Battery Size Calculator

Calculate required battery capacity for backup power, solar storage, and off-grid applications.

Load Power (W)

Backup Time (hours)

System Voltage (V)

Battery Type

Depth of Discharge (%)

Lithium: 80-90%, Lead-acid: 50%

System Efficiency (%)

Typical: 85-95%

Type	Max DoD	Cycle Life	Cost/kWh	Maintenance
Lithium-Ion	80-90%	3000-5000	$400-600	None
AGM	50%	400-800	$200-300	None
Flooded LA	50%	300-700	$100-200	Regular

Energy Needed: Wh = Power (W) × Time (h)

With Efficiency: Wh_total = Wh_needed / Efficiency

With DoD: Capacity = Wh_total / DoD

Amp-Hours: Ah = Wh / System Voltage

Lithium batteries allow deeper discharge (80-90%) vs lead-acid (50%)
Cold temperatures reduce battery capacity - add 25% for cold climates
Battery aging reduces capacity over time - plan for 80% capacity after 5-10 years
Continuous discharge should be limited to C/5 or C/10 for lead-acid
Lithium batteries have higher upfront cost but better lifecycle economics
Size batteries 25-30% larger than calculated for margin of safety
Series connections increase voltage, parallel connections increase capacity
Use battery management system (BMS) for lithium batteries
Flooded batteries require ventilation for hydrogen gas
Temperature compensation is critical for lead-acid charging