Ampere to Amp Hour Calculator

Converting amperes to amp hours tells you how much electric charge a device will consume over a given period. One amp hour equals one ampere of current flowing for one hour. If a device draws 5 amps and runs for 4 hours, it consumes 20 Ah of battery capacity.

Amps measure current at a single instant. Amp hours measure the total charge delivered across time. The distinction matters because the same 100 Ah of capacity can be drained slowly over many hours or quickly over a few, depending entirely on the load you connect.

In practice, most people reach for an ampere to Ah calculator when they know a load’s current draw and need to pick a battery that will last a specific number of hours. Solar installers do this daily when sizing off-grid battery banks. RV owners do it when planning how many hours their 12V fridge can run overnight. UPS technicians do it when specifying backup runtime for server racks.

Ampere to Ah Formula and How to Use It

The ampere hour formula is straightforward: multiply the current in amperes by the time in hours.

The reverse form is just as useful. If you already have a battery and want to know how long it will last:

h = Ah / A

A 100 Ah battery supplying 4 amps will last 100 / 4 = 25 hours under constant load. In real-world conditions, that number drops because of factors covered in the Peukert’s effect section below.

Worked Examples: Ampere to Ah at Different Voltages

The amp hour calculation itself does not depend on voltage. Ah = A × h works the same whether the system runs at 12V, 24V, or 48V. Voltage becomes relevant when you need to convert amp hours to watt-hours for energy comparisons: Wh = Ah × V.

Here are three worked examples at 12 V, 24 V, and 48 V.

Example 1: 12V RV Refrigerator (USA)

A 12V compressor fridge draws 3.8 amps on average. You want it to run for 14 hours overnight without shore power.

Ah = 3.8 A × 14 h = 53.2 Ah

Energy consumed: 53.2 Ah × 12 V = 638.4 Wh.

For a LiFePO4 battery with 90% usable depth of discharge (DoD), you need a minimum rated capacity of 53.2 / 0.90 = 59.1 Ah. A 75 Ah or 100 Ah battery gives comfortable headroom.

Example 2: 24V Solar Battery Bank (230V Region, Europe)

A small off-grid cabin in Germany runs LED lighting, a router, and a phone charger drawing a combined 2.4 amps from a 24V battery bank for 18 hours per day.

Ah = 2.4 A × 18 h = 43.2 Ah

Energy consumed: 43.2 Ah × 24 V = 1,036.8 Wh (about 1.04 kWh per day).

With two days of autonomy and 50% DoD on AGM batteries, the bank needs: (43.2 × 2) / 0.50 = 172.8 Ah. A pair of 12V 100 Ah AGM batteries wired in series (24V, 100 Ah) falls short. Two series pairs in parallel (four 12 V 100 Ah batteries, giving 24 V and 200 Ah) covers the requirement with margin.

Example 3: 48V UPS System (230V Region, India)

A rack-mounted UPS protecting network equipment draws 6.25 amps from a 48V battery string. The required backup time is 3 hours.

Ah = 6.25 A × 3 h = 18.75 Ah

Energy consumed: 18.75 Ah × 48 V = 900 Wh.

IEEE 485 recommends sizing lead-acid strings with an aging factor of 1.25 and a temperature correction factor for environments above 25°C. At 35°C in a Mumbai server room, IEEE 485 sizing applies a 1.25 aging factor: 18.75 × 1.25 = 23.4 Ah minimum. Standard practice takes no capacity credit above 25°C, and sustained heat shortens VRLA service life, so round up and specify at least 25 Ah.

Ampere to Ah Conversion Table for Common Durations

The table below shows amp hour values for common current draws across 1, 2, 4, 8, 10, and 20 hours. Each cell is A × h.

Amps vs Amp Hours: What Each Unit Measures

Amps and amp hours measure two different things. An ampere measures the rate of current flow at any given moment. An amp hour measures the total amount of electric charge transferred over time.

A 100 Ah battery is not a 100-amp battery. It can deliver 100 amps for one hour, or 1 amp for 100 hours, or any combination that multiplies to 100 Ah. The distinction trips up DIY users more than any other concept in battery sizing.

When sizing batteries, always start with amp hours. Once you have the Ah figure, you can convert to watt-hours using our Ah to Wh calculator for energy comparisons across different voltages.

How Peukert’s Effect Changes Real Amp Hour Capacity

Rated amp hours assume a specific discharge rate, almost always the C/20 rate (full discharge over 20 hours). A 100 Ah battery at C/20 delivers 5 amps for 20 hours. Draw 50 amps from that same battery and it will not last 2 hours. It will die sooner because of internal resistance and heat.

Peukert’s Law quantifies this loss. The Peukert exponent (k) varies by chemistry:

This is why lithium iron phosphate (LiFePO4) batteries deliver closer to their rated Ah across a wide range of discharge currents, while flooded lead-acid batteries fall far short of their label at anything above the C/20 rate. When converting amps to amp hours for lead-acid systems, add a 20-30% capacity margin to compensate for Peukert losses.

Battery Chemistry and Amp Hour Behavior

The amp hour formula gives an ideal value. Real batteries behave differently depending on their chemistry, and two batteries with the same Ah rating on the label can deliver very different amounts of usable energy.

