Battery Backup Time
Solve · Swap · 12 Battery Types
t = Ah ÷ I
· Solve for Runtime · Capacity · Current
Solve for Runtime, Capacity, or Load Current. Swap inputs and outputs. Supports 12 battery types with Peukert correction.
Ah
A
hours
V
Advanced Options
n
%
%
°C
Common Batteries:
— hours
—
0h
0h
12h+
Runtime
— h
Effective Capacity
— Ah
Energy (Wh)
— Wh
DOD Used
— %
How to Use This Tool
1
Select Battery Type
— Choose from 12 battery types including Tubular (solar/inverter),
LiFePO₄ (lithium), AGM, Lead Acid, and more.
2
Choose What to Solve
— Select "Runtime", "Capacity", or "Load Current"
from the dropdown. The calculator will rearrange the formula to solve for your chosen variable.
3
Enter Values
— Enter the two known values. The third value (marked with Result) will be calculated.
Use the Swap button to exchange input and output values.
4
Advanced Options
— Click "Advanced Options" to fine-tune Peukert exponent, efficiency, DOD limit, and temperature.
5
Calculate
— Click "Calculate" or press Enter to see the result with
detailed breakdown and recommendations.
6
Use Presets
— Click any "Common Batteries" preset to instantly load typical values.
Pro Tip:
Use Swap to quickly flip between solving for Runtime and Capacity.
For Tubular batteries, set DOD to 60-70% for longer life.
For LiFePO₄, use 90% DOD safely.
Battery Type Reference
Typical values for each battery type
| Type | Efficiency | Peukert (n) | DOD Limit | Typical Use |
|---|
t = Ah ÷ I · Corrected for Peukert, efficiency, DOD, and temperature. Estimates only. Always consult your battery manufacturer's specifications.
Variables
| Symbol | Quantity | Unit |
|---|---|---|
| t | Battery backup time (runtime) | hours (h) |
| Ah | Battery capacity rating (amp-hours) | Ah |
| I_load | Load current drawn by the system | A |
What it means
The backup time of a battery is given by t = Ah / I_load, where Ah is the battery capacity and I_load is the current drawn by the load. This assumes a constant current and 100% efficiency, which is not the case in practice (inefficiencies, Peukert effect, etc.). Example: A 100Ah battery supplying a 5A load will theoretically last 100/5 = 20 hours. In reality, it might be less due to factors like temperature and discharge rate. This calculation helps in estimating how long equipment can run on battery power during outages, and it guides the selection of battery capacity for critical loads.
Worked example
Home Inverter System
Tubular Lead Acid
Scenario: A 150 Ah tubular battery powers essential home loads (TV, lights, fans) during a power outage.
| Parameter | Value |
|---|---|
| Capacity | 150 Ah |
| System Voltage | 24 V |
| Load | 480 W |
| Load Current | 20 A |
| Max DOD | 65% |
| Efficiency | 85% |
| Formula | t = (Ah × DOD × Eff) ÷ I |
1Usable Capacity = 150 × 0.65 = 97.5 Ah
2Apply Peukert (n=1.2): Effective = ~80.5 Ah
3Apply Efficiency (85%): Usable = 68.4 Ah
4Runtime = 68.4 ÷ 20 = 3.42 hours
✅Result: ~3.5 hours of backup for essential home loads.
Runtime
3.42 h
≈ 3h 25m · 65% DOD
Why Tubular: Designed for deep-cycle inverter use. Recommended DOD: 60–70% for 1500+ cycles.
Solar Storage System
LiFePO₄
Scenario: A 200 Ah LiFePO₄ battery bank powers off-grid appliances (fridge, TV, lights, laptop).
| Parameter | Value |
|---|---|
| Capacity | 200 Ah |
| System Voltage | 48 V |
| Load | 1,200 W |
| Load Current | 25 A |
| Max DOD | 90% |
| Efficiency | 96% |
| Formula | t = (Ah × DOD × Eff) ÷ I |
1Usable Capacity = 200 × 0.90 = 180 Ah
2Apply Peukert (n=1.03): Effective = ~196 Ah
3Apply Efficiency (96%): Usable = 188 Ah
4Runtime = 188 ÷ 25 = 7.55 hours
✅Result: Over 7.5 hours of reliable off-grid power.
Runtime
7.55 h
≈ 7h 33m · 90% DOD
Why LiFePO₄: 96% efficiency, 2000+ cycles, 90% DOD safe. Ideal for solar storage.
Automotive Starting Battery
Flooded Lead Acid
Scenario: A 60 Ah starting battery powers camping accessories (lights, phone chargers, DVD) while preserving engine start.
| Parameter | Value |
|---|---|
| Capacity | 60 Ah |
| System Voltage | 12 V |
| Load | 120 W |
| Load Current | 10 A |
| Safe DOD | 50% |
| Efficiency | 75% |
| Formula | t = (Ah × DOD × Eff) ÷ I |
1Safe Capacity = 60 × 0.50 = 30 Ah
2Apply Peukert (n=1.3): Effective = ~44 Ah
3Apply Efficiency (75%): Usable = 33 Ah
4Runtime = 33 ÷ 10 = 3.32 hours
✅Result: ~3.3 hours of use while preserving engine start.
Safe Runtime
3.32 h
50% DOD · Start‑Safe
Warning: Do not exceed 50% DOD on starting batteries — you risk not being able to start the engine.
