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TYPES OF PV GENERATORS



There are three basic types of residential photovoltaic (PV) electrical systems:

Off the grid;
Grid-tie;
Grid-tie with battery backup.

All of them have pretty much the same working principle- they take the DC voltage produced by PV panels during daylight hours and convert it to conventional household AC voltage by using solid state DC-AC converters (inverters). However in each configuration the inverter has a different logic of operation. Note that sometimes the PV systems are casually called solar powered generators. Technically, this is not the most adequate term because the word "generator" usually refers to an electro-mechanical device that converts mechanical energy into electricity.

Stand-alone (or off-grid) setups are intended to operate independent of the electric utility grid. Note that PV panels do not store energy and can generate electricity only during daylight hours.




Therefore for continuous flow of electricity the energy generated by the PV panels have to be stored somewhere. Normally, it is stored in the batteries. For more details on how to build a home solar powered generator see off the grid wiring diagram and principals of operation. If your residential system has no other power source, the battery bank has to be significantly oversized to account for possible 4-5 days of bad weather. To reduce the size of the battery bank, solar power for off-grid homes is usually supplemented by wind-powered or electric generators. Such a multi-source system is called hybrid.

Grid-Tied systems are wired parallel to the utility grid lines. They generate electricity during the day, reducing the energy consumption from the utility and thus lowering your electric bills. Whenever the wattage generated by the PV panels is greater than the household needs, the grid-tie inverter sends the surplus to the grid. The utility provides AC power to the house at night and during times when the loads requirements exceed the output of the panels.
The UL-approved grid tie AC sources include automatic AC line disconnect switches that prevent backfeeding into unenergized utility lines. Not only such inverter will not energize a "dead" line, it will also disconnect from it when the level of input voltage or frequency varies substantially from nominal. Therefore, contrary to common misconception, a regular on grid system will not provide any back up during blackouts even when the sun is shining.

Grid tied systems with battery backup likewise generate power for homes whenever there is sunlight, thus reducing electric bills just like batteryless systems. Under normal conditions, the grid acts as an additional energy source charging the storage batteries. When it is down, the inverter will automatically go into backup power mode and will continue powering your entire home or selected loads from the energy accumulated in the battery bank.

BASIC REQUIREMENTS



Regardless on the type of the system, the wiring and all the electric installations must be done in accordance with NEC® 2014 (particularly Articles 690 and 705) and local electric codes in conjunction with UL1741 and UL1703 safety requirements. For example, to avoid electric shock, a protective earth grounding of the individual PV panels must be done by securing them to the mounting frames. The framing system shall be grounded in accordance with NEC® Article 250. The grounding wire should be copper wire minimum #10 AWG. The wiring cables generally have to be protected by metal conduits. A DC disconnect switch should be installed near the place where the cables from PV arrays enter the house. In addition, it is desirable to have a fuse or a circuit breaker for each string of series-connected modules to prevent their damage from the reverse currents from other parallel strings. It is also important to verify the structural integrity of the roof and the durability of the roof materials, to be sure the roof can support the PV array. Residential building regulations often require a safety factor of 1.5 or greater. The system specification should include maximum wind speed and snow load. A roof-top solar panel installation may not be possible in high wind areas where the wind pressure exceeds 45 PSF or in the regions with possible heavy snowfall.




REFERENCE AND ADDITIONAL INFORMATION:
National Electrical Code: Suggested practices for photovoltaic power systems (a detailed guide, but does not refer to the latest NEC®);
Solar Power and Energy: PV power news, information, tools, and resources.