Estimated Runtime
10.37 hours at the entered steady load.
Battery Runtime Calculator
The Battery Runtime Calculator estimates how long a battery can power a steady load using battery capacity, nominal voltage, planned depth of discharge, and system efficiency. Use it for LiFePO4, lithium-ion, AGM, lead-acid, solar storage, RV, marine, backup, and portable power planning.
Start with the ratings on your battery and the average watts of the device or inverter load. The calculator shows nominal energy, usable energy, delivered energy, approximate battery-side current, and estimated runtime so you can see exactly where each assumption affects the answer.
Calculate Battery Runtime in Minutes
Enter the battery bank rating and your average load. The calculator converts amp-hours to watt-hours, applies your usable discharge percentage and efficiency, then divides the delivered energy by load watts.
Battery Runtime Estimator
Use nominal battery voltage and average running watts. Startup surge and equipment current limits must be checked separately.
How to read your Battery Runtime Calculator results
The runtime number is the main answer, but the intermediate results are just as important. Nominal energy tells you what the battery stores on paper at its rated Ah and nominal voltage. Delivered energy shows what remains after the usable-discharge and efficiency assumptions. If the gap between those figures looks unexpectedly large, review those percentages before changing the battery size. The Battery Runtime Calculator is most useful when each input has a clear reason behind it.
The current estimate is another useful sense check in the Battery Runtime Calculator. It approximates the battery-side current needed to support the entered load after the efficiency allowance. A 1,000W AC load on a 12.8V battery can require far more current than the same power on a 51.2V battery. That matters for BMS limits, voltage drop, cable size, connectors, and fusing even though those items are not sized by this tool. If the current looks high, review the complete electrical path before assuming the runtime result is usable in practice.
Use average load power rather than the highest number printed on an appliance. A 1,500W microwave may run for only a few minutes, while a refrigerator may cycle between near-zero and a few hundred watts. For a mixed system, add the energy used by each device over a representative period and convert that back to an average watt figure. This gives the Battery Runtime Calculator a better input than simply adding every nameplate wattage as though all loads ran continuously.
It is also helpful to test more than one scenario. Run a normal case with your expected load, then a conservative case with slightly higher watts, lower usable capacity, or lower efficiency. If both cases still meet your required operating time, your plan has more resilience. If a small change causes the runtime to fall below your target, consider more battery capacity, a lower load, or a different system voltage and verify those options with the related sizing tools linked throughout this page.
Nominal Energy
1.28 kWh
Battery Ah multiplied by nominal voltage.
Usable Battery Energy
1,152 Wh
Nominal energy after the planned usable DoD percentage.
Delivered Energy
1,036.8 Wh
Usable energy after the entered efficiency factor.
Approx. Battery Current
8.68 A
A steady-load estimate; not a conductor or fuse sizing result.
How the Battery Runtime Calculator Works
Battery runtime is energy divided by power. The important part is converting the battery rating into energy and then reducing that energy by realistic operating assumptions before dividing by the load.
The first step is converting amp-hours into watt-hours. Amp-hours alone do not tell you how much energy a battery stores because voltage matters. A 100Ah battery at 12.8V contains 1,280Wh nominally, while a 100Ah battery at 25.6V contains 2,560Wh. If you mainly need unit conversion, the Battery Capacity Calculator and Watt Hour Calculator are useful companion tools.
Nominal Battery Energy (Wh) = Battery Capacity (Ah) × Nominal Voltage (V)
Next, the Battery Runtime Calculator applies the depth-of-discharge percentage you choose. This is a planning reserve, not a universal battery recommendation. A 90% usable setting on a 1,280Wh battery leaves 1,152Wh in the model. For a deeper explanation of this input, use the Battery Depth of Discharge Calculator.
Usable Battery Energy (Wh) = Nominal Wh × (Usable DoD ÷ 100)
System efficiency then accounts for conversion losses between the battery and the load. An AC appliance running through an inverter typically needs an efficiency allowance, while a direct DC load may have fewer conversion stages. The calculator lets you enter the percentage instead of assuming one value fits every inverter and operating point.
Delivered Energy (Wh) = Usable Wh × (Efficiency ÷ 100)
Estimated Runtime (hours) = Delivered Energy (Wh) ÷ Average Load (W)
For the default example, 100Ah × 12.8V = 1,280Wh. At 90% usable depth of discharge, the model has 1,152Wh. At 90% efficiency, 1,036.8Wh is delivered to the load. Dividing by 100W gives 10.368 hours, or about 10 hours and 22 minutes.
Battery Ah and Voltage
Use the capacity and nominal voltage for the complete bank, not a single cell unless the load is connected to that cell. Series and parallel arrangements change bank voltage and capacity differently. If you are building a bank, verify the final configuration with the Series / Parallel Battery Calculator.
