Deep Freezer Inverter and Battery Sizing Calculator

Estimate the inverter size and battery bank needed to keep one or more deep freezers running through a power cut, including startup surge.

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What this calculator does

It estimates the inverter and battery capacity for backing up one or more deep freezers. It accounts for the startup surge of the compressor, battery depth of discharge, inverter efficiency and an allowance for heat and ageing, which simple watt-hour sums leave out.

The inputs

• Running watts and number of freezers: from the rating labels.
• Startup surge multiplier: compressors briefly draw several times their running power when they start. The default of 4 is a placeholder; use the manufacturer's data if you have it.
• Backup hours and duty cycle: how long you need power and how much of that time the compressor runs.
• Battery voltage and type: lead-acid batteries are commonly discharged only to about 50%, lithium to about 80% or more. Check the datasheet.
• Efficiency, allowance and surge rating: inverter losses, spare capacity for temperature and ageing, and how many times its continuous rating the inverter can surge.

The formulas

• Running load = watts × freezers; surge = running load × multiplier
• Continuous inverter watts = the larger of running × 1.25 or surge ÷ surge rating; VA = watts ÷ 0.8
• Energy = running load × duty cycle × backup hours
• Battery bank (Wh) = energy ÷ (usable fraction × efficiency × (1 − allowance))
• Amp-hours = battery Wh ÷ system voltage

Worked example (illustrative)

One 150 W freezer, 4× surge, 8 hours, 50% duty cycle, lead-acid at 12 V, 90% efficiency, 10% allowance. Surge is 600 W, so the inverter needs about 300 W continuous (375 VA), and the next common size is 1,000 VA. Energy is 600 Wh, so the bank is about 1,480 Wh, or about 123 Ah at 12 V: one 200 Ah battery.

Safety and limits

Batteries and inverters can cause fires and shocks. Have a qualified electrician design and install the system. This tool excludes other loads such as lights, fans and chargers, and real startup surges vary by freezer.

FAQ

Why not size the inverter from running watts alone?

The compressor's startup surge can overload an inverter that is sized only for the running load.

Why is the battery bigger than the energy I need?

Batteries should not be fully drained, and inverters, heat and ageing all reduce what you can actually use.

Results are estimates, not professional advice. See our disclaimer. Calculator and guide by Borniventure Ugochukwu Nwakamma.

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