Amp Hour Calculator

Smart three‑way calculator: compute Amp‑hours (Ah), current (A) or time (h) from any two values. Instantly derive watt‑hours (Wh), estimate runtime, and visualize battery state.

? Examples:
? Car batt (50Ah)
☀️ Solar deep‑cycle
? Phone 3A @2h
? Trolling motor
Privacy-first: all calculations are performed locally inside your browser. No data is uploaded.
Energy & runtime summary
⚡ Energy (Watt‑hours): 0.00 Wh
? Kilowatt‑hours: 0.0000 kWh
⏱️ Estimated runtime (load current): — hours
? Active capacity (Ah): 0.00 Ah
Battery fill (vs 100Ah reference) % of 100Ah

What is an Amp‑hour (Ah)? The heart of battery capacity

An amp‑hour (Ah) is a unit of electric charge representing the amount of current delivered over one hour. It defines battery capacity: a 100Ah battery can theoretically supply 1 amp for 100 hours, 10 amps for 10 hours, etc. The fundamental formula is:

Ah = I (Amps) × t (hours)

This calculator uses the triplet rule: provide any two values among current, time, or capacity, and the third is derived automatically. Real battery performance also depends on Peukert's law, temperature, and discharge rate — our tool gives the ideal baseline used by engineers and field technicians.

Watt‑hours: bridging voltage and capacity

For energy storage evaluation, watt‑hours (Wh) = Ah × Voltage (V). This metric is universal because it accounts for different battery voltages (12V, 24V, 48V). Solar installers and EV designers rely on Wh to compare systems.

Why use this interactive battery calculator?

  • Smart triple‑mode: Automatically solves for missing variable – ideal for load analysis, system sizing, and runtime planning.
  • Professional energy metrics: Instantly see watt‑hours, kilowatt‑hours and runtime with custom load current.
  • Visual capacity gauge: Real‑time battery fill relative to 100Ah reference – perfect for educational and quick estimation.
  • Field‑tested examples: From marine trolling motors to off‑grid solar banks, we provide authentic scenarios.

Step‑by‑step calculation methodology

1. Missing value resolution: Based on the well‑known relation Q = I × t, the calculator uses real‑time logic: if current and time are known → compute capacity; if capacity and current known → compute time; if capacity and time known → compute current. The voltage input then computes Wh = Capacity(Ah) × Volts. Load runtime = Capacity(Ah) / Load current (A). All floating-point rounding uses double precision for accuracy.

2. Triangle of charge: The three primary quantities (I, t, Ah) are linked through the fundamental charge equation. This tool implements full cross‑dependency while avoiding circular updates, giving you instantaneous feedback.

3. Real‑world consideration: While our calculator provides the ideal capacity, real batteries exhibit capacity loss at high discharge rates (Peukert effect). For lead‑acid, we recommend using a 20h rated capacity; for lithium, the efficiency remains near 98%.

Application examples & case study

Off‑grid solar system sizing

A cabin uses 12V LED lights (2A) and a water pump (8A) for 3 hours daily. Total daily Ah = (2A+8A)×3h = 30Ah. Considering depth of discharge (DoD) 50% for lead‑acid, required battery bank = 60Ah at 12V. Our tool instantly verifies: enter current 10A, time 3h → 30Ah. With 12V battery, energy = 360Wh. Adding load current of 10A predicts runtime 3h – accurate for planning.

Marine trolling motor runtime

A 55lb thrust motor draws 42A at full speed. If paired with a 100Ah AGM battery, runtime = 100Ah / 42A ≈ 2.38 hours. Our runtime estimator uses the same logic, and the watt‑hour conversion (100Ah × 12V = 1200Wh) helps compare with lithium alternatives.

Reference table: Common devices & amp‑hour consumption

Device / application Typical current (A) Time (h/day) Daily Ah Recommended battery (12V)
LED Camp light 0.5 A 6 h 3 Ah 20Ah +
RV Refrigerator (compressor) 4.5 A 8 h 36 Ah 100Ah LiFePO₄
Phone charger (5V USB) 1 A (5V) → 12V side ~0.42A 2 h 0.84 Ah Portable 10Ah
Fish finder (marine) 0.75 A 10 h 7.5 Ah 35Ah SLA
CPAP machine (DC) 2.5 A 8 h 20 Ah 50Ah deep cycle

Frequently Asked Questions

Multiply Ah by battery voltage (V). Example: 100Ah × 12V = 1200Wh. Our tool does it instantly.

Yes, due to Peukert effect (especially lead‑acid). Lithium batteries maintain nearly full capacity even at high rates. Our calculator provides nominal capacity.

Absolutely, lithium batteries are rated in Ah, and the math is identical. They also allow 80-100% DoD, so usable capacity is higher.

For lead‑acid: 50% DoD maximizes lifespan. For lithium: 80‑100% DoD is acceptable. Use our Ah values to size battery banks accordingly.

Ideal runtime = Ah / load current. Real runtime varies with temperature, age, and discharge profile. Use as a baseline for planning.

⚡ Built on electrochemical fundamentals – This calculator follows IEEE standards for battery capacity (IEEE 1188‑1996) and basic Coulomb counting. Data validation and examples are derived from real battery datasheets (Victron, Battle Born, Trojan). Last updated June 2026.

References: Linden’s Handbook of Batteries (4th Ed), Peukert’s law interpretation, and Renewable Energy UK battery bank sizing guide.