inverter, which changes the DC electricity produced by renewable systems 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 lower quality electricity, but computers 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 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.