Battery Sizing Calculator
Size battery storage packs based on daily consumption Wh, autonomy days, nominal voltage, and low temperature capacity derating.
Sizing Criteria
Number of days the battery system must support the load without recharging.
Safe continuous limit. Recommend: 80% for Lithium, 50% for Lead-Acid.
Nominal voltage of pack (e.g. 12.8V, 25.6V, 51.2V)
Inverter loss, cabling voltage drops, and battery charge acceptance efficiency.
Cold temperatures reduce chemical energy storage capability.
Estimated Outputs
Design Layout Suggestion
To achieve 138.5 Ah capacity at 25.6V, you could use a series/parallel string configuration of 8 series cells with multiple parallel strings.
Mathematical Formulas
The required battery energy capacity E_req is calculated by scaling the daily energy demand by autonomy and dividing by losses:
The temperature multiplier derates battery capability at low temperatures:
- T >= 25°C: Multiplier = 1.00
- 0°C <= T < 25°C: Multiplier = 0.90 + 0.10 x (T/25)
- -20°C <= T < 0°C: Multiplier = 0.70 + 0.20 x ((T+20)/20)
Variables: Daily Energy in Wh, Autonomy in days, DoD as percentage, Efficiency as percentage, Temperature in °C. Assumptions: constant daily load, standard discharge curves, 25°C reference temperature.
Worked Examples
Example 1: Small Cabin — 1.5 kWh/day, 25.6V
- Daily load: 1,500 Wh | Autonomy: 2 days | DoD: 80%
- Efficiency: 90% | Temperature: 10°C
Temp multiplier: 0.90 + 0.10 × (10/25) = 0.94
Required energy: (1,500 × 2) / (0.80 × 0.90 × 0.94) = 4,433 Wh (4.43 kWh)
Capacity: 4,433 / 25.6 = 173.2 Ah
Example 2: Off-Grid Home — 8 kWh/day, 48V
- Daily load: 8,000 Wh | Autonomy: 3 days | DoD: 85%
- Efficiency: 92% | Temperature: 20°C
Temp multiplier: 0.90 + 0.10 × (20/25) = 0.98
Required energy: (8,000 × 3) / (0.85 × 0.92 × 0.98) = 31,358 Wh (31.36 kWh)
Capacity: 31,358 / 48 = 653.3 Ah
Example 3: Telecom Tower — 5 kWh/day, 48V, Cold Climate
- Daily load: 5,000 Wh | Autonomy: 4 days | DoD: 80%
- Efficiency: 90% | Temperature: -10°C
Temp multiplier: 0.70 + 0.20 × ((-10+20)/20) = 0.80
Required energy: (5,000 × 4) / (0.80 × 0.90 × 0.80) = 34,722 Wh (34.72 kWh)
Capacity: 34,722 / 48 = 723.4 Ah
Frequently Asked Questions
What does "Days of Autonomy" mean?
Autonomy is the number of days a battery storage bank can support system loads without receiving any input charge (e.g., during cloudy days for solar, or windless periods for turbines). Solar off-grid systems typically use 2 to 3 days of autonomy to prevent total blackouts.
How do Lead-Acid and Lithium temperature characteristics differ?
Lithium chemistries perform slightly better than lead-acid at cold temperatures down to -10°C. However, lithium batteries suffer a critical constraint: they cannot be charged below 0°C without causing permanent cell degradation due to lithium plating, unless they incorporate pre-heating circuits.
How do I estimate my daily energy consumption?
Multiply the wattage rating of every electrical load by the number of hours it runs per day. Add all load results together. Example: A 100W TV running for 3 hours = 300 Wh. A 50W fridge cycling on for 10 hours = 500 Wh. Total daily energy is 800 Wh.
Why is system round-trip efficiency lower than cell efficiency?
While lithium cells are 98% efficient, the system includes wiring resistive losses, battery management system (BMS) standby power, fuse contacts, and DC-to-AC inverter losses. The overall system efficiency represents the net thermal loss from battery terminal to end-load.
How many batteries do I need for a 5kW load?
It depends on autonomy and voltage. For a 5kW load running 8 hours/day at 48V with 2 days autonomy and 80% DoD: Required energy = (5,000W × 8h × 2) / (0.80 × 0.90) = 111,111 Wh = 111.1 kWh. At 48V, that is 2,314 Ah — equivalent to about 8 large 280Ah LFP batteries in a 48V configuration.
What is the difference between battery capacity and usable capacity?
Rated capacity is the total energy stored at 100% charge. Usable capacity is what you can safely extract before hitting DoD limits. A 100Ah LFP battery at 80% DoD provides 80Ah usable. A 100Ah lead-acid at 50% DoD provides only 50Ah usable. Always size based on usable capacity.
How does battery chemistry affect sizing?
LFP allows 80–90% DoD with 5,000+ cycles, NMC allows 70–80% DoD with 2,000 cycles, and lead-acid is limited to 50% DoD with 500–1,000 cycles. LFP requires less total capacity for the same usable energy but costs more upfront per kWh.
Should I oversize my battery bank?
Moderate oversizing (10–20%) provides safety margin for temperature derating, aging, and load growth. Excessive oversizing (50%+) wastes capital and may cause undercharging issues. A 15–20% margin above calculated minimum is generally appropriate.
