Battery Runtime Calculator

Estimate runtime and calculate the best battery solution for your off-grid system

Battery Type

# of Batteries

Voltage (V)

Capacity (Ah)

DoD (%)

AC Load (W)

Minimal (100W)

Lighting (150W)

Basic + Fridge (300W)

Comfort (500W)

Full Load (800W)

Battery Price ($)

Suggested: $180

Show Advanced Settings

Inverter Efficiency (%)

Battery Age (years)

Solar Charging (W)

4.37 hours

(0.18 days)

4h 22m

Less than half a day of backup power.

Battery Level After Use: 10% remaining

Usable Capacity: 1080Wh

Est. Recharge Time: ~3.2 hours with 400W of solar input.

Battery Cost Analysis

Lithium-Ion (Current)

Upfront: $180 ($150/kWh) • Lasts 10+ years

$180

10-Year Total Cost Comparison

Lithium-Ion: $180 (1 replacement)

AGM Lead-Acid: $240 (2 replacements)

Flooded Lead-Acid: $225 (3 replacements)

Frequently Asked Questions

What is Depth of Discharge (DoD) and why does it matter?

Depth of Discharge represents how much of your battery's capacity you use before recharging. For example, 90% DoD means you use 90% of the battery's energy. Different battery types have different optimal DoD levels: Lithium can safely go to 90-95%, AGM to 50-70%, and flooded lead-acid to 50%. Using higher DoD than recommended will significantly reduce battery lifespan.

How accurate are these runtime calculations?

Our calculations provide estimates based on ideal conditions. Real-world performance can vary by ±15-25% due to factors like temperature, battery age, load variations, and discharge rates. Cold temperatures can reduce capacity by 20-40%, while high discharge rates (>0.5C) can reduce effective capacity by 10-15%.

Why do lithium batteries appear more expensive initially but cheaper long-term?

Lithium batteries (LiFePO4) cost 2-3x more upfront but last 10-15 years versus 3-5 years for lead-acid. They also allow deeper discharge (90% vs 50%), provide more usable capacity, and maintain consistent voltage throughout discharge. Over 10 years, total cost of ownership is typically 20-40% lower than lead-acid alternatives.

How does inverter efficiency affect my calculations?

Inverter efficiency represents energy lost when converting DC battery power to AC power for your devices. A 90% efficient inverter means 10% of your battery energy is lost as heat. Higher quality pure sine wave inverters typically achieve 90-95% efficiency, while cheaper modified sine wave units may only reach 80-85%.

Should I wire batteries in series or parallel?

Series wiring increases voltage (2x 12V = 24V system) while maintaining the same amp-hour capacity. Parallel wiring maintains voltage but doubles capacity (2x 100Ah = 200Ah total). Most off-grid systems use 12V or 24V configurations. Higher voltage systems (24V/48V) are more efficient for larger loads and longer cable runs.

How do I size my solar charging system?

A good rule of thumb is to have solar capacity equal to 20-25% of your battery bank capacity in amp-hours. For a 400Ah battery bank, aim for 80-100W of solar per 100Ah (320-400W total). This allows for full recharge in 4-6 hours of good sunlight while accounting for charging losses and less-than-ideal conditions.

References

Battery University - Guide to Battery Discharge Characteristics and DoD Recommendations

Comprehensive resource covering battery chemistry, discharge curves, and optimal depth of discharge practices for different battery technologies.

IEEE Standards for Battery Testing and Performance Evaluation

Industry standards IEEE 1188, IEEE 1106, and IEEE 450 defining standardized testing procedures and performance metrics for stationary battery systems.

Solar Power World - Battery Bank Sizing Guidelines for Off-Grid Systems

Practical guidelines for residential and commercial off-grid battery system design, including sizing methodologies and safety considerations.

National Renewable Energy Laboratory (NREL) - Battery Storage System Design

Technical reports and research on energy storage system integration, performance modeling, and economic analysis for renewable energy applications.

International Electrotechnical Commission (IEC) - Battery Performance Standards

International standards IEC 61427 and IEC 62619 covering performance requirements and test methods for batteries in renewable energy applications.

Formulae Used

Total Battery Capacity (Wh)

Total Capacity = Voltage × Capacity (Ah) × Number of Batteries

Calculates the total energy storage capacity of your battery bank in watt-hours.

Usable Capacity (Wh)

Usable Capacity = Total Capacity × (DoD ÷ 100) × Age Factor

Determines how much energy you can actually use based on recommended depth of discharge and battery aging.

DC Load Requirement (W)

DC Load = AC Load ÷ (Inverter Efficiency ÷ 100)

Converts your AC power requirements to DC load, accounting for inverter losses.

Runtime Calculation (Hours)

Runtime = Usable Capacity (Wh) ÷ DC Load (W)

Estimates how long your battery bank will power your loads before reaching the minimum DoD.

Age Degradation Factor

Age Factor = 1 - (Age × Degradation Rate)

Accounts for capacity loss over time. Lithium: 2%/year, AGM: 5%/year, Flooded: 7%/year.

Solar Recharge Time

Recharge Time = Used Capacity ÷ (Solar Watts × 0.85)

Estimates time to fully recharge based on solar input, with 85% charging efficiency factor.

How It Works

Battery Configuration Analysis

The calculator first determines your total battery bank capacity by multiplying voltage, amp-hour rating, and number of batteries. It then applies battery-specific parameters like optimal depth of discharge and degradation rates based on the selected battery chemistry.

Load Conversion & Efficiency

Your AC load is converted to DC requirements by factoring in inverter efficiency losses. Most inverters lose 5-15% of energy during DC to AC conversion. This ensures accurate runtime calculations based on actual battery drain.

Usable Capacity Determination

The system calculates how much energy you can safely use without damaging your batteries. This involves applying the appropriate depth of discharge limit and age-related capacity degradation to determine real-world usable capacity.

Runtime & Economic Analysis

Finally, runtime is calculated by dividing usable capacity by DC load requirements. The economic analysis compares upfront costs with long-term value, factoring in replacement cycles and total cost of ownership over 10 years.

Important Considerations

                    These calculations assume steady loads and ideal conditions. Real-world performance varies based on temperature, discharge rates, battery quality, and load patterns. For critical applications, size your system 25-50% larger than calculated minimums to account for these variables.

Battery Runtime Calculator

Battery Cost Analysis

Frequently Asked Questions

References

Formulae Used

How It Works

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