Battery Backup Calculator
Estimate backup time from battery bank voltage, amp-hours, usable depth of discharge and efficiency.
Interactive Electrical Calculator
Battery Backup Calculator
Estimate backup time from battery bank voltage, amp-hours, usable depth of discharge and efficiency.
Result
Enter values and calculate.
About the Battery Backup Calculator
The Battery Backup Calculator is a free, browser-based tool for battery capacity, runtime, charging time and series/parallel bank sizing. Estimate backup time from battery bank voltage, amp-hours, usable depth of discharge and efficiency. Charging time depends on charger current, battery capacity and charge efficiency together, not on any one of those numbers alone.
Solar and off-grid system builders, RV and marine owners, backup-power installers, and hobbyists working with lithium or lead-acid packs typically reach for a tool like this to double-check a number before it goes into a real installation.
How the calculation works
Behind the scenes, the calculation follows:
Runtime = V × Ah × DoD × efficiency / load.
Where each input maps to a field in the calculator above:
- Bank Voltage in V.
- Bank Capacity in Ah.
- Usable Depth of Discharge in %.
- System Efficiency in %.
- Load in W.
Every one of these values can be edited above, and the calculator updates the result the moment you press Calculate.
A worked example
For example, start with 48 V for bank voltage, 200 Ah for bank capacity, 80 % for usable depth of discharge, 90 % for system efficiency and 1000 W for load. Those defaults are only a starting point — replace them with your own measured or nameplate values, then press Calculate to get a result specific to your situation.
When to use the Battery Backup Calculator
Typical reasons people reach for the Battery Backup Calculator:
- Sizing a battery bank for a specific backup runtime target.
- Working out how long a charger will take to top up a given battery.
- Converting a battery spec sheet between Ah, Wh and mAh so it can be compared with another product.
- Planning a series/parallel wiring configuration to hit a target system voltage and capacity.
Good practice: Depth of discharge matters: a lead-acid battery rated for 100 Ah cannot safely deliver anywhere near 100 Ah of usable capacity, unlike most lithium chemistries.
Also worth knowing: Wiring batteries in series adds their voltages while capacity (Ah) stays the same as a single cell; wiring in parallel adds capacity while voltage stays the same.
Frequently asked questions
What does the Battery Backup Calculator calculate?
Estimate backup time from battery bank voltage, amp-hours, usable depth of discharge and efficiency.
How accurate is this calculator?
The formula above is applied exactly to the values you enter, so the math itself is precise. Real-world results can still differ due to component tolerances, temperature, installation method, equipment efficiency and local code requirements, so treat the output as a strong planning estimate and verify final designs against manufacturer data and the electrical code that applies to your installation.
Is this calculator free to use?
Yes. The Battery Backup Calculator is completely free, with no account, download or installation required.
Why does this calculation matter in practice?
Charging time depends on charger current, battery capacity and charge efficiency together, not on any one of those numbers alone.
Can I use this on a phone, or does it need a desktop?
The Battery Backup Calculator works on any modern browser, phone or desktop, and recalculates as soon as you press Calculate. If a result looks off, double-check that every field is filled in with the unit shown next to it (mixing up units, such as W with kW, is the most common cause of an unexpected answer) and re-enter the values.