Calculation details
Equipment estimate only. Verify voltage, load profile, losses, duty cycle, manufacturer specifications, and electrical safety requirements before relying on the result.
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Use this result
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What does this calculator estimate?
A 100 amp-hour 12-volt battery stores 1,200 watt-hours; at 85 percent usable depth and 90 percent efficiency feeding a 100-watt load, runtime is about 9.2 hours. Runtime = amp-hours x volts x usable fraction x efficiency / load watts.
- Nominal energy in watt-hours = amp-hours x volts.
- Lead-acid batteries should not be discharged below about 50 percent (Battery Council).
- Real runtime drops as load rises because of Peukert-type losses.
How runtime is estimated
Battery capacity in watt-hours is the total energy stored; dividing by the load's wattage gives the ideal runtime. Amp-hour ratings need the voltage to convert to watt-hours — a 100 Ah 12 V battery stores 1,200 Wh.
Limitations to watch for
The estimate is ideal: inverters, converters, and battery chemistry lose 10–25% in practice. Batteries shouldn't be fully drained (lithium stops ~20% for longevity). Loads vary — a fridge cycles, it doesn't draw continuously.
How to use it in practice
Check the battery's Wh (or Ah × V) and the device's wattage (printed on the label or plug). Divide, then discount by 20–30% for real-world use. Use it to size power banks, solar batteries, and backup systems.
['Enter the battery capacity in Wh (or Ah with voltage).', "Enter the device's power draw in watts.", 'The tool computes ideal runtime.']
How runtime is calculated
Runtime = battery capacity ÷ load. A 100 Ah battery at 12 V holds 1,200 Wh; a 240 W load runs 5 hours. The calculator converts capacity, voltage, and load into hours with the efficiency loss included.
Capacity units matter
Batteries are rated in amp-hours (Ah) at a voltage; energy is watt-hours (Wh) = Ah × V. A 100 Ah 12 V battery holds 1,200 Wh; a 100 Ah 48 V battery holds 4,800 Wh. Comparing capacity across voltages requires the Wh conversion.
The 50 percent rule for lead-acid
Lead-acid batteries should not be discharged below about 50 percent — a 100 Ah lead-acid battery gives about 50 Ah of usable capacity. Lithium batteries discharge to 80–90 percent. The calculator applies the depth-of-discharge limit for the battery type.
Efficiency losses are real
Inverters lose 5–15 percent converting DC to AC, and wiring and temperature add losses. A 90 percent efficient system extends runtime less than the raw math suggests. The calculator includes the efficiency factor.
A worked example
A 12 V 100 Ah lithium battery (90% usable) with a 300 W load at 90% inverter efficiency: usable energy 1,080 Wh, effective draw 333 W → about 3.2 hours. The calculator returns the honest runtime.
How this calculator works
Formula
Runtime (hours) = battery capacity (Wh) ÷ load (watts). For amp-hour batteries: Wh = Ah × voltage.
Worked example
A 100 Wh battery powering a 20 W device: 100 ÷ 20 = 5 hours of runtime.
Assumptions to verify
- The load draws constant wattage.
- Conversion losses are ignored (discount the result manually).
- The battery is fully charged.
Frequently asked questions
How do I calculate battery runtime?
Divide watt-hours by the load's watts: 100 Wh ÷ 20 W = 5 hours.
What is a watt-hour?
A unit of energy: 1 Wh powers a 1 W device for 1 hour. Battery capacity is often rated in Wh (or Ah × voltage).
How do I convert amp-hours to watt-hours?
Multiply by voltage: 100 Ah × 12 V = 1,200 Wh.
Why is real runtime shorter?
Inverter/converter losses, battery chemistry, temperature, and not draining to zero all cut real runtime — often 20–30%.
Can I drain a battery to zero?
Not safely for most chemistries — lithium batteries stop at ~20% to protect lifespan; lead-acid shouldn't go below ~50%.
Does a fridge draw continuously?
No — compressors cycle. Use the average running watts, not the peak, for runtime estimates.
What size battery do I need?
Multiply your load's watts by the hours needed, then add 25%+ for losses and depth-of-discharge limits.
How long will a battery last?
Capacity (Wh) ÷ load (W) × efficiency × usable depth.
What is the difference between Ah and Wh?
Wh = Ah × voltage; compare energy, not amp-hours, across voltages.
Can I discharge a battery fully?
Lead-acid: no, keep to ~50%. Lithium: 80–90% depth is typical.
Do inverters waste power?
Yes — 5–15% conversion loss, which shortens runtime.
Cite this tool
BoringToolsKit. “Battery Runtime Calculator.” boringtoolskit.com/battery-runtime-calculator/ (reviewed 2026-08-25). Free to reference in articles, syllabi, and answer posts with a link.
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