Critical Load Systems – Backup Power When Utility Power is Out

Backwoods gets more and more calls each year from folks that have homes with utility power, but are also searching for a solution to seasonal or emergency power outages. It may only be called upon a couple times per year, but they want the peace of mind that a few key critical loads in their household (such as refrigerator, lighting, well pump, or an outlet to charge cellphones) will keep running until utility power is restored. Critical load systems are typically NOT used for net-metering, or selling extra power back to the grid. The arrays are typically sized smaller for the application, and would not justify the additional cost of components to implement the net-metering.

Many people understand that a good generator and 5 gallons of gasoline will keep the lights on and a refrigerator running for a few hours. We applaud you for taking this step. HOWEVER, this approach only works for a few hours. What happens when your 5 gallons of gasoline runs out and the local gas station is either out of fuel or out of power to pump the fuel they have?  Power outages can last for days and weeks, not just hours. You NEED another option. Backwoods Solar knows exactly what you need for these inevitable situations. Do call us for a more robust solution. 208-263-4290 or email [email protected]. We want to help you develop a critical load system.

Critical load systems are essentially battery based solar electric systems, sized only to support the designated loads that the homeowner chooses. This keeps the cost and size of the system much more manageable versus sizing a system to support the entire household. The circuits for the critical loads are isolated to a secondary breaker panel separate from the main house panel. This secondary panel is then the tie-in point for the new Critical Load System.

The system would have all the components of an off-grid system; solar panels charging a battery bank, with an inverter drawing power off the batteries to run the critical loads. By using an inverter with a built-in battery charger, the critical load system itself can be backed up by a generator, or even utility power when present. There are a number of options to choose from on how to balance and operate such a system depending on the application.

Several options exist in a Critical Load system’s design and operation. One of the simplest configurations would consist of a properly sized inverter/charger operating with a set of sealed batteries. Utility power is provided to the inverter which then holds the battery bank in a float state, only to be called upon during outages. The inverter also passes through the utility power to run the loads on a day to day basis. During a utility outage the inverter switches automatically to the battery power to run the loads. The battery bank can be sized to support however many days of reserve capacity are desired. Sealed batteries are a good choice for this type of standby application where the batteries are held at float most of the year, only to be called upon occasionally. A configuration like this doesn’t necessarily even need solar panels. You avoid the extra cost, but are limited to the back-up capacity of the battery bank. Or, add the solar panel component in the future as budget allows.

Another option would be to configure the system operation to run just like an off-grid system. Solar panels and battery power would be used on a daily basis, cycling the batteries a bit each day. Utility power, or a generator, would only be called upon during extended periods of overcast weather. This type of configuration is well served with flooded lead acid batteries that like to see a bit of daily exercising, and have a lower cost than their sealed counterparts.


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