Amp Hour Calculator

What is an amp-hour?

An amp-hour (Ah) is one amp of current flowing for one hour. It measures how much charge a battery can deliver, so a 100 Ah battery can in theory supply 5 A for 20 hours, 10 A for 10 hours, or 1 A for 100 hours.

You'll often hear "amps per hour", but that isn't a real unit. Amps are already a rate (charge per second), so a battery doesn't deliver "amps per hour". It delivers a current in amps for a number of hours, and current multiplied by time gives amp-hours.

How to use the calculator

1. Choose what you want to find: amp-hours, current, runtime, or an Ah ↔ Wh conversion.
2. Enter the values shown. Enter the load in amps, or in watts if that's what the device label gives.
3. Set the battery's nominal voltage. You need it for any watt or watt-hour figure.

The result is ideal arithmetic, with no allowance for depth of discharge, temperature or losses. The Working line under the result shows the numbers that were used.

Amp-hour formulas

Where A is current in amps, h is time in hours, Ah is capacity in amp-hours, V is the battery's nominal voltage in volts, W is power in watts, and Wh is energy in watt-hours.

A battery's Ah rating is measured at a specific discharge rate. For lead-acid and AGM batteries this is usually the 20-hour rate, so a 100 Ah battery is rated for about 5 A over 20 hours. At higher currents, lead-acid batteries deliver noticeably less than their label (the Peukert effect). Lithium (LiFePO4) batteries are much less affected.

Worked examples

Amp-hours vs watt-hours

Amp-hours measure charge, and watt-hours measure energy. The two are linked by voltage (Wh = Ah × V), so you can only compare amp-hour ratings between batteries of the same voltage. A 12V 100 Ah battery and a 24V 50 Ah battery both store about 1,200 Wh. The 24V battery has half the amp-hours but the same energy.

Use the battery's nominal voltage for conversions: 12 V for a 6-cell lead-acid battery and 12.8 V for a 4-cell LiFePO4 battery. Actual voltage falls during discharge, so any Wh figure is an estimate.

Series vs parallel battery banks

Series wiring adds voltage and keeps the amp-hours the same. Parallel wiring adds amp-hours and keeps the voltage the same. Either way the total watt-hours is the same, which is another reason to compare banks in Wh. Build a bank from identical batteries of the same chemistry, capacity, age and state of charge. With lithium batteries, check that the manufacturer supports series or parallel connection and how many units are allowed.

Amp-hours vs CCA

CCA (cold cranking amps) is a rating for starting batteries. It's the current a 12V battery can deliver for 30 seconds at −18 °C (0 °F) while staying at or above 7.2 V. It describes a short, high-current burst, not how much charge the battery stores.

There's no formula that converts CCA to amp-hours. Two batteries with the same CCA can have very different capacities, because starting batteries are built with many thin plates for burst current and deep-cycle batteries with thicker plates for sustained discharge. If a starting or marine battery lists reserve capacity (RC), use that instead. RC is the number of minutes the battery can supply 25 A before falling to 10.5 V, so RC × 25 ÷ 60 gives the amp-hours delivered at that 25 A rate. For standby or backup power, choose a deep-cycle battery with a stated Ah rating.

Battery sizing calculator

The calculator above gives the amp-hours a load actually draws. The battery you buy needs to be bigger, because most chemistries shouldn't be fully drained and some energy is lost in wiring and inverters. This calculator adds those allowances.

How the sizing calculator works

A common mistake is buying a battery whose rated capacity exactly matches the raw Ah calculation — that only works if you plan to fully drain the battery every cycle, which drastically shortens its life (especially for lead-acid and AGM). That's why the result already factors in your chemistry's safe depth of discharge — what you see here is what to buy, not just what you'll consume.

Sizing worked example

150W continuous load at 12V, running for 8 hours, with a 15% system-loss allowance and a lead-acid/AGM battery (50% depth of discharge).

Buy a 230 Ah battery (or bank smaller batteries in parallel to reach it) — not a 100 Ah one, which is only the raw energy consumed and would push a lead-acid battery well past its safe 50% discharge limit every cycle.

