Battery Ah Calculator

What Is a Battery Ah Calculator?

A battery Ah calculator takes an electrical load and a backup-time target and works backward to tell you how much amp-hour capacity you need. It's the same math an electrician does on paper when sizing a battery bank for an inverter, UPS, or solar system — this just does it faster and lets you adjust the assumptions.

The calculator on this page goes a step further than a bare watts-times-hours-divided-by-volts formula. It factors in system efficiency and usable depth of discharge, because those two numbers are usually the difference between a battery that works and one that leaves you short an hour before the sun comes up or the grid comes back.

What Is Battery Ah?

Ah stands for ampere-hours, usually just called amp-hours. It describes how much electrical charge a battery can deliver over time under a specific set of test conditions. A 100Ah battery, in the simplest theoretical sense, could deliver 5 amps for 20 hours, or 10 amps for 10 hours — current multiplied by time equals the rated capacity.

Real batteries don't follow that relationship as neatly as the arithmetic suggests. How much current you draw, the temperature the battery sits at, its chemistry, its age, and the exact conditions the manufacturer used to test it all change how many amp-hours you actually get. Lead-acid batteries in particular deliver noticeably less usable capacity at high discharge currents — an effect known as the Peukert effect, covered further down this page.

What Does a 100Ah Battery Mean?

A 100Ah rating does not mean the battery supplies exactly 100 amps for one hour. That's a common misreading of the label. What it actually means is that, under the manufacturer's test conditions — typically a slow, steady 20-hour discharge for lead-acid batteries — the battery can supply a current that adds up to 100 amp-hours before it's considered discharged.

In practice that could look like:

• 5A drawn continuously for around 20 hours
• 10A drawn for roughly 10 hours
• 20A drawn for somewhere less than 5 hours, because higher currents pull more from the battery than the simple math predicts

That last point is the part that trips people up. The 100Ah number sounds precise, but it's a rating under one specific discharge rate — not a fixed energy bucket that behaves identically no matter how fast you draw from it.

How the Battery Ah Calculator Works

The calculator above runs your numbers through four steps: it multiplies your load by your backup time to get the raw energy you need, divides that by your system efficiency to account for losses in the inverter and wiring, converts the result into amp-hours at your chosen voltage, and finally divides by your usable depth of discharge so the battery isn't sized as if you could drain it completely.

Battery Ah Formula

There are really two formulas worth knowing here. The simple one converts stored energy to amp-hours:

And the practical one, which starts from a load and a backup time and accounts for real-world losses:

The first formula tells you what a battery's nominal capacity represents. The second is what you actually use to size a battery for a job.

How to Calculate Battery Ah

Start with the theoretical version — it's a good sanity check before you add in the real-world adjustments:

Now the practical version, which is what the calculator on this page actually computes:

Ah vs Wh: What's the Difference?

Ah measures charge — the amount of current a battery can push out over time. Wh measures energy — the actual work that charge can do, which depends on voltage as well. Two batteries can share an Ah rating and store completely different amounts of energy if their voltages differ.

So a 12V 100Ah battery stores 12 × 100 = 1,200Wh nominal energy. A 24V 100Ah battery, despite carrying the same "100Ah" label, stores twice that: 2,400Wh. That's why comparing batteries by Ah alone only makes sense when the voltage is the same.

Nominal energy isn't the same as usable energy, either — depth of discharge and efficiency losses mean you won't draw the full 1,200Wh or 2,400Wh out in practice.

How Many Ah Do I Need for an Inverter?

This is the question most people actually came here to answer, and the honest answer is: it depends on what you're actually running, not what your inverter is rated for. A 2,000W inverter tells you the maximum it can output — it doesn't tell you what your appliances are drawing right now. If your fridge, a few lights, and a fan add up to 600W, that 600W is the number to plug into the calculator, not 2,000W.

Battery sizing for an inverter comes down to five things: the actual load, how long you need backup, the battery voltage, the inverter's efficiency, and how much of the battery's rated capacity you're willing to use before recharging. Get the actual load right and the rest of the calculation follows the same formula used throughout this page.

Examples at Different Loads (12V, 4-hour backup, 90% efficiency, 80% DoD)

Notice how quickly the required Ah climbs — this is usually the point where people switch to a higher battery voltage or a multi-battery bank rather than one enormous 12V battery.

12V Battery Calculations

12V is the most common voltage for small inverters, RVs, camping setups, and single-battery backup systems. Because voltage is low, current draw is comparatively high for a given power level, which is one reason 12V systems don't scale well past a few hundred watts without thick cabling and large batteries.