Lead-acid (flooded and AGM): Should not be discharged below 50% state of charge (SoC) regularly. A 100 Ah flooded battery gives roughly 50 Ah of usable capacity. AGM improves on this slightly. Both lose capacity in cold weather and at high discharge rates. IEC 60896-21 covers testing requirements for stationary VRLA (valve-regulated lead-acid) batteries.

LiFePO4 (lithium iron phosphate): Tolerates 80-90% DoD routinely. A 100 Ah LiFePO4 battery gives 80-90 Ah of usable capacity. Peukert losses are negligible. Cycle life ranges from 2,000 to 5,000+ cycles at 80% DoD. IEC 62619 covers safety requirements for secondary lithium cells used in industrial applications.

Li-ion (NMC, NCA): Common in consumer electronics and EVs. Higher energy density than LiFePO4 but less thermally stable. Typical DoD is 80-90%. IEC 62133-2 covers safety requirements for portable sealed lithium cells. UN 38.3 applies to all lithium batteries shipped by air, sea, or ground.

NiMH (nickel-metal hydride): Used in hybrid vehicles and some power tools. Lower energy density than lithium. Suffers from self-discharge (15-30% per month at room temperature). IEEE 1625 covers battery safety for laptop and portable computing devices.

Temperature Effects on Amp Hour Capacity

Cold temperatures reduce a battery’s effective amp hours. The chemical reactions that produce current slow down as temperature drops. At 0°C (32°F), a lead-acid battery delivers roughly 80% of its rated capacity. At -20°C (-4°F), that can fall to 50%.

Lithium batteries handle cold better but are not immune. Most LiFePO4 cells cannot be charged below 0°C without risk of lithium plating, which permanently damages the cell. Discharge performance drops less, typically 10-15% at 0°C.

When converting amps to amp hours for outdoor or cold-climate installations, apply a temperature derating factor. IEEE 485 (vented lead-acid) and IEEE 1189 (VRLA) provide temperature correction guidance. For a system in Winnipeg or Helsinki, sizing the battery for only the calculated Ah value without temperature correction will result in undersized backup.

Global Standards for Battery Amp Hour Ratings and Testing

Standards bodies define how manufacturers must test and report capacity. The most relevant standards are:

Manufacturers in the USA, EU, UK, Australia, and most of Asia must comply with IEC 62133-2 for consumer lithium products. Batteries that ship internationally require UN 38.3 certification. When you see an Ah rating on a battery, the testing method behind it should reference one of these standards. If a datasheet lists Ah capacity without referencing a test standard or C-rate, treat the number with caution.

Industry Applications for Amps to Amp Hours Conversion

Converting amps to amp hours applies across a wide range of industries and use cases:

Solar and off-grid storage: Sizing battery banks for daily load profiles. A system drawing 8 amps from a 24V bank for 6 hours of evening use needs at least 48 Ah of usable capacity, plus DoD and temperature margins.

UPS and data center backup: Calculating battery string capacity to support critical loads for a defined runtime. IEEE 485 is the standard method for lead-acid sizing in these applications.

Electric vehicles: EV battery packs are typically rated in kWh, but individual cell-level sizing starts with Ah calculations at the cell’s nominal voltage.

Marine and RV: 12V house batteries sized for lighting, pumps, and electronics draw. Amp hours consumed per day determines whether one 100 Ah battery suffices or whether two in parallel are needed.

Portable electronics: Smaller devices use milliamp hours (mAh). A phone battery rated at 4,500 mAh is 4.5 Ah. To convert between scales, use our Ah to mAh calculator.

Common Mistakes When Converting Amps to Amp Hours

The most frequent error is treating the entire rated capacity as usable. A 100 Ah lead-acid battery does not give you 100 Ah. After accounting for DoD limits (50%), Peukert losses at your actual discharge rate, and temperature derating, the real usable figure might be 40-45 Ah.

A second common mistake is ignoring variable loads. The formula Ah = A × h assumes constant current. A refrigerator compressor cycles on and off, drawing 5 amps when running and 0 amps when idle. Using the peak draw (5 A) for the full duration will overestimate consumption. Using the average draw (measured over a full cycle) gives a more accurate Ah figure.

The third mistake is mixing up amps and amp hours entirely. Saying “my battery is 100 amps” is incorrect. The battery has a capacity of 100 amp hours. Its maximum discharge current (in amps) is a separate specification, often listed as the C-rate or the maximum continuous discharge rating.

For the reverse conversion, use our Ah to amps calculator to find how much current a battery can deliver for a specific runtime. To convert amp hours into energy units, try the Ah to Wh calculator. For smaller batteries measured in milliamp hours, our Ah to mAh calculator handles the scale conversion. And for full battery system design including load profiling and autonomy, see the battery capacity calculator.

Professional Disclaimer

The calculations, values, and guidance on this page are for educational and planning purposes only. Battery sizing for safety-critical systems must be performed or verified by a licensed electrical engineer in accordance with local codes and applicable standards (NEC, IEC, IEEE, AS/NZS, BS 7671, or CSA). Always follow manufacturer specifications and consult a qualified professional before designing, installing, or modifying battery systems.