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Battery Capacity Calculator

Find how many amp-hours a battery bank needs to support a given load, using voltage and maximum depth of discharge as sizing inputs.

Result

200 Ah

You need about 200 Ah of battery capacity.

Quick Answer

A battery capacity calculator applies the IEEE 485 formula: Capacity (Ah) = Total Watt-Hours ÷ (Nominal Voltage × Max Depth of Discharge). A 1,200 Wh load on a 12V system with 50% max DoD needs 1,200 ÷ (12 × 0.50) = 200 Ah of rated battery capacity.

Battery Capacity Calculator: What It Sizes And Why

A battery capacity calculator sizes a battery bank for a known electrical load. It tells you how many amp-hours of rated capacity you need to store enough energy to run a given load at a given voltage, without exceeding the battery's maximum depth of discharge. The depth of discharge limit protects the battery from damage and preserves its cycle life. Get this number right before you buy batteries and you avoid the two most common mistakes: undersizing (running out of power) and oversizing (paying for capacity you cannot safely use). This is the pillar calculator for the battery cluster. Use it to find the Ah starting point, then use the linked calculators below to refine your system: Solar Battery Calculator adds the autonomy-days dimension for off-grid systems, Battery Charge Time Calculator tells you how long a charger takes to refill the bank you just sized, and UPS Battery Calculator sizes a backup battery for a critical AC load.

The IEEE 485 Battery Capacity Formula And How It Works

A battery capacity calculator applies IEEE 485 battery sizing principles to determine required amp-hour capacity from load energy, voltage, and maximum depth of discharge.

  • Capacity (Ah) = Total Watt-Hours ÷ (Nominal Voltage × Max Depth of Discharge)
  • Total Watt-Hours: energy load needs to run (watts × hours)
  • Nominal Voltage: system voltage of the battery bank (12V, 24V, 48V)
  • Max Depth of Discharge: safe usable fraction (0.50 lead-acid, 0.80-0.90 lithium)

No temperature derating or Peukert-effect loss at high discharge rates is applied. Add a 10 to 20% margin (IEEE 485 planning guidance) to the result for real-use conditions.

How To Use The Battery Capacity Calculator In Five Steps

Inputs

  • Total watt-hours: total energy the battery must supply (watts × hours)
  • Nominal voltage: battery nominal voltage (12V, 24V, 48V)
  • Max depth of discharge: maximum fraction of rated capacity usable (0.50 lead-acid, 0.80-0.90 lithium)

Steps

  1. Calculate your total watt-hours: multiply load power in watts by runtime hours needed.
  2. Enter the total watt-hours into the calculator.
  3. Select your system voltage.
  4. Enter your battery type's max depth of discharge as a decimal.
  5. Read the required bank capacity in amp-hours from the result. Add 15% as a sizing margin before purchasing.

Battery Capacity Formula Applied To Real Numbers Here

A 600W load runs for 2 hours (1,200 Wh) on a 12V system using lead-acid batteries at 50% max depth of discharge, or on a 12V lithium system at 80% DoD.

  1. Confirm total energy: 600W × 2 hours = 1,200 Wh.
  2. For lead-acid at 50% DoD: 1,200 / (12 × 0.50) = 200 Ah.
  3. For lithium at 80% DoD: 1,200 / (12 × 0.80) = 125 Ah.

Required capacity: 200 Ah for lead-acid, or 125 Ah for lithium (rounded up to nearest whole Ah).

When A Battery Capacity Calculator Gives The Right Answer

Use this calculator when selecting batteries for off-grid, RV, marine, or backup-power systems where you know the load in watts and runtime in hours.

For off-grid systems where autonomy matters, pass your result to the solar battery calculator. For AC loads with inverters, see the UPS battery calculator, and for charging times check the battery charge time calculator in our engineering calculators hub.