Emergency Medical Backup
AGM
Scenario: A 75 Ah AGM battery powers critical medical equipment (CPAP, oxygen concentrator, monitor).
| Parameter | Value |
|---|---|
| Capacity | 75 Ah |
| System Voltage | 12 V |
| Load | 200 W |
| Load Current | 16.7 A |
| Max DOD | 55% |
| Efficiency | 88% |
| Formula | t = (Ah × DOD × Eff) ÷ I |
1Usable Capacity = 75 × 0.55 = 41.25 Ah
2Apply Peukert (n=1.15): Effective = ~62.5 Ah
3Apply Efficiency (88%): Usable = 55 Ah
4Runtime = 55 ÷ 16.7 = 3.29 hours
✅Result: ~3.3 hours of critical medical support.
Runtime
3.29 h
≈ 3h 17m · 55% DOD
Why AGM: Maintenance‑free, spill‑proof, reliable. Ideal for critical medical backup with 55% DOD for long cycle life.
Small Solar Cabin
Gel Lead Acid
Scenario: A 100 Ah gel battery powers a weekend cabin with LED lights, a small fridge, and USB chargers.
| Parameter | Value |
|---|---|
| Capacity | 100 Ah |
| System Voltage | 12 V |
| Load | 150 W |
| Load Current | 12.5 A |
| Max DOD | 65% |
| Efficiency | 86% |
| Formula | t = (Ah × DOD × Eff) ÷ I |
1Usable Capacity = 100 × 0.65 = 65 Ah
2Apply Peukert (n=1.18): Effective = ~77.5 Ah
3Apply Efficiency (86%): Usable = 66.7 Ah
4Runtime = 66.7 ÷ 12.5 = 5.34 hours
✅Result: ~5.3 hours of power for a small cabin setup.
Runtime
5.34 h
≈ 5h 20m · 65% DOD
Why Gel: Deep‑cycle capable, maintenance‑free, low self‑discharge. Perfect for small solar installations.
Common mistakes
Watch for unit mismatches (W vs kW, single- vs three-phase) and remember to include power factor or efficiency where the formula requires it.Applications
Battery backup time t = Ah / I_load estimates how long a battery can supply a given load. This is used to size batteries for UPS, solar storage, and portable devices. Engineers use it to ensure adequate runtime for critical loads. This formula is essential for backup power design.
- UPS and emergency power system design
- Solar battery storage sizing
- Portable device battery life estimation
- Load profiling and runtime planning
- Educational understanding of battery run time
Frequently Asked Questions
Find answers to common questions about this calculator, its inputs, results, calculations, and practical use.
12 questionsWhat is a Battery Backup Time Calculator?
A Battery Backup Time Calculator estimates how long a battery or battery bank can supply power to a connected electrical load. It uses battery capacity, battery voltage, load power, efficiency, and other applicable factors to estimate runtime.
How is battery backup time calculated?
A basic estimate can be calculated using Backup Time (hours) = Battery Capacity (Ah) × Battery Voltage (V) × Efficiency ÷ Load Power (W). For more realistic calculations, usable depth of discharge, inverter efficiency, battery characteristics, and system losses should also be considered.
What does Ah mean on a battery?
Ah stands for ampere-hour and represents a battery's electrical charge capacity. For example, a 100 Ah battery is theoretically capable of supplying 5 A for 20 hours under specified test conditions, although actual performance depends on battery chemistry, discharge rate, temperature, age, and other factors.
How long will a 100 Ah battery last?
The runtime of a 100 Ah battery depends on its voltage, connected load, allowable depth of discharge, efficiency, and battery type. A 100 Ah rating alone is not enough to determine an accurate backup time.
How do I convert battery Ah into watt-hours?
For a simple nominal-energy estimate, multiply ampere-hours by battery voltage: Wh = Ah × V. For example, a nominal 12 V, 100 Ah battery contains approximately 1,200 Wh of nominal energy before accounting for usable depth of discharge and system losses.
Does inverter efficiency affect battery backup time?
Yes. An inverter consumes some energy while converting DC battery power into AC power. Lower inverter efficiency means more battery energy is required for the same AC load, which reduces the available backup time.
What is Depth of Discharge (DoD)?
Depth of Discharge indicates the percentage of a battery's capacity that has been discharged. If only part of the nominal battery capacity is intended to be used, the usable energy and resulting backup time will be lower than calculations based on 100% of the rated capacity.
Can I calculate backup time for multiple batteries?
Yes. A battery bank can be evaluated by determining its resulting system voltage and ampere-hour capacity according to its series and parallel configuration, then calculating the usable stored energy available to the load.
Why is actual battery runtime different from calculated runtime?
A calculated runtime is an estimate. Actual backup time can vary because of battery age, temperature, discharge rate, battery chemistry, inverter losses, wiring losses, battery condition, load variation, and manufacturer-specific capacity ratings.
Can this calculator be used for lead-acid and lithium batteries?
Yes, provided the calculation accounts for appropriate parameters for the selected battery chemistry. Lead-acid and lithium batteries can have different usable depth of discharge, discharge behavior, efficiency, and manufacturer recommendations.
Does connecting batteries in series increase backup time?
Series connection increases the battery-bank voltage while the ampere-hour rating normally remains the same for identical batteries. The total nominal stored energy increases when additional identical batteries are added, but the resulting runtime depends on the system voltage, load, efficiency, and usable capacity.
Does connecting batteries in parallel increase backup time?
For identical batteries, a parallel connection keeps the nominal voltage the same while increasing the total ampere-hour capacity. This increases the battery bank's nominal stored energy and can provide longer backup time for the same load.
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