What voltage should I choose for my battery bank?
Higher voltage reduces current for the same power, meaning thinner cables and lower losses. 12V suits small systems (<1kW), 24V suits medium systems (1–5kW), and 48V is standard for residential and commercial systems (>5kW). Most modern inverters are designed for 48V.
How do I account for future load growth?
Add 15–25% capacity margin to account for additional appliances, increased usage, or family growth. Alternatively, design for modular expansion — use a battery system that allows adding parallel strings later without replacing the entire bank.
Can I mix old and new batteries?
Never mix batteries of different ages, chemistries, or capacities in the same bank. Older batteries have higher internal resistance and lower capacity, forcing newer batteries to work harder and degrade faster. Always replace entire strings or banks at once.
What happens if I undersize my battery bank?
Undersized banks cause deep discharge cycles that accelerate degradation, voltage sag under load triggering premature low-voltage disconnects, and insufficient autonomy during extended cloudy periods. The cost of premature battery replacement often exceeds the savings from undersizing.
Parallel String Tool
Arrange series/parallel packs to match this output capacity.
Runtime Calculator
Verify run hours under a specific continuous Watt load.
Energy Conversion
Convert battery capacities between Ah and Wh.
Solar Battery Sizing
Size solar battery banks from daily consumption data.
Home Backup Calculator
Size home battery backup for critical loads.
Battery Pack Calculator
Design series/parallel cell configurations for target specs.
Marine Battery Sizing
Size marine house banks for hotel loads.
RV Battery Calculator
Plan RV power budgets for off-grid trips.
What Is Battery Sizing?
Why This Calculation Matters
→ Oversized battery banks waste capital — a 30% oversized bank can cost thousands in unnecessary lithium cells that sit unused.
→ Undersized banks cause unexpected power loss, forcing loads to shut down before the intended autonomy period ends.
→ Ignoring temperature derating can leave systems short 10–30% of expected capacity in cold climates.
→ Confusing nominal and usable capacity leads to banks that appear sufficient on paper but fail under real DoD constraints.
→ Neglecting round-trip efficiency means the battery must supply more energy than the loads consume, creating a hidden deficit.
Practical Applications
Residential Energy Storage
Size home battery systems for solar self-consumption, backup power, and time-of-use optimization.
Off-Grid Solar
Determine battery bank capacity for homes and cabins entirely disconnected from the utility grid.
Telecom Backup
Ensure telecom tower batteries meet minimum autonomy requirements during extended grid outages.
Marine Power Systems
Size house banks for vessels with significant hotel loads during extended cruising.
RV & Mobile Living
Calculate battery capacity for full-time RV living with solar charging infrastructure.
Industrial UPS
Size battery banks for critical industrial processes requiring guaranteed backup duration.
Common Mistakes to Avoid
✗ Forgetting temperature derating — lithium batteries cannot be charged below 0°C without permanent damage. Lead-acid capacity drops significantly in cold conditions. Always apply temperature correction factors.
✗ Confusing rated and usable capacity — a 100Ah lead-acid battery at 50% DoD only provides 50Ah usable. A 100Ah LFP battery at 80% DoD provides 80Ah. Size based on usable capacity.
✗ Ignoring system round-trip efficiency — battery charging and discharging loses 5–15% of energy. Your load requirement must be divided by system efficiency to get the true battery capacity needed.
✗ Using average instead of peak load — your battery must handle peak loads, not just averages. A well pump may draw 1,000W for 5 minutes even if average consumption is 50W.
✗ Mixing battery ages or chemistries — never combine old and new batteries, or different chemistries, in the same bank. Mismatched internal resistance causes uneven loading and accelerated degradation.
✗ Ignoring self-discharge losses — over multi-day autonomy periods, self-discharge (1–5%/month lithium, higher lead-acid) reduces available energy. Factor this into long-autonomy calculations.
✗ Sizing only for today's loads — future load growth from new appliances, family changes, or increased usage can quickly outpace an undersized bank. Include 15–20% growth margin.
✗ Choosing wrong system voltage — a 12V system for a 5kW load requires 400+ amps, demanding extremely thick cables. Use 48V for loads above 2kW to keep current manageable.
Why Trust These Calculations?
This calculator uses publicly documented battery engineering formulas. The sizing model accounts for energy scaling by autonomy, derating by depth of discharge, round-trip efficiency losses, and temperature-dependent capacity reduction. All intermediate values are displayed for independent verification. Our methodology follows IEEE 1188 and manufacturer application notes.
View our full methodology →Parallel String Tool
Arrange series/parallel packs to match this output capacity.
Runtime Calculator
Verify run hours under a specific continuous Watt load.
Energy Conversion
Convert battery capacities between Ah and Wh.
Solar Battery Sizing
Size solar battery banks from daily consumption data.
Home Backup Calculator
Size home battery backup for critical loads.
Battery Pack Calculator
Design series/parallel cell configurations for target specs.
Marine Battery Sizing
Size marine house banks for hotel loads.
RV Battery Calculator
Plan RV power budgets for off-grid trips.
References & Further Reading
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