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Category Electronics

Battery Chemistry

System Voltage

Autonomy Days

Depth of Discharge (%)

Inverter Efficiency (%)

Ambient Temperature (°C)

Single Battery Capacity (Ah)

Single Battery Voltage (V)

Appliance Load List

Appliance	Watts (W)	Qty	Hrs/Day	Wh/Day
Total Daily Consumption				0 Wh

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About

Undersizing a battery bank causes premature cell degradation and unexpected blackouts. Oversizing wastes capital on capacity that never cycles. This calculator determines the minimum usable battery capacity C in amp-hours required to sustain a defined load profile over D autonomy days. It factors in depth of discharge (DoD), inverter efficiency (η), and an optional temperature derating coefficient (k_T). The formula applies to lead-acid, AGM, gel, and LiFePO4 chemistries - each with different safe DoD limits that directly affect cycle life.

Results assume steady-state loads. Inrush currents from motors or compressors can spike draw by 3× to 7× nameplate wattage for several seconds. Account for this by adding a safety margin or selecting an inverter with adequate surge rating. The tool approximates ideal conditions: ambient temperature near 25°C, no significant cable losses, and batteries at stated nominal capacity. Real-world capacity degrades with age, temperature extremes, and high discharge rates (Peukert effect on lead-acid cells).

Formulas

The total daily energy consumption E_daily is the sum of each appliance’s power draw multiplied by its usage duration:

E_daily = n∑i=1 P_i × Q_i × H_i

where P_i = power rating in W, Q_i = quantity of that appliance, H_i = hours of daily use. Result is in Wh/day.

The required battery bank capacity in amp-hours accounts for system voltage, depth of discharge, inverter losses, and desired autonomy:

C_bank = E_daily × DV_sys × DoD × η × k_T

where C_bank = minimum battery capacity in Ah, D = days of autonomy (backup duration without recharge), V_sys = system voltage (12, 24, or 48 V), DoD = maximum depth of discharge (decimal, e.g. 0.5 for 50%), η = inverter efficiency (typically 0.85 - 0.95), k_T = temperature derating factor (1.0 at 25°C, lower at cold temps).

The number of batteries in parallel:

N_parallel = C_bankC_battery

where C_battery = rated capacity of a single battery in Ah. The result is rounded up to the nearest whole number. Series strings are determined by V_sys ÷ V_battery.

Reference Data

Battery Chemistry	Nominal Voltage	Recommended DoD	Cycle Life (at rec. DoD)	Energy Density	Self-Discharge / Month	Temp. Range
Flooded Lead-Acid (FLA)	2V/cell	50%	500 - 1000	30 - 40 Wh/kg	5 - 15%	−20 to 50°C
AGM (Absorbed Glass Mat)	2V/cell	50%	600 - 1200	35 - 45 Wh/kg	1 - 3%	−20 to 50°C
Gel	2V/cell	50%	700 - 1400	35 - 45 Wh/kg	1 - 3%	−20 to 45°C
LiFePO4 (LFP)	3.2V/cell	80%	2000 - 5000	90 - 120 Wh/kg	< 2%	−20 to 60°C
Li-Ion NMC	3.6V/cell	80%	1000 - 3000	150 - 250 Wh/kg	1 - 2%	−20 to 55°C
Common Appliance Reference Wattages
LED Light Bulb	10 W		Refrigerator (avg.)	150 W
Laptop	60 W		Microwave	1000 W
Phone Charger	10 W		Washing Machine	500 W
TV (LED 50″)	100 W		Electric Heater	1500 W
Ceiling Fan	75 W		Air Conditioner (window)	1200 W
Wi-Fi Router	12 W		Water Pump (0.5 HP)	375 W
Desktop Computer	200 W		Coffee Maker	800 W
Toaster	850 W		Hair Dryer	1800 W
Iron	1100 W		Vacuum Cleaner	700 W
Security Camera	15 W		Chest Freezer	100 W

Frequently Asked Questions

Cycle life is inversely related to DoD. A lead-acid battery discharged to 50% DoD daily may deliver 1000-1500 cycles, but the same battery at 80% DoD may only last 300-500 cycles. LiFePO4 cells tolerate 80% DoD routinely and still achieve 2000+ cycles. This calculator defaults to chemistry-safe DoD values to balance capacity cost against replacement frequency.

Batteries store DC energy, but most household appliances run on AC. An inverter converts DC to AC with typical losses of 5-15%. A 90% efficient inverter means for every 1000 Wh drawn from the battery, only 900 Wh reach the load. Omitting this factor undersizes the bank by 10% or more. If your system is purely DC (e.g., 12V RV lighting), set efficiency to 1.0.

Battery capacity drops in cold environments. Lead-acid cells lose roughly 1% capacity per degree below 25°C. At 0°C, effective capacity may be only 75-80% of nameplate. The temperature derating factor k_T corrects for this. At 25°C use 1.0, at 0°C use approximately 0.80, at −20°C use 0.60. LiFePO4 is less affected but still loses capacity below freezing.

Mixing chemistries (e.g., AGM with LiFePO4) is unsafe due to different charge profiles and voltages. Mixing capacities within the same chemistry causes uneven charge distribution - the smallest battery limits the bank. Always use identical batteries from the same manufacturer, batch, and age for parallel and series strings.

Compressor motors can draw 3-7× their rated wattage for 1-3 seconds during startup. This calculator sizes based on continuous draw. For inrush protection, ensure your inverter surge rating exceeds the startup spike. A common rule: select an inverter with a surge capacity at least 3× the largest motor load. The battery bank itself must also sustain the C-rate; lead-acid is generally limited to 0.2C continuous discharge.

Higher system voltage reduces current for the same power, allowing thinner cables and lower resistive losses. Systems under 2000 W typically use 12V. Systems between 2000-5000 W benefit from 24V. Above 5000 W, 48V is standard. The trade-off: higher voltage requires more cells in series, and a single cell failure can disable the entire string. This calculator adjusts the required Ah based on your voltage selection.