Amperage Calculator

How to Calculate Amps Using Ohm's Law

Ohm's Law is the foundation of every electrical calculation. It describes the relationship between voltage (V), current (I, measured in amps), and resistance (R, measured in ohms). The core formula is:

By rearranging Ohm's Law, you can solve for any of the three values:

• Calculate amps: I = V ÷ R
• Calculate voltage: V = I × R
• Calculate resistance: R = V ÷ I

In practical van build wiring, you'll most often use the power formula — a close relative of Ohm's Law — to calculate amps from watts and volts:

For example, a 600-watt microwave running on a 120V circuit draws 5 amps (600 ÷ 120 = 5A). That same microwave pulling power from a 12V battery bank would draw 50 amps (600 ÷ 12 = 50A) — a critical difference when sizing your wires and fuses.

Why You Need an Amperage Calculator for Your Van Build

Knowing the amperage of every device in your van is not optional — it's the starting point for a safe, functional electrical system. Here are the key reasons you'll need to calculate amps during your build:

1. Sizing Your Wire Gauge

Every wire has a maximum current rating. Running too many amps through an undersized wire creates heat and is a fire hazard. Once you know the amperage of a circuit, you can select the correct wire gauge using an ampacity chart. For example, a 20-amp circuit typically requires 12 AWG wire, while a 40-amp circuit needs 8 AWG or larger.

2. Selecting the Right Fuse Size

Fuses protect your wiring from overcurrent. To pick the correct fuse, you need to know the maximum amps the circuit will draw. A fuse that's too small will blow constantly; one that's too large won't protect your wires. The standard practice is to size your fuse at or just above the expected amp draw, but always below the wire's rated ampacity.

3. Sizing Your Battery Bank

Once you know the amps each device draws and how long you'll run it daily, you can calculate amp-hours — which directly determines how large your battery bank needs to be. Use our calculator above to find amp-hours, then check out our energy demand calculator to size your full battery system.

4. Designing Your Inverter Setup

An inverter converts 12V DC battery power to 120V AC for household devices. The amp draw on the 12V side is much higher than on the 120V side. For instance, a 1,500W load on the AC side pulls 12.5 amps at 120V, but the inverter draws roughly 125 amps from your 12V battery bank. Knowing these numbers is essential for sizing cables between your battery and inverter.

Understanding The Basics Of Electricity: Watts, Volts and Amps

Before using an amperage calculator, it's important to familiarize yourself with the electrical basics. Understanding the basic principles of power and electricity are vital to success in your DIY van build process.

Volts

Voltage is the electrical pressure that pushes electrons through a circuit. Think of it like water pressure in a pipe: the higher the voltage, the stronger the push. Measured in volts (V), this force is created by the difference in electrical potential between two points.

A standard household outlet in the US provides 120 volts, while a dryer outlet typically provides 240 volts. Most DIY van builds will have an auxiliary battery bank of 12 or 24 volts. The voltage determines how much energy each electron carries as it flows through the system.

Amps

Amperage measures the volume or quantity of electrical current flowing through a circuit. If voltage is like water pressure, then amps are like the amount of water flowing through the pipe. Measured in amperes or amps (A), this tells us how many electrons are moving past a point per second.

A typical household circuit might be rated for 15 or 20 amps. When too many amps flow through a wire, it can overheat, which is why circuit breakers exist to cut power when the current exceeds safe levels.

Watts

Wattage represents the actual power being consumed or produced, combining both voltage and current. It's calculated by multiplying volts by amps (Watts = Volts × Amps). Measured in watts (W), this tells us the rate of energy transfer. A 60-watt light bulb uses 60 watts of power, meaning if it runs on 120 volts, it draws 0.5 amps of current.

Wattage is what appears on your electric bill because it represents the actual energy being used. Higher wattage means more power consumption and typically more work being done, whether that's producing light, heat, or running a motor. Understanding watts helps you estimate energy costs and ensure your electrical system can handle your devices.

Understanding Energy Storage Measurements

Amp Hours

Amp hours (Ah) measure the capacity of a battery to deliver current over time. This unit tells you how many amps a battery can provide for one hour before it's depleted. For example, a 100Ah battery can theoretically deliver 100 amps for one hour, 50 amps for two hours, or 10 amps for ten hours. Think of amp hours like the size of a fuel tank: a larger Ah rating means the battery can power devices longer before needing a recharge. This measurement is commonly used for car batteries, marine batteries, and solar power systems. However, amp hours alone don't tell the complete energy story because they don't account for voltage. A 12-volt 100Ah battery stores different total energy than a 24-volt 100Ah battery, even though both have the same amp hour rating.

Watt Hours

Watt hours (Wh) provide a more complete picture of energy storage by combining both voltage and current capacity over time. Calculated by multiplying amp hours by voltage (Wh = Ah × V), this measurement tells you the actual energy available. A 12-volt 100Ah battery contains 1,200 watt hours of energy. This is the standard measurement for comparing batteries with different voltages, making it easier to understand how long a battery will power your devices.

For instance, if you have a 1,200Wh battery and a laptop that consumes 60 watts, you can run that laptop for approximately 20 hours. Watt hours appear on portable power stations, smartphone batteries (often shown in milliwatt hours), and electric vehicle specifications. This measurement is more practical for consumers because it directly relates to how much work the battery can perform, regardless of its voltage configuration.