of the building. In large commercial applications, there can be hundreds of circuits located within the array that require dozens of boxes to be strategically located and specified to minimize conductor size and overall length.
Power electronics — The output of the PV array will be connected to some form of power electronics. In systems that employ battery backup, both utility-interactive and stand-alone, the array will connect to a charge controller before going into the batteries. These battery-based systems will also use one or more inverters to then invert the energy stored in the batteries to AC for use by the loads. For grid-direct PV systems, the array will go directly into an inverter that will invert the DC energy from the array into AC for interconnection with the electric utility. An example wiring diagram for grid-direct systems is shown in Fig. 3 (click here to see Fig. 3).
Charge controllers — Charge controllers are only necessary for systems that use batteries. The primary purpose of a charge controller is to keep the battery bank from becoming overcharged, which could damage the batteries. Today’s charge controllers also have multiple auxiliary features that can be used to do things like engage relays and power up loads when the battery bank reaches meets certain conditions. One of the most common applications is a relay that sends power to a vent fan when the batteries are charging so the buildup of hydrogen gas does not occur.
In stand-alone systems, when the battery bank has been fully recharged, it is the charge controller’s job to isolate the PV array from the battery bank by opening the circuit and stopping all current flow from the PV array. For PV systems that use battery storage as a form of backup to the electric utility, the charge controllers are now manufactured with the ability to “talk” to the inverters connected to the other side of the battery bank and make sure the PV array is able to send as much energy back into the electric utility when possible.
Inverters — The inverters used in PV systems primarily are used to invert DC energy into AC that can be used within the building to run loads or be pushed back onto the grid. The inverters used in stand-alone and utility-interactive battery backup applications are very similar in their appearance and function, whereas the inverters used in grid-direct systems are very different from the battery-based units.
Battery-based inverters are modular units, meaning multiple small units can be connected together to create a single large power source. The most commonly used battery-based inverters are actually inverter/chargers, meaning they have the ability to take the energy from the battery bank to run loads as well as connect to an AC source and recharge the batteries as necessary. A stand-alone inverter will commonly have a traditional generator connected to it to provide battery charging as needed. The generator is not a requirement in these systems, but it gives users greater flexibility in their use of energy.
A utility-interactive, battery-based inverter will connect to the electric utility instead of a generator for its AC source of power, allowing the user to send energy back to the grid and receive credits for that energy when the PV array is able to produce more energy than is being consumed. The major difference between a stand-alone inverter and a battery-based backup inverter is the ability to push current back toward the AC source. In stand-alone systems, pushing current back into the generator can quickly damage the generator and serves no good purpose. In utility interactive systems, pushing current toward the grid is a desirable situation and results in a financial benefit to the system owner.
Grid-direct inverters come in a wide variety of power output sizes — from hundreds of watts (micro-inverters) to kilowatts (string-inverters) to thousands of kilowatts (central inverters). These inverters are simply a way to invert the DC energy produced by the PV array and connect it directly to the utility grid. Multiple grid-direct inverter