LiFePO4 sizing guideAverage Load Watts
Use average running power over the period you care about. A refrigerator, pump, laptop charger, or compressor may cycle on and off, so nameplate watts can overstate or understate the true average. A plug-in power meter or battery monitor can produce a better input than guessing.
Check battery C-rateUsable Percentage and Efficiency
Keep reserve and efficiency separate. Depth of discharge represents how much stored energy you plan to use; efficiency represents losses while delivering that energy. Combining them into one guess can make troubleshooting harder when real runtime differs from the estimate.
Plan recharge timeBattery Runtime Examples and Comparisons
These Battery Runtime Calculator worked examples use fixed loads so the arithmetic is easy to follow. Actual runtime can be different, but the examples show how voltage, capacity, load, reserve, and efficiency interact.
| Battery | Load | Nominal Energy | Delivered Energy | Estimated Runtime |
|---|---|---|---|---|
| 12.8V 100Ah | 100W | 1,280Wh | 1,036.8Wh | 10.37 hours |
| 12.8V 200Ah | 500W | 2,560Wh | 2,073.6Wh | 4.15 hours |
| 25.6V 100Ah | 300W | 2,560Wh | 2,073.6Wh | 6.91 hours |
| 51.2V 100Ah, 92% efficiency | 1,000W | 5,120Wh | 4,239.4Wh | 4.24 hours |
Why watt-hours make runtime comparisons clearer
Watt-hours put batteries with different voltages on the same energy scale. This prevents a common comparison error: assuming two batteries with the same Ah rating store the same energy. The Battery Runtime Calculator converts to Wh first for exactly this reason. Once energy is in Wh, dividing by watts gives a direct time estimate under the chosen assumptions.
When comparing a proposed battery upgrade, keep the load and assumptions identical and change only the battery voltage or capacity. That isolates the effect of the battery itself. If you also change the load, DoD, and efficiency at the same time, it becomes harder to understand why the runtime moved.
RV Overnight Load
Suppose an RV battery bank is 12.8V and 200Ah, while lights, fans, electronics, and intermittent inverter loads average 180W. At 90% DoD and 90% efficiency, delivered energy is about 2,073.6Wh, giving roughly 11.52 hours. An RV load profile often varies, so use the RV Lithium Battery Calculator when you want application-specific planning.
Solar Backup Load
A 25.6V 100Ah bank stores 2,560Wh nominally. Under the same 90% DoD and 90% efficiency assumptions, it delivers about 2,073.6Wh. A continuous 300W backup load would run for about 6.91 hours before the modeled reserve is reached. For days of autonomy and array planning, continue with the Solar Battery Calculator.
Marine House Battery
Boat loads can be highly mixed: navigation electronics may be steady while pumps and refrigeration cycle. Estimate the average watts over a realistic period, then test a conservative scenario as well. The Marine Lithium Battery Calculator can help organize a boat-specific battery plan around those loads.
What Changes Real Battery Runtime?
The Battery Runtime Calculator is intentionally transparent, but real systems are dynamic. The closer your inputs are to measured conditions, the more useful the estimate becomes.
Temperature and Battery Condition
Available battery capacity can change with temperature, age, state of charge, and discharge rate. Cold conditions can reduce usable capacity, while an older battery may no longer deliver its original rated energy. Do not treat the nameplate capacity as guaranteed under every condition.
For one manufacturer example, Victron publishes model-specific capacity, discharge-current, temperature, and cycle-life data in its Lithium Battery Smart technical data. Use the documentation for your own battery rather than transferring another product's limits.
Inverter and Standby Losses
An inverter's efficiency is not always fixed. It can change with load, voltage, and operating mode, and the inverter can consume power even when the connected appliance is small or off. If standby draw matters over many hours, add that consumption to your load estimate instead of relying on a single efficiency percentage.
If your battery system is still being designed, the Wire Gauge Calculator can help with a separate voltage-drop discussion. Runtime math alone does not determine cable size or protection.
Variable and Cycling Loads
A constant 500W heater is simple to model. A refrigerator, water pump, air conditioner, or compressor is not. Those devices can switch between idle, normal operation, and startup. For runtime, use average energy over time; for equipment selection, separately verify startup surge and continuous current limits.
When comparing systems at specific voltages, the 12V Lithium Battery Calculator, 24V Lithium Battery Calculator, and 48V Lithium Battery Calculator provide focused checks.
BMS Cutoff and Discharge Limits
A battery management system can disconnect loads because of low cell voltage, excessive current, or temperature even when a simple energy calculation suggests some capacity remains. High current can also create more voltage sag and heat, making high-load runtime less predictable than a low-load calculation.
Manufacturer operating instructions should govern these limits. Victron's Lithium Battery Smart operation guidance is one example of why discharge current, depth of discharge, and temperature should be checked separately from runtime arithmetic.
