RESIDENTIAL PV POWER SYSTEMS

TYPES OF SOLAR POWERED SYSTEMS

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

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

All these systems take DC voltage produced by PV panels during daylight hours and convert it to conventional household AC voltage by using solid state power inverters. However the inverter in each type of the system has a different logic of operation. The PV systems sometimes are casually called solar generators, although the term "electric generator" usually refers to a device that converts mechanical energy into electric energy.

Stand-alone (or off-grid) systems are intended to operate independent of the electric utility grid. Since solar panels do not store energy and can generate electricity only during daylight hours, for continuous power flow the energy generated by the PV panels have to be stored in the batteries.
If your residential system has no other power source, the battery bank have to be significantly oversized to account for possible 4-5 days of inclement weather. To reduce the size of the battery bank, off-grid solar systems are often supplemented by wind turbines that can continue generating electricity at night and during cloudy periods, or by an auxiliary generator that is fueled from a large diesel or propane tank. For more details on how to build a home solar powered system see off the grid system wiring diagram and principals of operation.

Grid-Tied home systems are wired parallel to utility grid system. They generate power during the daylight hours, reducing the consumption of electricity from the utility and accordingly reducing electric bills. Whenever the power generated by the Grid-Tie PV system power is greater than the household needs, the inverter sends the surplus to the utility grid. The utility provides AC power to the house at night and during times when the loads requirements exceed the output of the PV panels.
The grid tie inverters 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 the line when the level of utility voltage or frequency varies substantially from nominal. Therefore contrary to common misconception, a grid tie system will not provide any back up during power outages even when sun is shining. For more information see Grid tie solar systems.

Grid-Tied systems with battery backup generate solar electricity whenever there is sunlight, thus educing utility bills just like grid tie systems. Under normal conditions, the grid acts as an additional energy source charging the storage batteries. If the grid 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.

The wiring of the entire system has to be done according and local regulations. Regardless on the type of the system, all the electric installations must be done in accordance with to National Electrical Code® (NEC) ANSI/NFPA 70, local electrical codes, as well as UL1703 safety requirements. Particularly, to avoid electric shock, 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 or flexible conduits. Per NEC, 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 important to verify the structural integrity of the roof and the durability of the roof materials before the installation to be sure the roof can support the PV array. Residential building regulations often require a safety factor of 1.5 or greater. Solar installation may not 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
Photovoltaic system for homes: design and installation guide

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