Battery Autonomy Days Calculator

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Table of contents

• How to use
• Common mistakes
• Definitions
• Methodology
• Limitations & assumptions
• Sources

Daily energy use: The energy your selected loads use in one day, measured in kWh per day.

kWh: Kilowatt-hour, a unit of energy used for home batteries and electric bills.

Backup time wanted: The number of days you want the battery to run the loads without recharging.

Battery percent you plan to use: The share of rated battery capacity you allow the system to use. It is often called depth of discharge.

System efficiency: The percent of stored energy that reaches the loads after inverter, wiring, and conversion losses.

Extra reserve to add: Extra installed battery capacity added after the minimum size is calculated.

mAh: Milliamp-hour, a small-battery capacity unit used by power banks and device batteries.

mA: Milliamp, a unit of electric current used for a device load.

Usable battery rating: The percent of rated mAh that is expected to be usable after battery and converter losses.

Mistake: Entering whole-house usage in Daily energy use when only a few loads need backup.
Fix: Put only the backed-up loads in Daily energy use unless you want whole-home backup.

Mistake: Typing 0 or leaving Backup time wanted blank.
Fix: Enter the number of days the battery must cover without recharging.

Mistake: Setting Battery percent you plan to use above 100 or treating it like energy used per day.
Fix: Enter the percent of the battery capacity you plan to use, such as 80.

Mistake: Using 100 for System efficiency when the battery output passes through an inverter or long wiring.
Fix: Use a lower System efficiency if energy is lost in conversion or wiring.

Mistake: Mixing Battery capacity in mAh with Average load in watts.
Fix: In device mode, enter Battery capacity in mAh and Average load in mA for the same voltage system.

Mistake: Reading Minimum installed battery size before reserve as the final buying target.
Fix: Use Recommended installed battery size with reserve when you want room for aging, weather, or small load changes.

• Home mode sizes battery storage only. It does not size solar panels, generators, inverters, charge controllers, or battery wiring.
• Daily energy use is treated as constant for every backup day. Real loads can change with weather, habits, and appliance cycles.
• System efficiency is a single percent. The calculator does not model separate inverter, charger, wiring, or temperature losses.
• Battery percent you plan to use must be greater than 0 and no more than 100. The calculator does not choose a safe depth of discharge for a specific battery chemistry.
• Device runtime assumes Battery capacity and Average load are for the same voltage system. It does not convert between volts, watts, Wh, and mAh.
• Device runtime uses average current. It does not model startup surges, sleep modes, cutoff voltage, cold temperature, battery age curves, or Peukert effect.
• Number display can change how many decimals you see, but it should not be treated as product-level precision.

Home backup size

The home mode first finds the energy the selected loads need over the full backup time.

usable_energy_needed_kwh = daily_energy_kwh * target_days

It then divides by the usable battery percent and system efficiency, because the installed battery must be larger than the energy the loads actually receive.

minimum_installed_capacity_kwh = usable_energy_needed_kwh / ((max_discharge_percent / 100) * (system_efficiency_percent / 100))

The practical target adds the reserve percent after the minimum size is found.

recommended_installed_capacity_kwh = minimum_installed_capacity_kwh * (1 + reserve_buffer_percent / 100)

Device runtime

The device mode reduces the rated mAh by the usable battery rating, then divides by the average load current.

device_usable_capacity_mah = battery_capacity_mah * (device_efficiency_percent / 100)

device_runtime_hours = device_usable_capacity_mah / average_load_ma

device_autonomy_days = device_runtime_hours / 24

Worked example

For home backup, 12 kWh per day for 3 days needs 36 kWh of usable energy. With Battery percent you plan to use at 80% and System efficiency at 90%, the minimum installed size is 36 / (0.80 * 0.90) = 50 kWh. With Extra reserve to add at 20%, the Recommended installed battery size with reserve is 60 kWh.

For a device, a 10,000 mAh Battery capacity with an 85% Usable battery rating gives 8,500 mAh usable. At a 250 mA Average load, runtime is 8,500 / 250 = 34 hours, or about 1.42 days.