Battery Backup Calculator
Calculate backup time and power requirements for battery systems
Battery Backup Calculator
Professional Battery Backup Time Estimation
Results:
Backup Time: 0.00 hours
Backup Time: 0 minutes
Usable Capacity: 0.00 Wh
Battery Energy: 0.00 Wh
How to Use the Calculator
Battery Specifications
- Select battery voltage (12V, 24V, 48V or custom)
- Enter battery capacity in Amp-hours (Ah)
- Input the total load power in watts
- Set system efficiency percentage
- Configure depth of discharge limit
System Efficiency
- Typical inverter efficiency: 85-95%
- DC systems: 90-98%
- Include all conversion losses
- Consider cable and connection losses
- Account for temperature effects
Depth of Discharge
- Lead-acid: 50-80% recommended
- Lithium: 80-95% safe range
- AGM batteries: 50-80%
- Gel batteries: 50-70%
- Higher DoD reduces battery life
How to Calculate Battery Backup Time
Use these manual steps alongside the calculator to validate runtime estimates.
1. Capture Battery Specs
Gather the nominal voltage, rated capacity, and chemistry for the bank you are sizing.
- Common system voltages: 12V, 24V, 48V, or custom stacks.
- Record nameplate capacity in amp-hours or watt-hours.
- Note chemistry (flooded, AGM, gel, lithium) to pick a safe depth of discharge.
2. Define Load Profile
Quantify every device that will run from the battery bank.
- Sum continuous watts plus any surge multipliers for motors.
- Separate critical loads from optional circuits for staged shedding.
- Account for inverter standby draw and DC auxiliaries.
3. Compute Stored Energy
Translate the battery specs into watt-hours of energy.
- Energy (Wh) = Battery Voltage x Capacity (Ah).
- Multiply parallel strings before using the formula.
- Temperature corrections may reduce the effective capacity.
4. Apply Usable Energy Limits
Only a portion of the stored energy is available for daily cycling.
- Usable = Energy x Depth of Discharge x System Efficiency.
- Lead-acid banks typically use 50-80% DoD; lithium can reach 90-95%.
- Include inverter, cabling, and temperature losses when choosing efficiency.
5. Convert to Backup Time
Divide usable energy by the expected load to get runtime.
- Backup Time (h) = Usable Energy (Wh) ÷ Load Power (W).
- Multiply the result by 60 for minutes or by 24 for days.
- Cross-check against mission-critical loads to confirm margins.
6. Validate and Iterate
Stress-test the plan with best and worst case scenarios.
- Simulate peak loads, inrush events, and temperature swings.
- Increase capacity or add parallel strings if runtime falls short.
- Document inspection intervals to maintain real-world efficiency.
Example Inputs
Assume two 12V 200Ah AGM batteries wired for 24V feeding a 600W backup panel.
- Total capacity: 24V x 200Ah = 4,800 Wh.
- Depth of discharge: 75% to protect lead-acid life.
- System efficiency: 0.85 to cover inverter and wiring losses.
Math Walkthrough
Translate those inputs into usable energy and runtime.
- Usable energy = 4,800 Wh x 0.75 x 0.85 = 3,060 Wh.
- Backup time = 3,060 Wh ÷ 600 W ≈ 5.1 hours.
- Minutes = 5.1 h x 60 ≈ 306 minutes of support.
Real-Life Takeaways
Convert the calculation into an actionable plan.
- Add another parallel string to double runtime past 10 hours.
- Drop the load to 400W and the same bank lasts ~7.6 hours.
- For 2 kW surge loads, confirm inverter rating and cable gauge.
Battery Backup Time Reference Chart
Estimate runtime for popular battery banks using 80-90% depth of discharge and realistic efficiencies.
12V 100Ah (Lead-acid)
| Load (W) | Backup Time (h) | Use Case |
|---|---|---|
| 100 W | ~ 8.1 h | LED lighting + modem |
| 200 W | ~ 4.1 h | Home office gear |
| 400 W | ~ 2.0 h | Workstation + router |
| 800 W | ~ 1.0 h | Sump pump cycle |
24V 200Ah (AGM Bank)
| Load (W) | Backup Time (h) | Scenario |
|---|---|---|
| 300 W | ~ 11.5 h | Lighting + networking |
| 600 W | ~ 5.8 h | Telecom rack |
| 1,200 W | ~ 2.9 h | Server + HVAC fan |
| 2,000 W | ~ 1.7 h | Essential appliances |
48V 150Ah (LiFePO4)
| Load (W) | Backup Time (h) | Scenario |
|---|---|---|
| 500 W | ~ 11.9 h | Critical office |
| 1,000 W | ~ 6.0 h | Mixed residential |
| 2,000 W | ~ 3.0 h | Inverter split-phase |
| 3,000 W | ~ 2.0 h | Short HVAC ride-through |
Times assume 80% DoD & 85% efficiency for lead-acid, 90% DoD & 92% efficiency for lithium. Adjust the variables in the calculator to match your installation.
Battery Backup Time FAQs
Straightforward answers for the most requested backup-runtime scenarios.
How many hours is a 200Ah battery backup?
A single 12V 200Ah lead-acid battery stores about 2.4 kWh. Using 80% DoD and 85% efficiency leaves roughly 1.6 kWh usable, which powers a 300W critical load for about 5.5 hours. Doubling the voltage or adding a second string scales runtime proportionally.
How long will a 100Ah battery last with a 200W load?
100Ah at 12V equals 1.2 kWh. After applying 80% DoD and 85% efficiency, you have ~0.82 kWh usable. Divide by 200W to get just over 4 hours of runtime, assuming the inverter and wiring are sized correctly.
How many hours is a 150Ah battery backup?
The hours depend on voltage and load. A 24V 150Ah bank (3.6 kWh) with 75% usable energy can run a 500W load for about 5.4 hours, while the same bank at 48V delivers roughly 10.8 hours at 500W.
Which is better, a 150Ah or a 200Ah battery?
Higher Ah ratings supply more energy, but "better" depends on discharge rate, size, weight, and budget. Two 150Ah lithium batteries with high DoD can outperform a single 200Ah lead-acid because more of their capacity is usable each cycle.
How much backup does a 100AH battery give?
At 12V you have about 1.2 kWh of stored energy. Expect 600-900 Wh usable depending on chemistry. That equates to 3-9 hours for typical 100-300W loads or roughly 30 minutes if you power a 2 kW appliance.
How long will a 200ah battery last with a 2000W inverter?
A 24V 200Ah bank feeding a 2,000W inverter supplies around 1.7 hours at continuous full load (3.5 kWh usable ÷ 2 kW). Surge events or lower efficiencies will shorten that runtime, so consider parallel strings if you need multiple hours at 2 kW.
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