Battery Amp Hour Calculator

How to use this battery amp-hour calculator

1. Enter the battery capacity in Ah (for a parallel bank, the total Ah).
2. Select the battery voltage, or choose Custom and enter it.
3. Enter the load in watts or amps, whichever the equipment label or your meter gives.
4. Enter an estimated system-loss percentage for wiring, inverter or UPS inefficiency.
5. Select the usable depth of discharge (DoD) for the battery chemistry.
6. Press Calculate to see the estimated runtime.
7. Compare the estimate with real operating conditions: see what affects real-world runtime below.

Battery runtime formula

• Battery capacity: the rated amp-hour (Ah) capacity on the battery label.
• DoD: depth of discharge, the fraction of rated capacity you plan to use (0.5 for 50%).
• Load watts / load current: the average power or current the equipment draws. If you enter amps, the watts-to-amps step is skipped.
• Battery voltage: the nominal battery or bank voltage.
• Losses: the system-loss percentage as a fraction (0.15 for 15%).

The loss percentage is treated as extra demand on the battery. Dividing the load current by (1 − losses) means a 15% loss setting reduces the estimated runtime by exactly 15%. The sizing calculator on our Amp Hour Calculator works in the opposite direction, from a required runtime to the battery size to buy, and adds losses as a margin (× (1 + losses)). A battery sized there for exactly 8 hours will therefore show slightly less than 8 hours here.

The result is an estimate for planning, not a guaranteed runtime.

Worked example

100Ah lead-acid/AGM battery (50% depth of discharge) at 12V, powering an 80W load, with a 15% system-loss allowance.

This 100Ah lead-acid battery runs the 80W load for about 6 hours 22 minutes before hitting its safe 50% discharge limit — not the roughly 15 hours you'd get by naively dividing the full 100Ah by the load current.

How long will a 100Ah battery last?

It depends on the load, the usable depth of discharge and the losses. For a 12V 100Ah battery with a 15% loss allowance, the calculator gives:

Doubling the load halves the runtime, and doubling the battery's Ah doubles it. At high loads, lead-acid batteries will usually fall short of these figures because of the Peukert effect.

There's no single answer to "how long will a 12V 300Ah battery last" — it depends entirely on the load. As an example, at the same 100W load and 15% losses used above, a 300Ah battery runs for about 15.3 hours (15h 18m) at 50% DoD, or about 24.5 hours (24h 29m) at 80% DoD. Use the calculator above with your own load to get a figure for your setup.

Why the number on the label is optimistic

A battery's rated Ah (say, "100Ah") is measured under lab conditions at a specific discharge rate, usually a slow 20-hour rate. Draw more current than that (a bigger load) and the usable capacity drops — plus you should never fully drain lead-acid or AGM batteries anyway. This calculator applies the depth of discharge you select and your stated system losses so the runtime figure reflects real use, not the lab number.

Depth of discharge by chemistry

Runtime scales directly with DoD: the same battery and load run 1.6 times longer at 80% DoD than at 50%. The selected DoD is only an assumption about how much capacity you'll use. The Custom / 100% option uses the full rated capacity, which suits only batteries whose manufacturer allows it. Always follow the manufacturer's discharge limits.

What affects real-world runtime

• Actual average load. Equipment rarely draws its nameplate figure all the time. Cameras with IR at night, PTZ movement, disk activity in an NVR, or a charging device all change the draw. A clamp meter or the UPS load reading gives a better average.
• Inverter or UPS efficiency. AC loads run through an inverter draw more from the battery than the load's rated watts, and most inverters also draw their own idle (no-load) power just to stay on. That idle draw can take a large share of the battery at light loads, so where possible use the inverter or UPS's actual measured input draw rather than just the connected load's rating.
• Battery age and condition. Capacity falls as a battery ages. A sealed lead-acid battery a few years into service in a warm cabinet may deliver well under its rating.
• Temperature. Cold reduces available capacity, especially for lead-acid batteries.
• Discharge rate. Ah ratings are usually quoted at a slow rate (often 20 hours). Faster discharge reduces the capacity lead-acid batteries deliver (the Peukert effect).
• Manufacturer limits. Battery-management cut-offs, low-voltage disconnects and UPS shutdown thresholds can end the runtime before the calculated DoD is reached.

Common uses

Solar battery banks (how long will stored power last through the night), RV and marine house batteries (how long between charges), portable power stations, and on-site backup batteries for alarm panels or NVRs where you already know the installed battery's rating and just need the runtime.

Common mistakes

• Treating the nameplate Ah as fully usable. Only the capacity down to your chosen DoD counts. For lead-acid, that's usually about half.
• Forgetting to convert watts to amps. Runtime uses current. Divide watts by the battery voltage, not by 120V or 230V mains voltage.
• Assuming a constant load. Use the average draw over the whole period, not a single reading taken when the equipment was idle.
• Entering an unrealistic loss figure. 0% is optimistic for any inverter or UPS system. Around 5–10% suits direct DC loads, and 10–20% suits inverter-fed AC loads.
• Ignoring battery age and temperature. An old battery or a cold location can both cut runtime noticeably.
• Confusing Ah with Wh. Ah only compares batteries of the same voltage. A 24V 100Ah battery stores twice the energy of a 12V 100Ah battery.
• Treating the result as guaranteed. Test the actual runtime on critical systems, such as fire, security or life-safety backup.

Frequently asked questions

Why is my real runtime shorter than the simple Ah divided by amps math?

Because that simple math ignores depth of discharge and system losses. Factor in your chemistry's safe discharge limit and your stated losses, and the runtime figure drops to what the battery will actually deliver, not the optimistic label number.

What's the Peukert effect and should I worry about it?

It's the drop in usable capacity that happens at higher discharge rates, a battery delivers less than its rated Ah when drained quickly. It matters most for loads drawing more than about a fifth of the battery's Ah rating in amps. For lighter loads, the effect is small enough to ignore.

Can I use this for a battery bank instead of a single battery?

Yes. Add up the Ah ratings of batteries wired in parallel and enter that total, the voltage and runtime math work the same way. For batteries wired in series, enter the bank's total voltage instead and keep the single battery's Ah rating.

Related tools

Need to find what size battery to buy for a required runtime, or convert between Ah, amps, hours and watt-hours? Use the Amp Hour Calculator.

Sizing backup for an alarm panel, NVR or PoE switch using a standby-plus-alarm specification? Use the Battery Backup Calculator.

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