RESIDENTIAL GRID TIE SOLAR SYSTEMS WITH BATTERY BACKUP

THE BASICS

Grid-Tie solar systems with battery backup are similar to batteryless grid tie systems, except they can additionally provide a limited power for critical loads in case of a utility power failure.

When sun is shining, the PV panels generate power, which reduces the consumption of electricity from the grid and cuts electric bills accordingly. The required balance of the power is automatically provided by the utility. Under normal conditions, a small portion of the power from the grid is used to keep the storage batteries charged. If the PV system generates more electricity than your house is using, the system will feed the excess of the power back to the grid after the batteries are fully charged and spin your electric meter backwards.

An obvious advantage of a solar generator backup is it provides a certain protection against short-term blackouts. Its disadvantages are larger up-front investment relative to a batteryless system and lower reliability due to the batteries' limited lifetime. Also, if "wet" batteries are selected, they need to be periodically checked for fluids. Sealed batteries do not need any maintenance but may not last as long as the wet types. For a comparison, a typical warranty for solar panels is 25 years, inverters- 3-10 years, batteries 1-3 years.

PRINCIPLES OF OPERATION

A simplified electrical wiring diagram below illustrates the system's operation.
Grid tie solar system with battery backup: wiring diagram

Note that separate earth grounds have to be provided for each AC and DC circuit. A DC ground fault interrupter disconnects the DC input when a certain leakage current from the ungrounded bus is detected.

The storage batteries should never be connected directly to solar arrays since constant overcharging reduces useful life of the batteries and can cause their damage. Also, with a direct connection, the battery will determines the voltage level at which the solar array will operate. This level most likely will not correspond to the peak power point of the panels (see Understanding characteristics of PV panels). That's why solar charge controllers are normally put in between the PV array leads and the battery bank. Note that since the batteries do not share loads equally, it is not recommended to run more than four parallel strings of the batteries.

In practice, a solar grid-interactive inverter for backup systems usually includes a DC ground fault interrupter, DC-AC inverter itself and the battery charger. It often also has several pairs of fused DC inputs, which can make an external combiner unnecessary.

The above schematic diagram shows an inverter with center-tapped output transformer for 120/240 V wiring. More often a low-power residential-grade inverter provides only 120V. In this case, you need to stack two devices for 120/240 output. Also, note that some models provide isolation in a high-frequency converter stage and do not include a low-frequency output transformer.

The shown system allows excess energy that is generated by solar panels to be exported to the grid. Under normal conditions, the grid acts as an additional energy source to keep the system's batteries charged. If the grid fails, the inverter will automatically disconnects from the grid and supplies energy from the batteries to the critical loads wired to an auxiliary panel. UL1741 requires a grid tie inverter to disconnect from the grid within 0.1 second when input voltage goes off or drops below 60AC. When grid power returns, an internal transfer relay will automatically connect the wiring system back to the utility.

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