Units
Wh = V × Ah; 1 kWh = 1,000 Wh
Use Wh to compare energy across different battery voltages.
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Free LiFePO4 Battery Calculator
Not sure what battery size you need—or how long a battery will run? Enter your load and target hours to estimate Wh, Ah, continuous current and startup surge.
No sign-up. Calculate first, then compare products only if you want to.
Calculate first. Choose later.
Use this as a sizing guide. Final battery fit, charging, installation and safety still need to be checked.
Step 01
Pick the question closest to yours. You can switch at any time.
When you’re ready
This step is optional. Open it when you want to compare your result with products currently listed on our site.
Automatic product matching is for deep-cycle use only. Starting, dual-purpose, regulated and other special applications need an engineer’s review. Before choosing a battery, also confirm wiring, current sharing, dimensions, polarity, installation and charging.
The numbers behind the result
We start with the energy your loads use, then allow for conversion losses, usable capacity and current limits. We do not use a one-size-fits-all Ah shortcut.
Wh = V × Ah; 1 kWh = 1,000 Wh
Use Wh to compare energy across different battery voltages.
Wh = Σ(W × hours × quantity × duty cycle)
Energy inputs in Wh are already totals for the entered profile period.
DC Wh ÷ DC efficiency + AC Wh ÷ inverter efficiency + standby Wh
Losses are applied to the correct power path for each load.
Required Wh = battery-side Wh × (1 + reserve) ÷ (DoD × temperature × health)
Lower usable factors produce a larger, more conservative range.
Runtime h = planned usable Wh ÷ average battery-side W
Real runtime changes with temperature, ageing, duty cycle and converter performance.
A ≈ simultaneous battery-side W ÷ minimum working V
When minimum voltage is unknown, 90% of nominal is used and disclosed.
Compare surge, inverter and BMS ratings at the same duration
A 5-second catalogue peak cannot be extended to a longer surge without evidence.
h = replace Wh ÷ (V × net charge A × efficiency) × completion factor
The nominal defaults are 95% charge efficiency and a 1.10 completion factor.
Delivered Wh = nominal Wh × DoD × temperature × health × rate factor
Measured delivered energy takes priority; no fixed Ah conversion ratio is used.
Try an example
Choose an example, then replace the values with your own. These are starting points, not product recommendations.
RUNTIME
A 12.8V 100Ah battery stores 1,280Wh on paper, but usable runtime is lower once discharge limits, temperature, battery health, reserve and DC losses are included.
SIZE
An RV may run lights and pumps on DC while other appliances use an inverter. This example combines both so you can see daily energy use and startup current separately.
SIZE
A 768W motor running for two hours uses 1,536Wh before losses. Higher system voltage reduces the Ah and current needed for the same job; reserve and conversion losses are added afterwards.
REPLACEMENT
Six 8V 225Ah batteries in series make a 48V, 225Ah, 10.8kWh nominal bank. Age, discharge depth, temperature and load rate reduce what it can actually deliver. We calculate a 51.2V LiFePO4 target but do not show an unverified 48V product match.
SIZE
See how a 1,000W AC load running for two hours affects battery size at 25.6V. The example also includes inverter loss, standby power, reserve and an unknown startup surge.
REPLACEMENT
There is no reliable “divide by two” rule. This example estimates what a 12V 200Ah lead-acid bank can actually deliver from its load, discharge rate, age, temperature and usable discharge depth.
Related guides
Replacing lead-acid, checking charging or planning an OEM battery? These guides walk through what to do next.
Review charging, wiring, protection, dimensions and installation before replacing a bank.
Read guide →Compare usable energy, cycle behavior, charging and application tradeoffs.
Read guide →Continue from general sizing into application-specific operating conditions and requirements.
Read guide →Prepare enclosure, BMS, communication, certification and production requirements for OEM review.
Read guide →FAQ
Quick answers to questions that often come up when sizing or replacing a battery.
Use Wh to compare total energy, then convert to Ah at the battery’s nominal voltage. The same Ah at different voltages does not represent the same energy.
Runtime depends on voltage, DoD, temperature, battery condition, reserve, conversion losses and the real average load. Continuous and peak current must be checked separately.
Not by a fixed rule. Estimate delivered Wh from measured load/runtime or from capacity, discharge rate, DoD, health, temperature and cutoff behavior, then size lithium for that energy.
Possibly, but only after checking its full charge profile, maximum voltage and current, float/equalization behavior, temperature control and the final battery/BMS limits.
Continuous limits cover sustained operation. Peak limits apply only for a stated duration; a motor or compressor may have a much higher startup surge.
Only if the battery is designed and protected for it. Check the cells, BMS settings and whether the self-heating system has been verified.
Series increases bank voltage while one string’s Ah stays the same; parallel increases total Ah in a simplified model. The exact product must permit the configuration.
Check the nameplate or manual, or measure it with a suitable meter. For DC loads, verified volts and amps can also be used. For AC equipment, volts × amps is VA unless power factor is known.
Efficiency, temperature, DoD, battery condition, power factor and the load itself may vary. A useful range is more honest than a single number that only looks exact.
Not for a final selection. These and other regulated or high-energy applications need an engineer to confirm the battery, protection, documentation and compliance requirements.
Before you rely on the result
Uncertain values stay visible, so you can change them and see how the result moves.
The calculation runs in your browser and asks for no contact details. Data is sent to this site only if you choose to compare products.
Before choosing a product, confirm charging, wiring, dimensions, installation and any safety or compliance requirements.
Need a second opinion?
If space, terminals, charging, low temperature, communication, certification or OEM production matter, send us your result and we’ll help you check the details.