GRID-TIED WITH BATTERY BACKUP
Most grid-tied solar-electric systems are "batteryless" and require the utility grid to function. The grid provides a "signal" for a grid-tied inverter to follow, creating an AC waveform from the DC PV system output. Once that signal disappears or goes too far out of voltage or frequency specifications, the inverter stops operating. That is the most efficient way to produce PV-made energy--more watts can be converted.
Theoretically, a PV array could produce useful energy any time the sun shines. So why can't we use it if the grid does not operate? It's because a PV array is a constant-current energy source--it cannot cut back or increase the energy available depending upon how much the household needs at any moment.
When hooked up to the utility, this same PV system has the grid available to make up for any deficiencies, like during appliance start-up surges. The grid also provides a place to go for any excess energy produced by the PV-powered home.
Including a battery in the system adds a source of energy that can vary according to the needs of the home. Special battery-based, grid-tied inverters are designed to disconnect from the grid, instantaneously switching internally to draw needed energy from the battery instead of the grid. These inverters still disconnect themselves from the grid when the grid goes down, so that they don't inadvertently energize the grid while utility workers are working on it--a potentially shocking hazard.
High frequency or long durations of electrical outages are the most common reasons to have battery backup. Many rural homeowners want battery backup with their systems because they live with a low quality grid or have affecting circumstances-trees near power lines, a long power line extension, wind or snowstorms--that make outages more frequent or of longer duration.
If there are appliances that absolutely must run, or have medicine or food that must be kept cold; lights and computer equipment for work; an oxygen generator or CPAP or BIPAP machine for your health; a backup electrical system is a must.
A grid-tied system with battery backup can cost $10 or more per watt, because of adding batteries and the extra equipment needed to charge them. The total size of the critical loads and duration of outages dictates the size of your battery, inverter, and charging source, and your system's cost. Rewiring your service panel for the critical loads adds another cost, since backed up loads must be separated from non-backed-up loads and placed in a dedicated service panel.