A common question for solar installers is whether or not the battery system will provide whole house backup power, often in an attempt to avoid installing a dedicated panel for backed up loads. However, installers should remember that a standard 200 amp residential service, for example, is capable of delivering 48-kW continuous power whereas most battery inverters have less than 10-kW of backup continuous power output.
While these inverters can be stacked to deliver more power, most residential homeowners only purchase one or two battery inverters. Backing up the whole building also reduces the overall storage duration of the battery. It may be nice to power every device during a power outage, but not if the system turns off due to overloading the inverter or draining the battery.
System designers are often working with incomplete modeling data, so finding the right balance between customer wants and needs can be overwhelming. But everyone can agree that the goal of a battery backup system is to make power more reliable, not less so.
Ultimately, a backup system should protect food in the refrigerator and freezer, keep the garage door functional, the internet on, power LED lighting zones in entrance/exists, the kitchen area, and the primary bedroom. Automatically backing up any further loads puts these loads at risk, but keeping the automatically protected backup panel small keeps the rewiring work to a minimum.
What is notably missing from this list are air conditioning and hot water heating circuits. These heavy loads can often be run off solar batteries when enough solar power is available – but the power outage might occur when the systems owners are gone from the home. You would not want unused hot water or air conditioning to be the reason the system crashes and thaws all the food in the freezer. While these issues can be solved with smart service panels, bigger battery banks, and more inverter power, these are expensive solutions.
Inexpensive design options
One inexpensive solution is to wire the grid side of the battery inverter as a “supply side connection” – a solution that solar installers should be well familiar with for larger solar project sites. Then the installer should proceed with a small, pragmatic backup panel for automatically protected loads.
Lastly, the installer should consider running a “generator interlock switch” between the backup panel and the main service panel. This affordable device is generally used to lock out the grid when powering a home with a portable generator.
In this case, the backup side of the battery inverter replaces the generator. This allows the system owner to supply power to any circuit in the house by manually throwing breakers at the main service panel. This technique separates out the most critical loads from the rest of the building loads, so that food in the freezer doesn’t thaw because of accidentally running the air conditioner. But in the middle of the day when the sun is up, the homeowner can manually turn on heavy appliances like hot water tanks, as well as any other load in the house, without draining the battery. This means that reliable whole house backup power can be delivered with a single battery and battery inverter at a budget that most solar owners can afford.
John Cromer is an applications engineer for Fortress Power