A common question for solar installers is whether the battery system will provide backup power for the entire house, usually to avoid installing special panels for backup loads. However, installers should keep in mind that, for example, a standard 200-amp residential service can provide 48 kilowatts of continuous power, while most battery inverters have standby continuous power outputs of less than 10 kilowatts.

Although these inverters can be stacked to provide more power, most residential owners only buy one or two battery inverters. Backing up the entire building also reduces the overall storage time of the battery. It may be good to power each device during a power outage, but not if the system shuts down due to an inverter overload or a depleted battery. System designers often use incomplete modeling data, so finding the right balance between customer needs and requirements can be overwhelming. But everyone agrees that the goal of a battery backup system is to make the power supply more reliable, not more reliable.

Ultimately, the backup system should protect the food in the refrigerator and freezer, keep the garage door functioning properly, the Internet is on, the entrance/existing power LED lighting area, the kitchen area and the master bedroom. Automatic backup of any further loads puts these loads at risk, but keeping the backup panel for automatic protection small can minimize rewiring work. Obviously missing from this list are the air conditioning and hot water heating circuits. When enough solar energy is available, these heavy loads can usually be operated on solar cells-but when the system owner leaves the house, power outages may occur. You don't want unused hot water or air conditioners to be the cause of the system crashing and defrosting all the food in the refrigerator. Although these problems can be solved by smart service panels, larger battery packs and more inverter power, these are expensive solutions.

An inexpensive solution is to use the grid-side wiring of the battery inverter as a "supply-side connection"-for large-scale solar project sites, solar installers should be very familiar with this solution. Then, the installer should continue to use small, practical spare panels to automatically protect the load.

Finally, the installer should consider operating a "generator interlock switch" between the backup panel and the main service panel. This economical device is often used to lock the grid when a portable generator is used to power the home.

In this case, the backup side of the battery inverter replaces the generator. This allows the system owner to power any circuit in the house by manually throwing a circuit breaker on the main service panel. This technology separates the most critical load from the rest of the building so that the food in the refrigerator will not be thawed by accidental operation of the air conditioner. But at noon when the sun rises, homeowners can manually turn on heavy equipment such as hot water tanks and any other loads in the house without draining the battery. This means that a single battery and battery inverter can provide reliable whole-house backup power at a budget that most solar owners can afford.

John Cromer is an application engineer at Fortress Power

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