Generators in Grid-Tied Systems

Batteryless grid-tied PV systems are in many ways the essence of simplicity. Reliable and essentially maintenance-free if skillfully installed, they are ideally suited to homes accustomed to utility power. But the chief limitation to batteryless PV systems is lack of backup power during utility outages. If you want backup power for “critical” loads, such as refrigeration or lighting, there are three common approaches.

The first approach is to install a battery-based grid-tied system. While a battery backup system works well if designed and installed well, it will add cost and complexity to the system. It will be slightly less efficient than a batteryless system, as it must balance AC production with charging and maintaining batteries, rather than solely maximizing AC production into the grid. It also will usually supply only enough stored power to support selected house loads during a typical short-duration utility outage, though RE-generated energy will extend the length. Off-the-shelf packages are available to support a modest load profile during an outage of short duration: A typical battery bank of four 100 amp-hour sealed batteries only provides 4 kilowatt-hours of usable power before it must be recharged; this would be a hard limit for most households at night or during a snowstorm.

Adding a generator to a battery-based grid-tied PV system will extend the system’s backup duration indefinitely—until the fuel runs out or the residents get tired of the noise. With good design and execution, this approach can work well. However, the system quickly reaches complexity beyond the skills of most homeowners and novice installers. It must be set up and programmed to work reliably and automatically, and must be tested and exercised regularly if it’s to be relied upon. Plus, not all manufacturers’ equipment is capable of integrating both generator and grid inputs—Xantrex’s XW system will; OutBack’s will not.

The third approach is to install a generator and transfer switch as a backup source during outages without tying the systems together. The disadvantage is that switchover to the backup source is not instantaneous, and with batteries eliminated, the generator must run continuously whenever backup power is needed. Advantages include the simplicity of separate, conventional systems, each supported as necessary by its separate provider and each optimized to a single task. An entire market has developed for generators for residential standby use.

All three approaches are considerably more costly than batteryless systems, for two main reasons. The first is simply the added hardware and complexity. The second is the cost of modifications to a home’s wiring. Since none of these approaches typically has the capacity to run an entire home, key loads are identified as critical to operate during an outage. These usually include some lights, a heating source, refrigerator and freezer, and communications (computer, television, and Internet). Key circuits must be identified and moved from the home’s main breaker panel to a “critical loads” panel to be fed by the backup source. Unless specifically planned during the home’s original construction, this is a labor-intensive job for an experienced electrician.

If a grid outage is no more than an inconvenience, install a batteryless system and enjoy the occasional meal by candlelight when the grid goes down. If power during an outage is critical and typical outages are infrequent and of short duration, a grid-tied system with battery backup is a good choice. If an outage would be potentially catastrophic, such as a winter ice storm that can cause power to be lost for days in subfreezing weather, the battery and PV recharge capacity of a battery-based grid-tied system will be insufficient, and a backup generator will be a wiser choice. Consult with a reputable local RE installer or generator dealer to determine the best approach for your needs.