Lithium Battery Backup Calculator
Calculate backup time and battery capacity for your power needs
Lithium Battery Backup Calculator
Professional Battery Sizing Tool
Results:
Backup Time: 0.00 hours
Required Capacity: 0.00 Ah
Energy Storage: 0.00 Wh
Current Draw: 0.00 A
How to Use the Calculator
Calculate Backup Time
- Select "Calculate Backup Time"
- Enter your power load in watts
- Input battery capacity in Ah
- Select or enter battery voltage
- Adjust efficiency and DoD settings
Calculate Battery Capacity
- Select "Calculate Battery Capacity"
- Enter your power load in watts
- Input desired backup time in hours
- Select or enter battery voltage
- Adjust efficiency and DoD settings
Key Parameters
- Power Load: Total watts to be powered
- Battery Voltage: System voltage (12V, 24V, 48V)
- Efficiency: Inverter/system efficiency (80-95%)
- DoD: Safe discharge depth (70-90% for lithium)
How to Calculate Lithium Battery Backup
Work through these checkpoints to confirm every manual step the calculator automates.
1. Map the Critical Load
Document the devices that must stay on, capture both their continuous draw and any inrush current, then sum the watts to define your design target.
Watts / Voltage = Current (A) keeps the DC bus loading in view so breakers, cabling, and
the inverter remain in their safe zones.
2. Translate Loads to Energy
Multiply every load by the hours it must run to uncover the true energy demand for the day or outage window.
Energy (Wh) = Load (W) x Hours is the baseline that lets you compare scenario runtimes
without guessing.
3. Determine Usable Storage
Start with the nameplate capacity, then apply the voltage, allowable depth of discharge, and realistic system efficiency.
Usable Wh = Capacity (Ah) x Voltage x DoD x Efficiency shows the energy you can repeatedly
pull without hurting lithium cycle life.
4. Size the Battery Bank
Divide the required energy by the product of voltage, DoD, and efficiency to discover how many amp-hours the bank must supply.
Capacity (Ah) = (Load x Hours) / (Voltage x DoD x Efficiency) converts runtime goals
directly into battery modules.
5. Configure Strings and Modules
Select the module size, determine how many go in series to hit the target voltage, and add parallel strings until the Ah requirement is satisfied.
Parallel Strings = Required Ah / Module Ah keeps the math transparent as you stack rack
batteries or wall units.
6. Validate Runtime and Growth
Run the calculator with the proposed bank, compare runtime against the target window, and include margin for seasonal or future load growth.
Runtime (h) = Usable Wh / Load (W) confirms the design, while
Reserve Margin >= 15% protects against aging and cold-weather losses.
Lithium Battery Backup Chart
Match common lithium stacks with practical loads to estimate runtimes before you finalize the build.
| Battery Setup | Typical Load | Expected Backup | Notes |
|---|---|---|---|
| 12V 100Ah LiFePO4 (1.2 kWh) | 150W (router + LED lighting) | ~ 5.3 - 5.8 h | Single module backup for cabins or telecom repeaters. |
| 24V 200Ah rack pack (5.1 kWh) | 500W (home office + networking) | ~ 8.0 - 8.5 h | Comfortably handles monitors, PCs, and ventilation. |
| 48V 100Ah wall battery (5 kWh) | 1,000W (critical household circuits) | ~ 4.2 - 4.6 h | Ideal for refrigerator, lights, and circulation pumps. |
| 48V 200Ah cabinet (10 kWh) | 2,000W (mini split + lighting) | ~ 4.1 - 4.4 h | Pairs with 5 kVA hybrid inverters for longer outages. |
| 48V 280Ah powerwall (14 kWh) | 3,000W (server rack + sump pumps) | ~ 3.8 - 4.2 h | High-DoD lithium leaves space for brief surges. |
| 51.2V 400Ah floor stack (20 kWh) | 5,000W (whole-home essentials) | ~ 3.0 - 3.3 h | Multiple cabinets balance HVAC, well pumps, and EV loads. |
Assumes 90% DoD and 92% round-trip efficiency — adjust the chart when your inverter or BMS targets different limits.
Lithium Battery Backup FAQs
Answers to the runtime questions that installers and homeowners ask most.
How to calculate lithium ion battery backup time?
Multiply the rated amp-hours by system voltage to get watt-hours, multiply again by allowable depth of discharge and the efficiency of your inverter or DC system, then divide by the connected load in watts.
Runtime (h) = (Capacity x Voltage x DoD x Efficiency) / Load
How long will a 100Ah battery last with 200W load lithium ion?
A 12V 100Ah LiFePO4 stores about 1.2 kWh. Using 90% DoD and 92% efficiency leaves roughly 1.0 kWh usable, so a steady 200W draw runs for just over five hours before the BMS asks for a recharge.
1,000 Wh / 200W ~ 5.0 h
How many kW is a 100Ah lithium battery?
Convert amp-hours to kilowatt-hours by multiplying by voltage and dividing by 1,000. A 48V 100Ah battery equals about 4.8 kWh, while a 12V version equals 1.2 kWh. Divide that stored energy by the discharge time to express it as kilowatts.
kW = (Voltage x 100Ah / 1,000) / Hours of Discharge
How much backup for a 100Ah battery?
The usable backup depends on voltage and load. At 24V you have roughly 2.4 kWh stored; using 85-90% DoD yields about 2.1 kWh, which covers a 300W networking rack for seven hours or a 1,000W tool for just over two hours.
Usable Wh = 24V x 100Ah x 0.9 ~ 2,160 Wh
How long will a 12V fridge run on a 100Ah lithium battery?
Most efficient 12V compressor fridges average 60-70W once cold. A 12V 100Ah lithium battery with 90% DoD provides around 1.0 kWh usable, so expect 14-16 hours of runtime, or longer if the fridge cycles off in cooler ambient conditions.
1,000 Wh / 65W ~ 15.3 h
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