RESIDENTIAL OFF THE GRID SOLAR SYSTEMS

OFF GRID BASICS

Stand-alone (or off the grid) PV systems are intended to operate independent of the electric utility.

Since solar panels do not store energy and can generate electricity only during daylight hours, for continuous power flow the electricity they generate have to be stored somewhere, usually in the batteries. If the off grid home has no other power source, the battery bank has to be significantly oversized by design to account for possibly 4-5 days of inclement weather. To reduce the size of the battery bank, an off-grid solar system should be supplemented by a wind turbine or by an electric generator that will produce electricity at night and during cloudy periods. An auxiliary energy source simplifies an off grid system's sizing.

PRINCIPLES OF THE DESIGN AND ELECTRICAL WIRING

Below is a simplified solar system wiring diagram for an off-grid home. The installations and grounding must be done in accordance with the National Electrical Code (NEC) ANSI/NFPA 70 and local electrical codes.

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Solar wiring diagram for an off-grid system

If you parallel several strings of solar panels, it is desirable to use a combiner with a fuse or a circuit breaker for each string to prevent the panels' damage from possible reverse currents. One of the two wires that carry the DC voltage from the PV panels must always be grounded. Although in theory you can ground either bus, most battery chargers and inverters come from the factory configured for negative ground solar systems. The combiner's frame or the PV arrays grounding conductor should be routed to a ground rod located as near as possible to the arrays. A DC disconnect switch should be installed near the place where the cables from the combiner enter the house. Since PV arrays are mounted outside, they can act like lightning rods. To reduce the possibility of a fire and to protect the system from a damage caused by lightings, it is recommended to have a voltage-clamping device across the DC bus. A metal oxide varistor (MOV) is commonly used in such applications.

The main DC disconnect switch is followed by a DC ground fault interrupter - a device that opens the circuit when a ccertain leakage current to ground from an ungrounded bus is detected.

The current from the PV array is charging the batteries. To protect the batteries from overcharging or discharging by reverse currents, the battery bank is normally connected to the PV array via a solar battery charger. To extract the maximum power out of the array, it is best to operate the PV panels near maximum power point of their I-V curve (see: Understanding characteristics of solar panels). This requires different load points depending on the illumination and ambient temperature. That is why it best to use a specially designed peak power tracker that charges the batteries while forcing a the array to operate at a maximum power point.

A DC voltage from the battery bank is then converted to AC voltage by a DC-AC inverter that operates as a switching mode power converter. The solar wiring diagram above shows an inverter that provide 3-wire split phase 120/240 VAC required for most US household wirings. Many inverter models provide only 120VAC. There are 120V models that by design can be synchronized and stacked in master-slave mode for 120/240 VAC output. In this case, the inverters inputs are paralleled, and outputs are connected in series. Note that some systems provide isolation between DC input and AC output in a high-frequency boost converter stage, and do not use a low-frequency output transformer.

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