Assumptions

  • Total watt-hours reflects a consistent average load; peak surge demand is not modeled.
  • System voltage is stable across the full discharge cycle.
  • Depth of discharge is uniform across all cells.
  • No Peukert-effect capacity loss at high discharge rates.
  • No temperature-based capacity derating applied.

This calculator does not apply temperature derating or Peukert-effect capacity loss at high discharge rates; add margin for real-world conditions.

Limitations

  • High-rate discharges (C-rate above 0.2C) deliver less than rated Ah; add 10 to 20% margin for fast-discharge applications (IEEE 485 planning guidance).
  • Cold temperatures reduce usable capacity significantly; a battery rated at 100 Ah at 25C may deliver 70 to 80 Ah at 0C.
  • This formula sizes the battery bank only; it does not size the solar array, charge controller, or inverter.
  • For AC loads, apply inverter efficiency before entering watt-hours (divide AC watt-hours by 0.90 or your inverter's rated efficiency).

In Practice

The most common mistake is entering the battery's nameplate Ah rating as the usable capacity. A 200 Ah lead-acid battery does not give 200 Ah of usable energy. It gives 100 Ah at 50% DoD. Always apply the depth of discharge limit when sizing. Size for usable capacity, not nameplate capacity. If a load needs 100 Ah of usable energy at 12V and you have lead-acid batteries, the formula tells you: buy 200 Ah of rated capacity.

Related Calculators

Solar Battery Calculator

Size your off-grid solar battery bank in amp-hours by entering your daily energy use, system voltage, autonomy days, and depth of discharge.

Open the Calculator

Battery Charge Time Calculator

Calculate how many hours a charger takes to refill a battery from the capacity in Ah, battery voltage, charger power, and charging efficiency.

Open the Calculator

UPS Battery Calculator

Find the amp-hours your UPS battery needs using load watts, backup runtime, system voltage, inverter efficiency, and power factor.

Open the Calculator

Related Guides

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  • How To Calculate Battery Capacity

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Frequently Asked Questions About Battery Capacity Sizing

What is depth of discharge and why does it affect the Ah I need?

Depth of discharge is the maximum percentage of a battery's rated capacity that can safely be used before recharging. Lead-acid batteries typically limit to 50% DoD to protect cycle life. Lithium batteries allow 80 to 90% DoD. Because you can only use a fraction of the rated capacity, the rated Ah must be larger than the usable Ah you need. The formula divides by DoD to automatically scale the required rated capacity up from the usable amount.

What system voltage should I choose for my battery bank?

For small loads under 500W, 12V is common and simplifies wiring. For loads from 500W to 3,000W, 24V reduces current and allows lighter cable. Systems above 3,000W benefit from 48V, which minimizes current further and supports higher-power inverters efficiently. Higher voltage also reduces I²R resistive losses in the wiring.

How do I convert my AC loads to watt-hours before entering them?

Multiply the AC load power in watts by the runtime hours to get watt-hours, then divide by your inverter efficiency (typically 0.90 per IEEE 485 sizing guidance). For example, a 300W AC load running for 4 hours through a 90% efficient inverter needs 300 times 4 divided by 0.90 = 1,333 Wh as the calculator input, not 1,200 Wh.

How is this different from the solar battery calculator?

This calculator sizes a battery for a known watt-hour load at a given voltage, without modeling how many days the battery must sustain the load without solar charging. The Solar Battery Calculator adds the autonomy-days dimension: how many consecutive low-production days the bank must cover. Both use the same Ah formula but with different inputs.

Why is no Peukert effect modeled?

The Peukert effect causes battery capacity to drop at high discharge rates. Modeling it requires knowing the battery's specific Peukert exponent, which varies by manufacturer and chemistry. Per IEEE 485, this calculator applies the simplified formula for planning purposes. For high-rate discharge applications, add 10 to 20% (IEEE 485 planning margin) to the result.

Sources

Last updated: . Reviewed for accuracy against the formula shown above.