What Size Battery Do I Need for Whole Home Backup?

Calculating Critical Loads

To size a battery bank, start by identifying the critical loads in your home, such as refrigerators, air conditioning units, and medical equipment. Add up the wattage of these loads during peak usage hours, and then multiply by the number of hours you want the battery to power them during an outage. For example, if you have a 10 kW air conditioning unit that runs for 8 hours during a heatwave, that’s 80 kWh of power required. Consider the total wattage of all critical loads for a minimum of 24 hours of backup.

Battery Bank Sizing

A typical whole home battery system consists of a battery bank with a capacity of 10-20 kWh. This can be achieved with multiple 48V batteries, such as the Powerwall Enphase EG4, or a single large battery. To calculate the number of batteries needed, divide the desired battery capacity by the capacity of each battery unit. For example, if you want a 15 kWh battery bank and each Powerwall EG4 unit has a capacity of 13.5 kWh, you would need 1.11 units (round up to 2 units for simplicity). Ensure the battery bank has a sufficient depth of discharge (DOD) rating for the expected usage.

System Design Considerations

When designing a whole home battery backup system, consider the automatic transfer switch (ATS) and the charge controller. The ATS will transfer power from the grid to the battery bank during an outage, and the charge controller will regulate the solar panel input to prevent overcharging the batteries. Ensure the ATS and charge controller are compatible with the battery system and can handle the peak current requirements of the critical loads. Consult with a professional solar installer to design a system that meets your specific needs and complies with local electrical codes.