How to Calculate Required Battery Capacity for Critical Loads?

Identifying Critical Loads and Daily Energy Demand

To calculate required battery capacity, first identify critical loads in your home, such as lighting, refrigeration, and medical equipment. Make a list of these loads and their associated power consumption. Next, calculate your daily energy demand by adding up the total watt-hours (Wh) of each load. For example, if you have a refrigerator using 200Wh per hour and a medical device using 20Wh per hour, and you use these loads for 8 hours, your daily energy demand for these critical loads would be 200Wh x 8 + 20Wh x 8 = 1,640Wh.

Considering System Losses and Efficiency

When calculating battery capacity, don’t forget to consider system losses and efficiency. A typical inverter efficiency is around 95%, but this can vary depending on the specific model and manufacturer. For this example, we’ll assume an efficiency of 95%. Additionally, consider any other system losses, such as battery round-trip efficiency and charge controllers. A 95% efficient inverter, 90% efficient battery, and 95% efficient charge controller would result in a total system efficiency of 76.5% (0.95 x 0.9 x 0.95). This means you’ll need to oversize your battery by 1/0.765 = 1.31 times to account for system losses.

Calculating Required Battery Capacity

Using a spreadsheet or calculator, plug in your daily energy demand and system losses to determine the required battery capacity. For our example, assuming a 1.31 times oversize factor to account for system losses, we would need a battery with a capacity of 2.14 x 1,640Wh = 3,500Wh. When choosing a battery, consider its depth of discharge (DOD) and round-trip efficiency to ensure it meets your needs. For example, a Powerwall Enphase EG4 battery has a capacity of 13.5kWh and a DOD of 50%. If you need a battery with a capacity of 3,500Wh, you would need to consider a battery with a higher capacity and adjust for its DOD.