Why depth of discharge (DoD) matters

Discharging lead-acid or AGM batteries deeper than 50% regularly cuts their usable lifespan sharply — it's the single biggest reason backup batteries "die early" on job sites. Lithium (LiFePO4) tolerates much deeper discharge without the same penalty, which is why it needs a smaller rated capacity for the same job — worth the higher upfront cost on systems that cycle daily (solar, daily-use equipment).

Don't forget system losses

If you're running AC loads through an inverter, expect 10–15% loss from inverter inefficiency alone. Add wiring resistance over longer cable runs and connector losses, and a 15–20% overall loss factor is a realistic default for most real-world installs — pure DC loads wired directly (like a 12V camera or alarm panel) can use a lower figure, closer to 5–10%.

Common mistakes

• Comparing Ah across voltages. A 24V 100 Ah battery holds twice the energy of a 12V 100 Ah one. Compare in watt-hours.
• Treating the label Ah as usable Ah. Lead-acid and AGM batteries should normally only be discharged to about 50%. Size with the calculator above.
• Using peak current instead of average. Ah depends on average draw over time. A camera's night-time IR current, a PTZ's motor current or a power supply's maximum rating overstates the average if you apply it to the whole day.
• Converting AC watts at the wrong voltage. For a load on an inverter, divide the AC watts by the battery voltage (not 120 V or 230 V) to get battery current, then allow for inverter losses.
• Reading CCA as capacity. A high CCA figure says nothing about how long a battery will run a load.

Frequently asked questions

How many amps is 200W at 12V?

About 16.7 A. Current is watts divided by volts, so 200 W ÷ 12 V = 16.67 A. Over one hour, that load uses about 16.7 Ah from a 12 V battery. If the 200 W is an AC load running through an inverter, the battery current will be higher because of inverter losses.

How many watt-hours are in a 100Ah 12V battery?

About 1,200 Wh nominal (100 Ah × 12 V). A 12 V LiFePO4 battery has a nominal voltage of 12.8 V, so 100 Ah is closer to 1,280 Wh. Usable energy is lower: roughly 600 Wh at the 50% depth of discharge recommended for lead-acid, or about 1,000 Wh at 80% for lithium.

How long will a 12V 300Ah battery last?

It depends on the load current, because runtime = Ah ÷ amps. At a 10 A load (about 120 W at 12 V), 300 Ah ÷ 10 A = 30 hours in theory. Usable runtime is shorter: about 15 hours from lead-acid at 50% depth of discharge, or about 24 hours from lithium at 80%, before wiring or inverter losses.

Should I size the battery around the raw Ah or the rated Ah?

Always buy based on the rated Ah figure, the one already divided by your chemistry's depth of discharge. The raw Ah number is only what you'll consume. Buying to that figure means draining the battery further than is safe every cycle.

What losses figure should I use for a mostly DC setup?

A pure DC load wired directly, like a camera or alarm panel, typically only needs 5 to 10 percent for wiring losses. Reserve the higher 15 to 20 percent figure for setups running through an inverter, since inverter inefficiency is usually the bigger loss source.

Is lithium always the better choice?

Not automatically. Lithium (LiFePO4) needs a smaller rated capacity for the same job and lasts far more cycles, but costs more upfront. It tends to pay off fastest on systems that cycle daily, like solar, rather than backup power that sits idle most of the time.

Related tools

Sizing backup power specifically for an alarm panel, NVR, or PoE switch with a standby + alarm current spec? Use the Battery Backup Calculator instead — it's built around the standby-hours-plus-alarm-minutes method used in fire and security code compliance.

Already own a battery and want a realistic runtime that allows for chemistry and losses, rather than the theoretical Ah ÷ A figure above? Use the Battery Amp Hour Calculator. It works in the opposite direction from the sizing calculator on this page.

Not sure whether to wire batteries in series or parallel once you've got your Ah figure, or which chemistry to pick? See our guide: How to Choose the Right Battery Size for a Security Panel.

Found a wrong result on this calculator? Send feedback →