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How to Size a Power Inverter

Ever craved a hot cup of coffee while on a camping trip? Or perhaps you need to keep essential medical devices running during a power outage? These scenarios, and many more, highlight the increasing demand for portable power solutions.

Enter the power inverter: a versatile device that bridges the gap between DC battery power and the AC electricity we rely on for most appliances.

This article equips you with the knowledge to select the perfect inverter size for your needs. We'll delve into the world of inverters, understand how they function, and most importantly, guide you through the step-by-step process of calculating the exact wattage requirement for your situation.

Power Inverter Connected To A Truck At A Campsite Charging A Cooler

Demystifying Power Inverters

There's something about inverters that confuses people. Despite being commonplace, power inverters seem to be widely misunderstood.

At its core, a power inverter is an electronic device that transforms the direct current (DC) electricity stored in batteries into usable alternating current (AC) electricity.

Imagine it as an adapter that allows you to utilize the power stored in your car battery or a dedicated battery bank to run appliances typically designed for household AC outlets.

Inverters come in various sizes and capacities, measured in watts. The fundamental principle is that the inverter's wattage capacity needs to be sufficient to handle the combined wattage requirements of the appliances you intend to power. Selecting the right size inverter is crucial to ensure smooth operation and prevent overloading or damage to the inverter itself.


There are two main types of inverters to consider

Modified Sine Wave Inverters

These inverters produce a stepped approximation of a true sine wave, the standard AC waveform used in household electricity. While more affordable, modified sine wave inverters may not be suitable for all appliances. Some sensitive electronics and appliances with motors (like refrigerators or power tools) might not function correctly or may even be damaged when connected to a modified sine wave inverter.

The real benefit of modified sine wave inverters is their lower price point.

Pure Sine Wave Inverters

These inverters generate a much cleaner AC output that closely resembles a true sine wave. This makes them ideal for powering any appliance, including sensitive electronics and those with motors.

While typically more expensive than modified sine wave inverters, pure sine wave inverters offer greater versatility and reliability.

Power Inverter Connected To A Truck Battery At A Construction Site

How Inverters Fit into an Electrical System

A basic inverter setup consists of three key components:

1. Battery Bank

This is the DC power source that provides the energy for the inverter to convert. Battery banks can be standalone units or a series/parallel combination of deep-cycle batteries, designed for discharging and recharging cycles. The voltage (usually 12V, 24V, or 48V) of your battery bank needs to be compatible with the input voltage of your chosen inverter.

2. Inverter

This is the electronic device that performs the DC to AC conversion. Inverters come in various sizes and capacities, with features like surge protection, overload protection, and battery low-voltage cut-off.

3. Appliance Connection

The inverter provides standard AC outlets that you can plug your appliances into, just like you would at home.

The wattage capacity of the inverter dictates the total amount of power (measured in watts) it can deliver to your appliances. Let's explore how to calculate the right inverter size to ensure your appliances run smoothly and safely.

Power Inverter On A Table In A Home In Front Of A Window

Calculating the Right Inverter Size: A Step-by-Step Guide

Selecting the right size inverter requires careful consideration of the appliances you intend to power. Here's a step-by-step approach to ensure you get the perfect match.

Step 1: Identify Your Wattage Requirements

The first step involves understanding the power consumption of the devices and appliances you plan to use with the inverter. This information is usually printed on a label on the appliance itself, often denoted in watts (W) or Volt-Amps (VA). It's important to differentiate between two key wattage ratings:

  • Continuous Wattage: This represents the amount of power an appliance typically uses during normal operation.
  • Surge Wattage: This is the initial burst of power an appliance needs to start up. Surge wattage can be significantly higher than continuous wattage, especially for appliances with motors (like refrigerators or air conditioners).

When selecting an inverter, prioritize the continuous wattage rating of your appliances. However, it's also wise to consider the surge wattage requirements, especially if you plan to run multiple appliances simultaneously during startup.

Step 2: Calculate Total Wattage Load

Once you have identified the continuous wattage ratings of your appliances, add them together to determine the total wattage load. Here's a formula to simplify this process:

Total Wattage Load = Wattage of Appliance 1 + Wattage of Appliance 2 + ... + Wattage of Appliance N

For example, if you plan to run a laptop (75W), a portable TV (50W), and a small fan (25W) simultaneously, the total wattage load would be:

Total Wattage Load = 75W + 50W + 25W = 150W

Step 3: Apply a Safety Factor (Optional)

It's recommended to incorporate a safety factor into your calculations to account for unexpected power surges or the potential addition of appliances in the future. A typical safety factor can range from 20% to 30%.

Here's a formula to calculate the adjusted wattage after applying the safety factor:

Adjusted Wattage = Total Wattage Load x Safety Factor (as a decimal)

Let's say you decide on a 20% safety factor. In the previous example, the adjusted wattage would be:

Adjusted Wattage = 150W x 1.2 (20% as a decimal) = 180W

Step 4: Consider Inverter Efficiency

No inverter is 100% efficient. A portion of the DC power is lost during the conversion process to AC power. Most inverters have an efficiency rating between 80% and 90%.

To ensure your inverter can handle the adjusted wattage load, you need to factor in its efficiency. Here's a formula to calculate the required inverter capacity:

Required Inverter Capacity = Adjusted Wattage / Inverter Efficiency (as a decimal)

Let's assume you're considering an inverter with an 85% efficiency rating. Using the adjusted wattage from the previous step:

Required Inverter Capacity = 180W / 0.85 (85% as a decimal) = 211.76W

Rounding Up: Since inverter capacities come in discrete values (e.g., 200W, 300W), it's advisable to round up the required inverter capacity to the nearest available size. In this case, the ideal inverter for this scenario would be a 250W inverter.

3 Additional Considerations When Sizing a Power Inverter

1. Continuous vs. Surge Power Needs

Some appliances, like power tools or refrigerators, have a higher surge wattage during startup compared to their continuous operating wattage. If you plan to run such appliances, consider an inverter with a capacity that can handle the peak wattage requirement.

2. Matching Inverter Input Voltage

Ensure the input voltage of your chosen inverter matches the voltage of your battery bank. For example, if you have a 12V battery bank, you'll need a 12V inverter.

3. Inverter Features

Consider features like overload protection, battery low-voltage cut-off, and USB charging ports when selecting your inverter.

By following these steps and considering these additional factors, you can confidently choose the perfect inverter size for your power needs.

Additional Tips

1. Consider future needs: If you think you might add more appliances in the future, factor that potential increase into your wattage calculations.

2. Think about your power source: Inverters are often used with batteries. Make sure your battery has enough capacity to run the inverter for your desired duration.

Consult a qualified electrician if you have any doubts about sizing an inverter for your specific application.

By following these steps, you can choose a power inverter that efficiently meets your needs and avoids overloading or malfunction.

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