Battery Runtime Calculator
How long will your battery run a given load?
Battery Runtime Calculator
Battery runtime answers a practical question: how long will a given battery run one appliance—or a steady load—before it is depleted to a safe depth of discharge? The inputs are battery amp-hours and voltage, appliance wattage, chemistry, and inverter efficiency.
Nameplate amp-hours alone are misleading. A 100 Ah battery does not deliver 100 Ah at useful voltage for every chemistry: lithium (LiFePO₄) typically allows deeper discharge than lead-acid, and an inverter converts DC to AC with losses. This calculator estimates usable watt-hours and runtime hours so you can plan backup power, camping setups, and off-grid loads.
Use the result to compare batteries, size a bank for overnight loads, or sanity-check claims like “how long will a 100 Ah battery run a fridge?” Pair it with the battery capacity and solar battery sizing tools when you are designing a full system rather than testing one appliance.
How this calculator works
- Enter battery amp-hours (Ah) and nominal voltage (V). The tool multiplies them to get nameplate watt-hours (Wh).
- Enter appliance watts—the continuous draw of the load you want to run through the inverter.
- Select chemistry: lithium uses ~90% depth of discharge in this model; lead-acid uses ~50% for typical cycle-life planning.
- Set inverter efficiency (default 90%). Lower efficiency reduces usable energy delivered to AC loads.
- Usable Wh = battery Wh × DoD × inverter efficiency. Runtime hours = usable Wh ÷ appliance watts.
Core runtime relationships
Battery Wh = Ah × V | Usable Wh = Battery Wh × DoD × inverter η | Runtime h = Usable Wh ÷ load W
Amp-hours must be multiplied by voltage to compare energy across 12 V, 24 V, and 48 V banks. Depth of discharge caps how much of that nameplate energy you can safely use—using more than recommended DoD on lead-acid shortens life. Inverter efficiency accounts for DC-to-AC conversion losses before dividing by appliance watts to get hours of run time.
Formulas used by this tool
- Usable Wh = battery Wh × DoD × inverter efficiency.
- Runtime hours = usable Wh ÷ appliance watts.
- DoD depends on chemistry (LiFePO₄ vs lead-acid).
Worked example: 100 Ah at 12 V
A 100 Ah, 12 V lithium battery runs a 100 W load through a 90% efficient inverter.
- Battery Wh = 100 Ah × 12 V = 1,200 Wh nameplate.
- Usable Wh = 1,200 × 0.9 DoD × 0.9 inverter efficiency ≈ 972 Wh.
- Runtime = 972 Wh ÷ 100 W ≈ 9.7 hours of continuous operation.
- Switch to lead-acid at 50% DoD: usable Wh ≈ 540 Wh → runtime ≈ 5.4 hours for the same load.
Practical tips
- Use measured appliance watts—a fridge cycles on and off, so average draw differs from compressor surge.
- Inverter idle draw reduces runtime; small loads on large inverters waste energy overnight.
- Do not plan to hit 100% DoD on lead-acid; voltage sag and sulfation accelerate below ~50%.
- For motor loads (pumps, tools), account for surge current separately—inverter must start the load, not just run it.
- Temperature affects capacity; cold batteries deliver less usable energy than lab-rated specs.
Frequently asked questions
How do I convert Ah to Wh?
Multiply amp-hours by nominal voltage: Wh = Ah × V. A 100 Ah, 12 V battery is 1,200 Wh nameplate. The same Ah at 24 V doubles the energy. Runtime and sizing comparisons should always use watt-hours, not Ah alone.
What depth of discharge does this calculator use?
Lithium is modeled at 90% DoD; lead-acid at 50%. These are planning defaults for LiFePO₄ and typical flooded/AGM cycle life—not every manufacturer’s warranty limit. Check your battery datasheet and BMS settings before routinely cycling deeper.
Why include inverter efficiency?
Most household appliances run on AC through an inverter. Conversion from battery DC to AC loses energy—often 5–15%. The efficiency slider adjusts usable watt-hours delivered to the appliance. DC loads wired directly to the battery can skip or set efficiency to 1.0.
Does runtime include surge power for motors?
No. The model assumes steady watts. Compressors, pumps, and power tools draw several times rated watts for a few seconds at start. Your inverter must handle surge; runtime only estimates duration once the load is running steadily.
How long will a 100 Ah battery last?
It depends on voltage, chemistry, inverter losses, and load watts. At 12 V lithium with a 100 W load, this model gives roughly 9–10 hours. At 200 W, about half that. Enter your exact values rather than using rules of thumb alone.
Can I run multiple appliances?
Add their simultaneous watt draws and enter the total as appliance watts, or run the calculator once per appliance to see individual runtimes. If loads do not run at the same time, plan for the highest combined load you expect at once.
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