What is depth of discharge (DoD)?

DoD is the percentage of battery capacity you should use before recharging. Using a lead-acid battery below 50% DoD shortens its lifespan. Lithium batteries can handle 80–100% DoD. The default here is 80% — a conservative figure suitable for most battery types.

Why does efficiency matter?

When running AC loads through an inverter, some energy is lost as heat. A typical inverter is 85–95% efficient. If you are running DC loads directly from the battery, set efficiency to 100%.

Amp-hours (Ah) is a measure of a battery's charge capacity. A 100 Ah battery can theoretically deliver 1 amp for 100 hours, or 10 amps for 10 hours — at the same voltage. The actual runtime depends on your load wattage.

How accurate is this calculation?

The result is an estimate. Real-world runtime can vary due to battery age, temperature, Peukert's effect (capacity drops at higher discharge rates), and cable losses. For critical systems, add a 20–30% safety margin.

Can I use this for a UPS battery?

Yes. Enter the UPS battery's rated Ah and voltage (usually 12V), your load wattage, and the UPS inverter efficiency (typically 85–92%). The result is the estimated runtime before the UPS shuts down.

What if I have multiple batteries?

For batteries wired in parallel, add the Ah ratings together (same voltage). For batteries in series, the voltage adds but Ah stays the same. Enter the totals into the calculator.

Battery Backup Calculator

Enter battery capacity, voltage, and load power to calculate estimated backup runtime in hours and minutes.

Battery & Load Parameters

Battery Capacity (Ah)

Rated amp-hour capacity printed on the battery label.

Battery Voltage (V)

Load Power (W)

Total wattage of all devices the battery will power.

Inverter Efficiency 90%

Set to 100% for direct DC loads (no inverter).

Depth of Discharge (DoD) 80%

How much of the battery capacity to use. 50% for lead-acid, 80–90% for lithium.

Estimated Backup Runtime

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Calculation Breakdown

Total energy (nominal) —

Usable energy (after DoD) —

Effective load (after efficiency) —

Draw current —

Runtime —

Tips for Accurate Results

Use the battery's rated Ah at 20-hour discharge (C20 rating) for best accuracy.
Older batteries lose capacity — reduce Ah by 10–20% for batteries over 3 years old.
Cold temperatures (below 0°C) can reduce lead-acid capacity by up to 30%.
Add all device wattages, including standby/idle draw for computers and monitors.

Summary

This battery backup calculator estimates how long a battery will power a connected load based on its capacity in amp-hours (Ah), system voltage (V), and the load wattage (W). It accounts for inverter efficiency and depth of discharge (DoD) to give a realistic runtime, not just a theoretical maximum.

How it works

Enter the battery capacity in amp-hours (Ah) — found on the battery label.
Enter the battery voltage (V) — commonly 6V, 12V, 24V, or 48V.
Enter the total load power in watts (W) — sum of all connected devices.
Optionally adjust inverter efficiency (default 90%) and depth of discharge (default 80%).
The calculator computes usable energy, then divides by load draw to give runtime.

Use cases

Sizing a UPS for a home office or server room.
Estimating solar battery storage runtime during a power outage.
Planning off-grid power systems for cabins or RVs.
Checking if a car battery will run a camping appliance overnight.
Verifying backup generator start battery life.
Calculating runtime for electric vehicles or mobility scooters.
Designing battery banks for amateur radio emergency equipment.
Comparing different battery configurations for a solar system.

Frequently Asked Questions

Last updated: 2026-06-04 · Reviewed by Nham Vu