For a 500W load over 4 hours at 12V with 90% efficiency and 80% DoD, the calculator returns roughly 231.5Ah — often split across two or three parallel batteries rather than one oversized unit, for reasons of cost, weight, and availability.

24V Battery Calculations

Stepping up to 24V halves the current for the same power, which means thinner cables and less resistive loss. A 24V bank is commonly built from two 12V batteries wired in series.

The Ah figure doesn't multiply just because you've added a battery — series wiring adds voltage, not capacity. That trips a lot of people up the first time they build a bank.

48V Battery Calculations

48V systems show up in larger inverter installations and solar setups where the total load is high enough that 12V or 24V would demand impractically thick cabling. Four 12V batteries in series produce a 48V bank at the same Ah rating as a single battery.

For a 2,000W load over 5 hours at 48V with 90% efficiency and 80% DoD, expect a requirement of roughly 289.4Ah — comfortably split across a well-matched bank rather than one very large battery.

Battery Series vs Parallel Connections

Two 12V 100Ah batteries in series give you 24V 100Ah. The same two batteries wired in parallel instead give you 12V 200Ah. Same two batteries, very different results — which is why it's worth being deliberate about which configuration your system actually needs.

Whichever way you connect them, use batteries that match in age, capacity, and chemistry. Mixing an old battery with a new one, or different capacities, in the same bank tends to shorten the life of the whole set and can create imbalanced charging.

How to Calculate Battery Bank Capacity

These are nominal figures — the number printed on the label, not the number you'll actually get out once efficiency losses and depth-of-discharge limits are applied.

Battery Backup Time

If you already know your battery's capacity and want to work out how long it'll last, flip the formula around:

Treat this as a starting estimate rather than a countdown timer — the next section explains why actual runtime tends to come in a bit lower.

Why Your Battery May Not Provide the Calculated Backup Time

The calculator gives you a solid planning number, but real systems have more variables than any formula captures cleanly. A few of the biggest factors:

• Inverter losses beyond the efficiency figure you entered, especially at low load or standby
• Discharge rate and the Peukert effect, which reduce usable capacity at high current draw
• Battery age and condition, since capacity fades with cycle count
• Temperature, which affects lead-acid and lithium batteries differently
• Cable losses, worse with long or undersized cabling
• Startup surges from motors, compressors, and pumps drawing far more than their running wattage for a moment
• Inverter low-voltage cutoff, which shuts things down before the battery is technically empty
• Manufacturer rating conditions that don't match how you're actually discharging the battery

Does Battery Type Affect Ah?

The Ah number on the label is a rating, but how much of it you can realistically use, and how well the battery holds up over time, depends heavily on chemistry.

• Flooded lead-acid — inexpensive, but typically recommends a shallower DoD (around 50%) for reasonable lifespan and needs ventilation and periodic maintenance.
• AGM — sealed, low maintenance, tolerates a somewhat deeper discharge than flooded, moderate cost.
• Gel — sealed and stable in temperature swings, sensitive to overcharging, moderate cost.
• Lithium-ion / LiFePO4 — supports much deeper discharge (often 80–100% depending on the pack), lighter, longer cycle life, but requires a battery management system (BMS) and different charging equipment.

Two batteries with an identical Ah rating won't perform the same if one is flooded lead-acid rated for 50% DoD and the other is LiFePO4 rated for 90% DoD. Always size around the usable capacity the manufacturer actually recommends, not the number on the case.

What Is Peukert's Law?

Peukert's Law describes something lead-acid batteries do that a simple Ah calculation doesn't capture: the faster you drain them, the less total capacity you actually get. A battery might deliver its full rated Ah at a slow, steady discharge, but pull much higher current and it can effectively lose 20–30% of that capacity before it hits the cutoff voltage.

You don't need to run the Peukert math yourself to make use of this. The practical takeaway is simpler: if your load is going to draw a lot of current relative to the battery's rating, build in extra margin rather than sizing right to the calculator's raw output.

Does Temperature Affect Battery Capacity?

Yes, noticeably. Cold temperatures slow the chemical reactions inside a battery and reduce the current it can deliver — a lead-acid battery can lose a significant chunk of its rated capacity in freezing conditions. High temperatures do the opposite for short-term output but accelerate long-term degradation and, for some chemistries, increase safety risk if charging isn't managed properly.

Follow the manufacturer's temperature guidance for both charging and discharging — it's usually printed right on the datasheet, and it matters more than most people expect.

Does Battery Age Affect Capacity?

