Estimated runtime0 hr
Battery Runtime Calculator
Estimate how long a battery can run a device or load.
Updated May 2026No signup requiredBuilt for mobile
Battery runtime decision result card
What the result means
Battery runtime estimates usable watt-hours divided by load. Inverter losses, battery chemistry, temperature, and safe depth of discharge can change the answer.
What to do next
Check the load in watts, then compare battery size, solar recharge, or generator runtime for longer outages.
When to round
Round runtime down for critical loads and round capacity up when cold weather or inverter losses matter.
Real-world examples
Router backup
Inputs: 500 Wh battery, 35 W load, 85% usable efficiency. Result: about 12 hours. Interpretation: good for internet backup; next step is checking recharge time.
CPAP night
Inputs: 700 Wh battery, 55 W CPAP load, 85% efficiency. Result: about 10.8 hours. Interpretation: one night is realistic; next step is confirming humidifier draw.
How to use this calculator
- Enter the real numbers for your situation.
- Review the main result and any secondary results.
- Adjust inputs to compare practical scenarios.
- Use the estimate as a planning number before making a purchase or decision.
Formula
Result = inputs applied to the calculator formula
Use the visible result box for the final estimate
Add a reserve when real-world conditions can vary
This calculator uses practical estimating math. The exact formula depends on the inputs shown in the calculator card.
Worked examples
Basic example
Enter the default values, then replace them with your real measurements or usage numbers to get a realistic estimate.
Planning example
Increase the main quantity or add a buffer to see how the result changes before buying supplies or committing to a plan.
Practical planning tips
- Double-check units before trusting the result.
- Round up when running short would stop the job or trip.
- Use manufacturer labels, local rules, or professional guidance when safety matters.
- Save or print the result before going to the store or job site.
Safety and disclaimer note
This is a practical planning estimate. Verify safety, code, health, financial, or equipment decisions with the right professional or official source.
FAQ
Battery Runtime Calculator questions
How accurate is the Battery Runtime Calculator?
It is a planning estimate based on the numbers you enter. Real-world results can vary.
Should I add a buffer?
Yes when waste, weather, terrain, safety, or product variation could change the result.
Can I use the result for buying supplies?
Use it as a starting point, then confirm supplier packaging, labels, and local requirements.
Why do results vary?
Measurements, product specs, user habits, and field conditions can all change the final outcome.
Does this replace professional advice?
No. Use professional guidance for safety, health, code, legal, or financial decisions.
What number should I use?
Use measured inputs first
Start with the actual number from your project, device, network, trip, or equipment label instead of a best guess.
Round in the safer direction
Round up for materials, food, water, storage, and capacity. Round down for runtime when running short would cause trouble.
Check related tools
Use the related calculators on this page to plan the next part of the job instead of treating one result as the whole answer.