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Battery Runtime Calculator
Estimate usable Wh and backup hours from Ah or Wh, voltage, battery count, depth of discharge, efficiency and load.
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Formula
nominal_wh = capacity converted to Wh * battery_count usable_wh = nominal_wh * DoD * efficiency runtime_hours = usable_wh / load_w
Assumptions
- Inputs are user-provided.
- Results are preliminary estimates.
- Load is constant and battery capacity is accurate.
Limitations
- Does not model age, temperature, BMS cutoff, voltage sag, surge loads, or Peukert effect.
Sources / methodology
Converts battery capacity to usable Wh and divides by load.
- Formula version
- 1.0.6
- Formula review date
- 2026-05-25
Source links
Quick answer: This battery runtime calculator estimates backup hours after converting Ah or Wh capacity into usable energy. It keeps voltage, battery count, depth of discharge, efficiency and load visible so 12 V, 24 V and 48 V runtime estimates stay transparent.
Input parameters
- Battery capacity (Ah/Wh/kWh): 100 Ah/Wh/kWh
- Capacity unit: Ah
- Battery voltage (V): 12 V
- Battery count (pcs): 1 pcs
- Depth of discharge (%): 80 %
- System efficiency (%): 90 %
- Load (W): 100 W
Output values
- Nominal energy (Wh)
- Usable energy (Wh)
- Runtime (hours)
- Runtime (minutes)
Step-by-step example
- Enter values into the calculator fields.
- Run the calculation and review the result values.
- Compare the result with the assumptions, limitations and sources below.
How long will a battery last?
Battery runtime depends on usable energy, not only the label capacity. After converting Wh or Ah into usable Wh, divide by the load power to estimate hours, then keep battery age, temperature and cutoff behavior as separate limitations.
Battery runtime calculator in Wh and Ah
If the label is already in Wh, the page can work directly from that value. If the label is in Ah, voltage is still required because Ah alone is not energy; the calculator first converts Ah x V into Wh.
Battery runtime formula
The simplified method is nominal_Wh = capacity converted to Wh x battery_count, usable_Wh = nominal_Wh x depth_of_discharge x efficiency, and runtime_hours = usable_Wh / load_W. This keeps the estimate transparent instead of hiding the main assumptions.
Worked example: 12 V 100 Ah battery
A 12 V 100 Ah battery has 1200 Wh of nominal energy. If depth of discharge is 80% and system efficiency is 90%, the usable energy is about 864 Wh. A 200 W load would therefore run for about 4.3 hours in the simplified model.
Comparing 12 V, 24 V and 48 V battery setups
Voltage does not create extra energy by itself, but it changes how Ah translates into Wh. That is why battery systems should be compared in usable Wh first and only then in runtime hours.
| Battery setup | Nominal energy | Usable energy at 80% DoD and 90% efficiency | Runtime at 200 W |
|---|---|---|---|
| 12 V x 100 Ah | 1200 Wh | 864 Wh | 4.3 h |
| 24 V x 100 Ah | 2400 Wh | 1728 Wh | 8.6 h |
| 48 V x 50 Ah | 2400 Wh | 1728 Wh | 8.6 h |
Why manufacturer runtime can still differ
Real batteries and UPS packs do not always deliver nameplate energy at every discharge rate. Age, temperature, voltage sag, BMS cutoff, inverter conversion loss and manufacturer test conditions can all move the real runtime away from the simplified estimate.
Battery runtime vs UPS runtime
Use the general battery runtime page when you know battery capacity and load. Use the UPS Runtime Calculator when VA, power factor, UPS limits and manufacturer runtime behavior matter.
Worked examples
12 V 100 Ah battery example
A 12 V 100 Ah battery starts with 1200 Wh of nominal energy. At 80% depth of discharge and 90% efficiency, the usable energy is about 864 Wh, so a 200 W load would run for about 4.3 hours.
48 V battery bank example
A higher-voltage battery bank can carry the same energy with a different Ah rating, which is why Ah alone is not enough. Always compare systems in usable Wh before comparing runtime.
Runtime caution example
Use the result as a planning estimate, then check battery age, discharge rate, inverter losses and manufacturer runtime data before relying on the number for backup or off-grid decisions.
24 V or 48 V system comparison example
Two battery banks can show very different Ah ratings while storing similar energy. Convert both systems to usable Wh first, then compare runtime, instead of assuming that the larger Ah number always means longer backup.
Common mistakes
- Keep all units consistent with the field labels.
- Check whether the input values are measured, nameplate values or planning assumptions.
- Review the stated assumptions and limitations before using the estimate.
What to check next
Check the inputs, limitations, sources and related calculators before using the estimate in a real decision.
FAQ
How long will a 12 V 100 Ah battery run a 200 W load?
A simplified estimate gives about 4.3 hours if you assume 80% depth of discharge and 90% system efficiency. Without those reductions, the nameplate energy would look higher than the usable energy.
Why do I need voltage when capacity is in Ah?
Because Ah alone is not energy. Voltage is needed to convert Ah into Wh so different battery systems can be compared on the same energy basis.
Should I include depth of discharge and efficiency?
Yes. Depth of discharge reduces usable capacity, and efficiency accounts for losses in the battery and the surrounding DC or inverter system.
Does higher system voltage automatically mean longer runtime?
Not by itself. Higher voltage changes how Ah converts to Wh, but runtime still depends on total usable energy and the actual load.
Can Ah alone compare two different battery banks?
No. Ah must be combined with voltage so both systems are compared on the same Wh basis before you compare runtime.
Can this replace manufacturer runtime data?
No. The page is a transparent planning estimate and does not replace manufacturer curves, battery aging data, cutoff behavior or application-specific testing.
Related calculators
This calculator provides an estimate for informational purposes only. It is not a certified engineering design, electrical safety approval, or professional installation recommendation. Always verify final decisions with a qualified professional and applicable local codes.