Limitations, Safety, and Accuracy Notes
A runtime estimate is useful for planning and comparison, but it does not certify that a battery, inverter, cable, fuse, connector, or installation is suitable for the load.
The Battery Runtime Calculator assumes the entered average load remains constant and that the entered depth-of-discharge and efficiency percentages are reasonable for the system. It does not model detailed battery voltage curves, Peukert behavior, cell imbalance, thermal derating, aging, transient loads, inverter low-voltage shutdown, BMS protection thresholds, or changing conversion efficiency. Those factors can make measured runtime shorter or occasionally longer than the simplified result.
For critical applications, leave an energy reserve and verify performance under the expected temperature and load. Medical devices, emergency communications, navigation equipment, safety systems, and essential home backup deserve redundancy and product-specific engineering. If the battery is part of a larger system, use the Lithium Battery Calculator for broader sizing and the Lithium Battery Charge Time Calculator to estimate recovery time after discharge.
Do Not Size Wires from Runtime
Average current is only one piece of conductor sizing. Cable length, insulation temperature rating, allowable voltage drop, installation method, fault current, ambient temperature, and protection rules also matter. Use a dedicated wiring calculation and applicable standards.
Cold Weather Can Reduce Capacity
If a battery is used in cold conditions, its available energy may be lower than its room-temperature rating. Entering the full nameplate Ah can therefore overstate runtime. Check the exact battery datasheet and build reserve into cold-weather planning.
Measure Before You Oversize
Runtime improves when your load estimate is accurate. Measure energy use over a representative period where possible. This can prevent buying unnecessary battery capacity or, more importantly, choosing a bank that is too small for real use.
Accuracy note: Results are educational estimates. Always verify battery, BMS, inverter, charger, conductor, fuse, and installation specifications before relying on a runtime figure. If you are uncertain about high-current wiring or protection, consult a qualified electrical professional.
Related Battery Tools
Use these published tools together with runtime calculations to check energy capacity, charging, system voltage, and battery-bank configuration from different angles.
Battery Capacity Calculator
Convert between Ah, Wh, and kWh so batteries at different voltages can be compared on the same energy basis.
Use CalculatorBattery Charge Time Calculator
Estimate how long it may take to replace discharged energy after you know your battery capacity and charger output.
Use CalculatorSolar Battery Bank Calculator
Move from single-load runtime to daily energy and battery-bank planning for solar and off-grid systems.
Use CalculatorBattery Runtime Calculator FAQs
These answers cover the most common questions about battery runtime math, Ah versus Wh, inverter losses, battery voltage, and why real-world results vary.
Convert the battery to watt-hours by multiplying Ah × nominal voltage. Multiply by the usable depth-of-discharge percentage, then multiply by system efficiency. Divide the resulting delivered watt-hours by the average load watts. The Battery Runtime Calculator performs those steps and shows the intermediate energy values.
Voltage and losses matter. With the default 12.8V example, 100Ah equals 1,280Wh nominally. At 90% usable depth of discharge and 90% efficiency, about 1,036.8Wh reaches the load. A 100W steady load is therefore estimated at about 10.37 hours.
Amp-hours do not include voltage. A 100Ah 12.8V battery stores about 1,280Wh, while a 100Ah 25.6V battery stores about 2,560Wh. Battery chemistry, usable discharge, temperature, condition, and system losses can further change how much of that energy reaches the load.
Use the average watts actually demanded by the load, then account for inverter losses with the efficiency input. Do not enter the inverter's maximum rated wattage unless the load continuously draws that much. Include meaningful inverter standby consumption in the load when it runs for long periods.
Not automatically. Runtime depends on total energy in watt-hours, not voltage alone. A higher-voltage battery with the same Ah stores more energy, but a lower-voltage bank with much more Ah can store more overall. Compare Wh before deciding which system has longer runtime.
Yes for the basic energy arithmetic, because Ah × volts = Wh applies across battery types. The usable depth of discharge, efficiency, voltage behavior, current capability, and temperature response can differ substantially, so use settings and limits appropriate to the exact battery you own.
Common reasons include a higher real load, inverter idle draw, cold temperature, an older battery, less than full starting charge, voltage sag, wiring loss, conservative BMS cutoff, or a duty cycle that was estimated too low. Re-measure the load and compare actual battery conditions with the datasheet.
No. It provides an approximate steady battery-current figure to help you understand scale, but equipment and protection sizing require surge current, cable length, voltage drop, terminal ratings, fault protection, installation environment, and product-specific limits. Treat those as separate design checks.
Ready to Test Another Battery Scenario?
Run the Battery Runtime Calculator with your actual battery label, measured load, planned reserve, and realistic efficiency. Then compare the result with capacity, charging, wiring, and application-specific tools before finalizing your system.
Calculate Another Runtime