Battery Calculators
Accurate, free calculators for battery capacity, charging time, backup duration, and bank sizing — built for solar installers, DIYers, and engineers.
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Battery System Basics
Selecting the right battery chemistry and configuration is key to a reliable and efficient power system.
Lead-Acid (AGM/Gel)
Robust and cost-effective batteries for backup power and deep-cycle solar applications.
Lithium-Ion (LiFePO4)
High energy density, longer life, and faster charging compared to traditional chemistries.
Charge Controller
Manages power from solar panels or the grid to prevent battery overcharging.
Battery Bank
A group of batteries connected in series or parallel to meet specific voltage and capacity needs.
How to Size a Battery Bank
Follow this process to calculate the exact battery requirements for your off-grid or backup system.
Calculate Daily Load
Sum up the wattage of all devices and multiply by their daily runtime to get Total Watt-Hours (Wh) per day.
Factor in Autonomy
Decide how many days of backup power you need during overcast or grid-down periods (Days of Autonomy).
Adjust for Depth of Discharge
To maximize battery life, don't use 100% of capacity. Adjust for DoD (e.g., 50% for Lead-Acid, 80% for Lithium).
Account for System Losses
Inverters and wiring create heat and energy loss. Multiply your required capacity by 1.15 to 1.25 for safety.
Select Voltage & Configuration
Choose 12V, 24V, or 48V based on your inverter and connect batteries in series/parallel to reach target Ah.
Core Battery Formulas
Mathematical equations used to convert capacity and predict performance.
Energy = Amp-Hours × Voltage.
Time = Capacity ÷ Current Draw.
Time = Capacity ÷ Charger Amps.
Maximizing Battery Lifespan
Batteries are a major investment. Proper care and precise calculation prevent premature failure and ensure you have power when you need it most.
Key Health Factors
- Temperature-controlled environment
- Correct Depth of Discharge (DoD) limits
- Balanced charging for multi-cell packs
- Appropriate cable sizing to prevent voltage drop
- Regular inspection of terminals and connections
Ah vs. Wh
Amp-Hours (Ah) measure charge, while Watt-Hours (Wh) measure total energy. Wh is the best way to compare different voltages.
Series vs. Parallel
Series increases voltage; Parallel increases capacity (Ah). You can combine both for large industrial banks.
Self-Discharge
All batteries lose charge over time even when not in use. Monitor your bank during long storage periods.
Common Sizing Mistakes
Avoid these frequent errors that lead to dead batteries and system downtime.
Over-Discharging Cells
Frequently running lead-acid batteries below 50% capacity causes permanent chemical damage and rapid failure.
Capacity LossMixing Old & New Batteries
Adding a new battery to an old bank causes unbalanced charging, leading to the new battery aging prematurely.
System ImbalanceIgnoring Peukert's Law
Lead-acid capacity drops as you draw current faster. A battery rated for 20 hours won't last 1 hour at 20x the current.
Physics ErrorInsufficient Cable Gauge
Thin wires cause voltage drop and heat, making the system think the battery is dead when it's actually just wire loss.
Efficiency DropComparisons & Use Cases
From small hobby projects to utility-scale energy storage — tools for every battery application.
Compare the upfront cost vs. long-term cycle life. Understand which chemistry fits your budget and weight requirements.
Calculate the storage needed to run a cabin or home entirely on solar power, including overcast day autonomy.
Precision sizing for critical backup systems to ensure zero downtime during power grid fluctuations or failures.
Battery Glossary
Key engineering terms encountered in battery sizing and energy management.
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Ah
- Ampere-Hour
- The amount of energy charge in a battery that will allow one ampere of current to flow for one hour.
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DoD
- Depth of Discharge
- The percentage of a battery's capacity that has been discharged relative to its total capacity.
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CCA
- Cold Cranking Amps
- A rating used in the automotive industry to define a battery's ability to start an engine in cold temperatures.
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Cycle
- Charge Cycle
- The process of discharging and then recharging a battery. One full cycle is 100% discharge and recharge.
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SG
- Specific Gravity
- A measurement used in flooded lead-acid batteries to determine the state of charge by checking electrolyte density.
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BMS
- Battery Management System
- An electronic system that manages a rechargeable battery pack to protect it from operating outside its safe window.
Why Use KWCalc?
Every battery calculator is built on verified IEEE and chemical performance standards — instant results, zero data stored, completely free.
Engineering Accuracy
Formulas verified against industry standards for reliable, professional-grade results.
Instant Results
All calculations run locally in your browser — no loading, no waiting, no server calls.
Private & Secure
Zero data stored or transmitted. Your inputs stay entirely on your device.
100% Free
No paywalls, no registration, no hidden fees. Every tool is completely free to use.
Chemistry Specific
Detailed support for Lead-Acid, AGM, Gel, and Lithium-Ion battery chemistries.
Frequently Asked Questions
Everything you need to know about battery capacity, runtime, and charging.
What is the difference between Ah and Wh?
Amp-Hours (Ah) measures the electrical charge capacity at a specific voltage. Watt-Hours (Wh) measures the total energy capacity regardless of voltage.
To compare batteries of different voltages, Wh is the most accurate metric. Formula: Wh = Ah × Voltage .
How do I calculate battery runtime?
To estimate how long a battery will last under a specific load:
Runtime (hours) = Battery Capacity (Ah) ÷ Load Current (Amps)
For example, a 100Ah battery running a 5-Amp load will last approximately 20 hours (100 / 5 = 20). Note that lead-acid batteries will last significantly less time under very heavy loads due to Peukert's effect.
Can I mix different capacity batteries in a bank?
No, it is highly discouraged. Mixing batteries of different capacities or ages leads to uneven charging and discharging. The weaker battery will be stressed more, causing the entire bank to fail prematurely.
How do I calculate charging time?
Basic charging time formula is:
Time (hours) = Capacity to add (Ah) ÷ Charger Current (Amps)
You should also add about 15-20% extra time to account for charging efficiency losses (heat and chemical conversion).
What is the 'Depth of Discharge' (DoD)?
DoD refers to how much energy you have taken out of the battery. For example, if you have a 100Ah battery and use 50Ah, your DoD is 50%. Most lead-acid batteries should not exceed 50% DoD daily, while lithium batteries can safely go up to 80-90% DoD.
What is a 'Series' vs 'Parallel' connection?
Series: Connect Positive to Negative. Increases Voltage (V), Capacity (Ah) stays the same. (e.g., two 12V 100Ah batteries in series = 24V 100Ah).
Parallel: Connect Positive to Positive and Negative to Negative. Increases Capacity (Ah), Voltage stays the same. (e.g., two 12V 100Ah batteries in parallel = 12V 200Ah).
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