How to Calculate Battery Capacity for a 12V Inverter to Run a Refrigerator Overnight

Having a refrigerator in your off-grid setup is a great convenience, but ensuring it stays cool through the night requires careful planning. This guide walks you through the steps to determine the right battery capacity needed to power a refrigerator with an inverter overnight.

Understanding Refrigerator Power Consumption

Before you begin, it's crucial to know your refrigerator's power consumption. Typically, this is listed on the fridge's label in watts (W). However, if the wattage isn't found, you can calculate it using the following formula:

Wattage Voltage (usually 120V) × Amperage listed on the label

Once you have the wattage, you can estimate the daily energy consumption in kilowatt-hours (kWh). For example, a fridge that uses 200 watts and runs 8 hours a day would consume about 1.6 kWh per day.

Estimating Overnight Energy Needs

If you need the fridge to run for 12 hours overnight, the energy needed can be calculated as follows:

Energy Needed 1.6 kWh/day × (12/24) 0.8 kWh (800 watt-hours)

Accounting for Inverter Efficiency

Not all the energy from the battery is transferred into useful work. Most inverters have an efficiency rate of 85–90%, so you need to account for this loss:

Adjusted Energy 0.8 kWh ÷ 0.9 0.89 kWh (890 watt-hours)

Selecting the Right Battery

The choice between battery types (lead-acid and lithium) depends on your specific needs and the extent of use you expect from your battery.

Lead-Acid Batteries

Lead-acid batteries can release only 50% of their capacity without causing damage. Therefore, to meet the adjusted energy requirement of 0.89 kWh, you need a battery capacity of:

Battery Capacity (Ah) Watt-hours ÷ Battery Voltage 890 ÷ 12 ≈ 74.2 Ah

Considering the 50% usable capacity:

Usable Battery Capacity 74.2 Ah ÷ 0.5 ≈ 148.4 Ah

For a 12V system, you would need a battery with a capacity of around 150 Ah.

Lithium Batteries

Lithium batteries, on the other hand, can release up to 90% of their capacity. Hence, to meet the adjusted energy requirement:

Battery Capacity (Ah) Watt-hours ÷ Battery Voltage 890 ÷ 12 ≈ 74.2 Ah

Considering the 90% usable capacity:

Usable Battery Capacity 74.2 Ah ÷ 0.9 ≈ 82.5 Ah

A 100 Ah 12V lithium battery should suffice for your needs.

Additional Notes and Guidance

Here are a few additional considerations to keep in mind:

Inverter Power Rating: Ensure the inverter's power rating exceeds the fridge's starting surge, usually 2–3 times its running wattage. Battery Configuration: Depending on your system voltage, you may need to wire multiple batteries in series or parallel.

If you need further assistance in selecting specific battery models or inverters, feel free to reach out for professional advice.