How to Calculate Amps Per Hour: A Complete Guide to Electrical Measurements

From computers and smartphones to household appliances and electric vehicles, electricity powers nearly every aspect of modern life. Whether you’re checking appliance consumption, evaluating battery capacity, or sizing circuits, electrical calculations are an essential tool. One of the most common questions people ask is: how do you calculate amps per hour?

This guide breaks down the terminology, explains the relationships between volts, watts, amps, and kilowatt-hours, and provides step-by-step methods for making accurate calculations.

Understanding the Basics: Amps, Volts, and Watts

Before diving into calculations, let’s review some fundamental electrical terms:

• Amps (A): A measure of electrical current, representing the flow rate of electrons. One ampere equals about 6.24 × 10^18 electrons passing a point in one second.

• Volts (V): The electrical potential or “pressure” pushing electrons through a circuit. Higher voltage means a stronger push.

• Watts (W): A unit of electrical power. One watt equals one joule of energy per second.

• Kilowatt-hour (kWh): A unit of energy used by utility companies, representing 1,000 watts consumed over one hour.

These values are interconnected through Ohm’s Law and the power formula, forming the foundation of all electrical calculations.

The Truth About “Amps Per Hour”

It’s important to clarify a common misconception:

• Amps measure the rate of current flow.

• Amp-hours (Ah) measure capacity over time.

In other words:

• Amps = speed (rate of flow)

• Amp-hours = distance (capacity over time)

For example, a 100Ah battery theoretically provides:

• 100 amps for 1 hour

• 50 amps for 2 hours

• 10 amps for 10 hours

• 1 amp for 100 hours

In real-world conditions, battery performance is not perfectly linear due to factors like Peukert’s Law, temperature, and efficiency losses.

Converting Watts, Kilowatts, and Kilowatt-Hours

Watts to Kilowatts

Since 1 kW = 1,000 W:

• A 1,500W heater = 1.5 kW

• A 500W device = 0.5 kW

• A 3,000W machine = 3 kW

Kilowatts to Kilowatt-Hours

To calculate energy consumption:

kWh = Power (kW) × Time (hours)

Examples:

• A 2 kW heater running for 3 hours consumes 6 kWh.

• A 100W bulb (0.1 kW) running for 10 hours uses 1 kWh.

Converting Kilowatt-Hours to Amp-Hours (Ah)

Amp-hours represent how much charge a battery can deliver. To calculate:

Ah = (kWh × 1,000) ÷ Voltage (V)

Examples:

• A 1 kWh battery at 12V = 83.3 Ah

• A 5 kWh battery at 48V = 104.2 Ah

This means a 104Ah battery could provide 104A for one hour or 10.4A for 10 hours. In practice, efficiency losses reduce actual usable capacity.

Converting Amp-Hours to Watt-Hours (Wh)

To find stored energy in watt-hours:

Wh = Ah × V

Examples:

• A 12V, 100Ah battery = 1,200 Wh (1.2 kWh)

• A 24V, 50Ah battery = 1,200 Wh (1.2 kWh)

Reversing the calculation:
Ah = Wh ÷ V

Calculating Current in DC and AC Circuits

DC Power

For direct current (DC) systems:

Amps (A) = Watts (W) ÷ Volts (V)

Example: A 120W load at 12V draws 10A.

AC Power

For alternating current (AC), you must include Power Factor (PF):

Amps (A) = Watts (W) ÷ (Volts (V) × PF)

Example: A 1,500W air conditioner at 120V with PF = 0.8 draws 15.6A.
Without correcting for PF, the result would be 12.5A—too low for accurate circuit sizing.

Three-Phase Power Calculations

In three-phase systems, the formula includes the square root of three (1.732):

Power (W) = 1.732 × Volts (V) × Amps (A) × PF

To solve for current:

Amps (A) = Power (W) ÷ (1.732 × Volts (V) × PF)

Example: A 30kW motor at 400V with PF = 0.9 requires 48.1A.

Quick Reference Conversion Formulas