Capacity fades over a battery's life, gradually at first and then faster as it nears the end of its usable cycle count. How quickly that happens depends on cycle count, charging habits, temperature exposure, how deep the battery is typically discharged, and how it's stored when idle.

An old 100Ah battery is not a new 100Ah battery. If you're sizing a system around a battery that's already a few years old, it's worth checking its actual measured capacity rather than trusting the original label.

Why Battery Voltage Matters

Power, voltage, and current are locked together by one relationship:

For a 1,000W load:

• At 12V: 1000 ÷ 12 ≈ 83.3A
• At 24V: 1000 ÷ 24 ≈ 41.7A
• At 48V: 1000 ÷ 48 ≈ 20.8A

Higher voltage means lower current for the same power — that's why larger systems generally move to 24V or 48V rather than staying at 12V. Real current draw from the battery tends to run a bit higher than this simple math because of inverter losses.

How to Convert Ah to Wh

How to Convert Wh to Ah

Example: 2,400Wh ÷ 24V = 100Ah. Voltage always has to be known for this conversion — Wh and Ah aren't interchangeable without it, since Ah on its own says nothing about how much energy is actually stored.

Common Battery Sizes

All figures are nominal energy values, not usable-after-DoD figures.

Battery Ah Calculation for Solar Systems

Solar battery sizing has an extra layer on top of the load-and-backup-time math: how much daily energy your panels can realistically replace, and how many days you want to ride through without much sun. The battery bank needs to be sized around your daily energy consumption and desired autonomy, not just a single backup event.

Work out your daily Wh consumption, decide how many days of autonomy you want, apply your DoD and efficiency assumptions the same way this calculator does, and check that against what your panels and charge controller can realistically replace on an average day for your location.

Battery Sizing for Off-Grid Systems

Off-grid sizing follows the same core formula but leans harder on getting your daily consumption number right, since there's no grid to fall back on if you undersize the bank. Key inputs: daily consumption, required autonomy in days, battery voltage, usable capacity after DoD, inverter efficiency, and how reliably your solar charging can top the bank back up given seasonal weather variation.

Battery Ah Calculation for UPS Systems

UPS battery sizing uses the same load-and-time approach, with a few UPS-specific details worth checking: the UPS's own conversion efficiency, how its internal batteries are configured (series, parallel, or both), the voltage at which it cuts off rather than risk damaging the batteries, and how the connected load actually behaves — steady IT equipment draws differently than equipment with periodic spikes. Always cross-check your estimate against the UPS manufacturer's own runtime charts where available.

RVs, Camping, and Portable Power

The same formula applies at smaller scale for RVs, camping setups, portable power stations, and small DC systems — the numbers are just lower. A 12V fridge and some LED lighting drawing 60W combined, run for 10 hours overnight: 60 × 10 = 600Wh, and at 90% efficiency and 80% DoD that comes to roughly 69.4Ah — comfortably inside a single mid-size 12V battery.

Common Battery Sizing Mistakes

• Using inverter rating instead of actual loadA 2,000W inverter doesn't mean you're drawing 2,000W — size around what's actually plugged in and running.
• Ignoring efficiencySkipping this step under-sizes the battery, sometimes by 10% or more.
• Assuming 100% of rated capacity is usableMost chemistries recommend well under full discharge for a healthy lifespan.
• Confusing Ah and WhThey're related but not interchangeable without knowing the voltage.
• Forgetting battery voltageAn Ah figure means little on its own without the voltage attached.
• Ignoring battery chemistryDifferent chemistries support very different usable DoD.
• Ignoring temperatureCold especially can meaningfully cut usable capacity.
• Ignoring battery ageAn old battery rarely still delivers its original rated capacity.
• Mixing old and new batteries in a bankShortens the life of the whole set and can unbalance charging.
• Mixing incompatible batteriesDifferent chemistries or capacities in one bank rarely age or charge evenly.
• Assuming calculated runtime is exactTreat the output as a planning estimate, not a guarantee.
• Ignoring cable lossesLong or undersized cabling adds resistive loss the formula doesn't see.
• Ignoring inverter low-voltage cutoffSystems shut down before the battery is technically empty.
• Selecting a battery by Ah aloneChemistry, DoD rating, and discharge behavior matter just as much.
• Ignoring manufacturer specificationsThe datasheet usually has the real answer for your exact battery.

Real-World Examples

Five complete examples, each at 90% efficiency and 80% DoD unless noted, worked through the same four steps: energy required, battery energy after efficiency, Ah at full discharge, and required Ah after DoD.

Battery Capacity Formula Summary

Battery Safety

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