Battery Capacity Calculator
Estimate required amp-hours and watt-hours from target runtime, load, voltage, efficiency, and discharge assumptions.
Capacity Planning Inputs
Required Capacity
⚠️ Engineering Caution:
This tool is intended for screening and pre-check workflows. Results are usually directionally useful, but they
can still shift with equipment selection, environmental conditions, naming conventions, revision status, or
interpretation rules. Confirm any value that affects ordering, substitution, compliance, or installation before
acting on it.
Frequently Asked Questions
What is the basic battery sizing formula?
A practical first estimate starts with load watts multiplied by required runtime to get watt-hours, then adjusts that energy for inverter efficiency and usable depth of discharge before converting to amp-hours at the selected system voltage.
Why does a higher system voltage reduce required amp-hours?
For the same watt-hour requirement, a higher voltage battery bank needs fewer amp-hours because watt-hours are the product of volts and amp-hours.
What This Calculator Is For
When planning a battery-backed system, users often know the load and the runtime they want — but not the battery size required to achieve it. This calculator reverses the runtime question and estimates how much battery capacity is needed.
Typical practical questions include:
- How many amp-hours are required for this load and runtime?
- What battery watt-hour capacity should I plan for?
- How much bigger does the battery need to be after efficiency losses?
- How does usable depth of discharge affect required capacity?
- What changes if the system runs at 12V, 24V, or 48V?
This calculator is designed for those early planning and sizing checks.
What It Calculates
The tool estimates:
- required load energy (
Wh) - required battery-side energy after efficiency and discharge assumptions
- required battery capacity in amp-hours (
Ah) - a simple recommended nominal battery energy target
It is useful for UPS planning, control panels, mobile systems, telecom backup, and general DC battery sizing discussions.
Core Relationships
Load Energy (Wh) = Load W × Runtime HoursRequired Battery Wh = Load Energy ÷ (usable depth × efficiency)Required Ah = Required Battery Wh ÷ System Voltage
These are planning formulas and do not replace manufacturer discharge curves or detailed battery engineering.
Practical Use Cases
This kind of calculator is useful for:
- sizing a backup battery for a known DC or inverter load
- comparing battery bank voltage options
- estimating the effect of conservative discharge limits
- generating quick engineering or procurement notes
- checking whether an existing battery bank is likely undersized
Important Limitations
This tool is intended for first-pass planning, not final design validation. Real required capacity can be affected by:
- battery chemistry and discharge curve behavior
- low-temperature performance
- inverter losses that vary with load
- startup surges
- battery aging and reserve margin requirements
- system-specific cutoff settings
For critical systems, always confirm sizing with vendor data, project requirements, and field-specific engineering review.
FAQ
Why does required battery capacity go up when depth of discharge is limited?
If only part of the nominal battery energy is considered usable, the total installed battery capacity must be larger to deliver the same required load energy.
Why is system voltage important?
For the same watt-hour requirement, a higher system voltage means fewer amp-hours are needed. That changes battery-bank configuration and current levels.
Should I add extra margin on top of this result?
Usually yes. Many real systems need margin for aging, temperature, surge loads, or future expansion.