Ah and Wh
Wh = V × Ah; 1 kWh = 1,000 Wh
Compare Wh, not Ah alone, when battery voltages differ.
Menu
Product lines, applications, and resources.
Enter your device power, hours of use and system voltage to estimate battery capacity or runtime. You can also calculate a LiFePO4 replacement for a lead-acid bank.
Choose capacity, runtime or lead-acid replacement.
If you want to choose a battery, compare your result with the specifications of our listed products.
Automatic matching covers deep-cycle use only. Starting, dual-purpose, regulated and other special uses need an engineering review. Also confirm charging, wiring, current sharing, dimensions, polarity and installation before choosing a battery.
Battery size depends on energy use, usable capacity and losses. Current limits and startup loads need a separate check.
Wh = V × Ah; 1 kWh = 1,000 Wh
Compare Wh, not Ah alone, when battery voltages differ.
Wh = Σ(W × hours × quantity × duty cycle)
A Wh input is already the total energy used during the selected period.
DC Wh ÷ DC efficiency + AC Wh ÷ inverter efficiency + standby Wh
DC devices and AC devices have different supply losses.
Required Wh = battery-side Wh × (1 + reserve) ÷ (DoD × temperature × health)
Less usable capacity means a larger battery is needed for the same load.
Runtime h = planned usable Wh ÷ average battery-side W
Temperature, ageing, duty cycle and conversion losses affect actual runtime.
A ≈ simultaneous battery-side W ÷ minimum working V
If the lowest operating voltage is unknown, the estimate uses 90% of nominal voltage.
Compare surge, inverter and BMS ratings at the same duration
A 5-second peak rating does not confirm that a longer surge is supported.
h = replace Wh ÷ (V × net charge A × efficiency) × completion factor
Nominal defaults: 95% charging efficiency and a 1.10 end-of-charge time factor.
Delivered Wh = nominal Wh × DoD × temperature × health × rate factor
Measured usable energy takes priority. There is no fixed Ah conversion ratio.
Load an example, then change the values for your equipment. These are sample calculations, not product recommendations.
Runtime
A 12.8V 100Ah battery has 1,280Wh of nominal energy. Estimate runtime at 100W DC, allowing for usable capacity, reserve and supply losses.
Capacity
A 12.8V system supplies an 80W DC load for 10 hours and a 600W AC load for 1.5 hours. The AC load has a 2× startup surge; its duration and inverter standby draw still need checking.
Capacity
A 768W DC motor runs for 2 hours on a 25.6V system: 1,536Wh before losses. Allow for usable battery capacity, reserve and DC supply losses.
Replacement
Six 8V 225Ah lead-acid batteries in series form a 48V, 225Ah bank with 10.8kWh of nominal energy. Estimate a 51.2V LiFePO4 replacement for a 2,400W DC load, allowing for ageing, discharge depth, temperature and discharge rate.
Capacity
A 25.6V system supplies a 1,000W AC load for 2 hours, plus 8W of inverter standby draw. Allow for inverter losses and reserve, and confirm startup surge before choosing a battery.
Replacement
A 12V 200Ah lead-acid battery has 2,400Wh of nominal energy. Estimate usable energy under a 600W DC load, allowing for ageing, discharge depth, temperature and discharge rate—not a fixed 2:1 Ah conversion.
Check charging, installation and application requirements before choosing a battery.
Check the charger, wiring, protection, dimensions and installation space before replacing a bank.
Read guide →Compare usable energy, service life, charging and day-to-day use.
Read guide →Check the operating conditions for RVs, boats, golf carts and other applications.
Read guide →Plan the enclosure, BMS, communication, certification and production requirements for an OEM project.
Read guide →Capacity, charging and replacement questions.
Use Wh to compare energy, then convert to Ah at the battery voltage. A 12.8V 100Ah battery and a 25.6V 100Ah battery have the same Ah rating, but the second stores twice the nominal energy.
The Ah rating alone is not enough. Runtime depends on voltage, usable discharge depth, temperature, battery condition, losses, reserve and average load power. Also check whether the battery can supply the continuous and startup current.
Sometimes, but there is no fixed 2:1 rule. Compare the energy the lead-acid bank can actually supply, using measured load and runtime where possible. Otherwise, allow for discharge rate, discharge depth, ageing, temperature and cutoff voltage before sizing LiFePO4.
Only after confirming compatibility with the chosen battery. Check the complete charge profile, maximum voltage and current, float and equalization settings, temperature control and battery/BMS limits; the voltage label alone is not enough.
Continuous current determines whether the battery can keep the equipment running. Peak current is available only for a specified time and must cover the startup surge of motors, pumps or compressors.
Do not assume it can. Use the charging-temperature limits for the exact model and check its cells, BMS and heating controls. Never bypass low-temperature charging protection.
With compatible batteries, series raises voltage while Ah stays the same; parallel raises total Ah. Confirm that the model allows the configuration, then check battery versions, charge levels, BMS, wiring, fuses and current sharing.
Check the nameplate or manual, or measure running power with a suitable meter. For DC, voltage and current can be used; for AC, voltage × current gives VA, so power factor is also needed to estimate watts. Check startup power separately for motors and compressors.
Efficiency, temperature, usable discharge depth, battery condition, power factor and load power are not always fixed. The range reflects those uncertainties, and the result lists the values used. Replace defaults with measured or confirmed values when available.
Engine starting, high-voltage EVs, medical or life-safety loads, fire protection, legally required backup, grid-connected storage and other regulated or high-energy systems need project-specific engineering. Use this calculation only to estimate load and energy needs, not to replace protection design or compliance checks.
Send your requirements for help with battery fit, charging or an OEM project. You can include your calculation with the enquiry.
