The most recent innovation in stand-alone power is the AC (Alternating Current) coupled system. Energy generators (E.g. Solar array) are coupled into the system via regular 230V AC wiring.
• Solar arrays can directly power household loads during daylight hours resulting in higher efficiency. Directly powering appliances during the daytime can bypass batteries which reduces battery cycling; the solar inverter and other AC inputs are added/layered onto the capacity of the main battery inverter/charger. Excess solar power during daylight hours is then used to charge the batteries via the inverter/charger.
• AC coupled systems work at higher operating voltages which typically results in much lower solar array wiring losses and increased system efficiency.
• In AC coupled systems greater flexibility is available about the placement of solar arrays or other electrical equipment.
• Battery bank life expectancy is enhanced; all energy inputs are controlled via one central control system (Inverter/Charger). Battery overcharging or undercharging caused by separate control of different energy sources common to ordinary DC-coupled systems is avoided.
• The fuel-powered generator is also fully AC coupled; its generation capacity can also be additive to the battery inverter/charger capacity providing a significant system advantage at times of high system demand or low renewable energy input.
- The equipment cost is typically a little higher; in the case of small systems, the additional cost may not be justified however for most customers the costs are quickly recovered in lower cable and installation costs and enhanced system performance.
- Additional safety measures are required for AC coupled systems and the DC solar array component; these requirements add some additional cost but this is not excessive compared with the extra system performance that can be gained.
The SMA AC coupled system uses a frequency shift control system; in some cases, very sensitive devices such as some inverter-type air conditioners may not operate at certain times of the day because of frequency variation. Appliances should be checked for frequency tolerance range to make sure they will operate consistently.
Until recently the traditional method has been to use a DC (Direct Current) connection method in which energy generators are directly coupled into the system battery bank via a regulator or charge controller.
• DC coupling of system inputs for small systems may have an advantage in simplicity and lower component cost where the power transmission distance is short and the system power is fairly low.
• The solar array and associated wiring operate in the extra low voltage class mode therefore it could be considered by some to have some safety advantages.
• Working with the DC extra low voltage class typically means high electric currents need to flow in the solar array and sub-array cables to obtain the same power as systems working at higher voltages. High currents often need to be carried by very heavy costly copper cables which can also be difficult to manage and very costly to install.
• Where the solar array is located well away from the other standalone equipment problems of prohibitive cable cost or voltage loss can be very problematic; often undersized cables may be installed to cut the installation cost; this often results in inferior system performance.
• In the DC coupled system all power must be delivered to the home via the main battery inverter; the solar generating capacity can’t add onto the capacity of the main battery inverter/charger therefore the inverter/charger having limited capacity may at times limit the system performance because it can’t transfer all available solar array power when it is available in the daytime.