amp hour calculator

What Is an Amp Hour?

An amp hour (Ah) is a unit of electric charge commonly used to describe battery capacity. If a battery delivers 1 amp for 1 hour, that is 1 Ah. A 100 Ah battery can ideally supply 100 amps for 1 hour, 10 amps for 10 hours, or 5 amps for 20 hours, depending on conditions.

In practical battery sizing, amp hours are one of the most useful metrics because they directly connect to how long your loads can run.

Amp Hours vs Amps vs Watt-Hours

• Amps (A): Instant current draw at a moment in time.
• Amp-hours (Ah): Current over time (capacity).
• Watt-hours (Wh): Energy (power over time), which includes voltage.

You can convert between Ah and Wh if you know voltage:

• Wh = Ah × V
• Ah = Wh ÷ V

Formulas Used by This Calculator

1) From Current and Time

Ah = A × h

2) From Power, Time, and Voltage

Ah = (W × h) ÷ V

3) From Energy and Voltage

Ah = Wh ÷ V

4) Convert Capacity to Energy

Wh = Ah × V

How to Size a Battery Bank Correctly

Step 1: List your electrical loads

Write down each device and either its current draw (amps) or power rating (watts). Include realistic daily run-time hours.

Step 2: Calculate daily energy use

Sum total watt-hours or amp-hours per day. If your loads are mixed AC and DC, convert them into one common unit for cleaner planning.

Step 3: Account for losses and reserve

Real systems are not 100% efficient. Inverter losses, wiring losses, and charging inefficiency all matter. Add a reserve margin to avoid chronic under-sizing.

Step 4: Consider allowable depth of discharge

If you only use part of a battery’s rated capacity, divide required Ah by the usable fraction. For example, if you need 100 Ah usable and you only use 80% depth of discharge, plan for roughly 125 Ah rated capacity.

Real-World Example

Suppose a 12V setup runs a 60W device for 10 hours:

• Energy needed = 60 × 10 = 600 Wh
• Required Ah (ideal) = 600 ÷ 12 = 50 Ah
• At 90% system efficiency: 50 ÷ 0.90 = 55.56 Ah

So, a battery around 56 Ah minimum is needed in this simplified scenario, before adding reserve capacity and depth-of-discharge limits.

Factors That Affect Actual Runtime

• Temperature: Cold weather reduces available capacity, especially with lead-acid batteries.
• Battery age: Older batteries often hold less charge than their original rating.
• Discharge rate: High current draw can reduce effective capacity.
• Chemistry: Lithium batteries typically provide more usable capacity than lead-acid at similar nameplate ratings.

Battery Chemistry Quick Notes

Lead-Acid (Flooded, AGM, Gel)

Lower upfront cost, but typically lower usable depth of discharge and shorter cycle life.

Lithium Iron Phosphate (LiFePO4)

Higher upfront cost but generally better cycle life, lighter weight, and higher usable capacity percentage.

Frequently Asked Questions

Is a bigger Ah battery always better?

Not always. Bigger capacity can help runtime, but cost, weight, charging time, and physical space may become limiting factors.

Can I compare Ah across different voltages directly?

No. Convert to watt-hours first. A 100Ah battery at 24V stores about twice the energy of a 100Ah battery at 12V.

Should I include inverter efficiency?

Yes, especially for AC loads. If you skip inverter losses, battery capacity estimates are often too low.

Final Takeaway

An amp hour calculator helps you quickly estimate battery capacity and runtime, but the best designs also account for efficiency losses, battery chemistry, depth of discharge, and real operating conditions. Use the tool above as a solid first-pass estimate, then add practical safety margins for reliable performance.