Planning Battery Power for Cold Food Storage

Reliable refrigeration is one of the more difficult conveniences to maintain when living off-grid or preparing for extended power outages. Compressors cycle frequently, and even modern efficient models can consume hundreds of watt‑hours per day. This calculator helps homesteaders, van dwellers, and emergency planners determine how long a given battery bank can keep a refrigerator running without recharging. By entering the fridge’s typical energy use in kilowatt‑hours per day, the amp‑hour capacity of the battery bank, system voltage, allowable depth of discharge, and inverter efficiency, the script computes the runtime in days and hours. Knowing this number guides decisions about battery size, solar panel array sizing, and backup generation.

The central energy balance is straightforward. Battery capacity is typically specified in amp‑hours at a nominal voltage. Multiplying these yields total stored energy in watt‑hours: , where is capacity and is voltage. However, discharging a battery completely shortens its life and may damage certain chemistries. The usable energy therefore equals , with the permitted depth of discharge in percent. Additionally, inverter and wiring losses mean not all stored energy reaches the appliance. An overall efficiency factor adjusts for this, resulting in . Dividing available kilowatt‑hours by daily energy consumption yields runtime in days.

For example, consider a 12‑volt off‑grid system with four 100‑Ah deep‑cycle batteries wired in parallel, yielding 400 Ah of capacity. At 12 V, the bank stores watt‑hours or 4.8 kWh. Limiting discharge to 50% to prolong battery life leaves 2.4 kWh usable. If the inverter is 90% efficient, the available energy drops to 2.16 kWh. Suppose the refrigerator consumes 0.6 kWh per day; runtime becomes days. In hours, that is roughly 86 hours of cooling before the bank must be recharged or supplemented.

Manufacturers sometimes rate energy use in watt‑hours per day, while others list average current draw. Converting between units clarifies expectations. A fridge drawing 5 amps continuously on a 12‑V system uses 60 watt‑hours per hour, or 1.44 kWh per day. Many appliances cycle, meaning they run intermittently. Measured energy consumption using a plug‑in watt meter or the energy guide label provides a realistic average that accounts for cycling. The calculator assumes a constant daily use value, but savvy users can adapt by estimating a worst‑case hot weather consumption and a typical cool season value to see how runtime varies.

Battery chemistry significantly influences depth of discharge limits. Flooded lead‑acid cells tolerate occasional deep discharges but last longest when kept above 50% state of charge. Absorbed glass mat (AGM) and gel cells share similar constraints. Lithium iron phosphate (LiFePO₄) batteries, while more expensive, allow up to 80% or even 90% discharge without significant degradation, dramatically extending runtime for a given capacity. The table below highlights common chemistries and recommended DOD values:

Chemistry	Typical DOD	Notes
Flooded Lead‑Acid	50%	Requires ventilation and maintenance
AGM/Gel Lead‑Acid	50‑60%	Sealed, lower maintenance
LiFePO4	80‑90%	High cycle life, light weight

Temperature also affects both fridge efficiency and battery capacity. Batteries deliver fewer amp‑hours in cold conditions, while refrigerators work harder in hot climates. To account for seasonal swings, you might derate capacity by 20% during winter or increase expected daily energy use during summer. Because the calculator runs entirely in the browser, you can easily adjust inputs to explore these scenarios without losing previous data.

Some users integrate solar panels or wind turbines to recharge the battery bank. To maintain continuous operation, the renewable system must, on average, supply at least as much energy as the refrigerator consumes. Suppose sunlight provides 4 peak sun hours per day and panels deliver 400 watts. Their daily production is roughly 1.6 kWh before losses. If the fridge uses 0.8 kWh per day, the panels not only cover the load but also replenish the batteries. However, during cloudy periods the fridge draws from storage, so runtime estimates from this calculator help size the battery bank to ride through poor weather.

Efficiency considerations extend beyond the inverter. Thick wiring reduces voltage drop, improving overall system performance. Positioning the refrigerator in a cool, shaded area lowers its energy use. Adding insulation or a thermal mass—such as frozen water bottles—inside the fridge can maintain temperature longer during power interruptions. While such strategies do not alter the mathematical runtime directly, they effectively reduce daily energy consumption, allowing the same battery bank to last longer. Experiment by adjusting the daily energy use field to reflect improvements like upgraded insulation or reduced door openings.

This calculator intentionally keeps the model simple to remain broadly applicable. Real systems may exhibit nonlinearities: inverter efficiency often varies with load, and battery voltage can sag as it discharges. Yet the approximation offers a valuable baseline, translating abstract amp‑hours into an intuitive metric of days of refrigeration. Whether you are planning an off‑grid homestead, outfitting a camper van, or preparing for emergencies, quantifying runtime enables informed decisions. Combine this tool with actual energy measurements and battery monitoring for the most accurate results, and always allow a safety margin to protect food and equipment.