Calculating Battery Size for Your Caravan or 4WD

If you're preparing to travel off-grid in your caravan, 4WD, boat, or RV, chances are you'll need an auxiliary battery to power your devices and appliances. A battery allows you to stay off-grid for longer periods, but how do you know which battery size to choose? In this blog, we'll share our bombproof method to calculate battery size for your 4WD or caravan setup.

What is a battery?

A battery is a device we can store energy in. Stored energy in batteries can be discharged to power electrical devices when you need it. Batteries have a maximum capacity that is expressed in ampere-hours (Ah), which is a measure of the amount of electrical energy the battery can store. 

There are many different types of batteries, including:

• Lithium (LiFePO4)
• AGM/Gel (Lead-acid)

Lithium batteries have significantly decreased in price over the past five years, making them a highly recommended option for those looking to add an auxiliary battery to their caravan or 4WD. Their long lifespans and overall reliability make them an excellent choice. 

When designing a system with lithium batteries, you must consider the following factors:

• Maximum discharge and recharge ability vs recommended
• Temperature rating
• Series/Parallel ability
• Installation location (outside or under caravan mounts?)

Read our comprehensive lithium battery guide below.

Read more: A guide on auxiliary batteries

What do you need a battery for?

Batteries are a versatile solution for storing energy, enabling you to have power on hand even when you're away from a power outlet. Batteries can be charged using various sources, including standard power outlets, solar panels, and even your vehicle's battery.

Read more: Benefits of a caravan solar system

Stored energy from batteries can be used to run appliances and devices while you're off-grid in a caravan, 4WD, RV, or boat, providing you with the convenience of electricity wherever you go. Essentially, batteries provide the freedom to stay powered and maintain your lifestyle during extended off-grid adventures, ensuring that your devices and appliances remain operational no matter how remote your location. 

How do you calculate what size battery you need?

To calculate what size battery you need for your caravan or 4WD, you need to determine how many days of autonomy you want and your daily energy load. 

This is less daunting than it sounds - we'll explain exactly how to determine days of autonomy and energy load soon, but all you need to do is consider the following:

• How much time you like to spend off-grid
• How remote you travel (how far away from reliable power you are)
• What appliances and devices do you use while travelling

Every powered device in your vehicle will tell you its electrical load draw on the label or packaging. Dedicate an hour to list each device and record its load draw until you have a complete overview of your appliances and devices. Then, you can complete the final calculation to determine which size battery you need for your adventures. Simple!

Read more: Why shunts and battery monitors are critical to your off-grid system

What is autonomy?

Autonomy refers to the duration your system and battery can operate without needing additional charging. 'Days of autonomy' indicates the number of days your battery can power essential appliances without receiving a charge. 

Read more: DC-DC chargers

While this may not be a daily concern when you're off-grid (since it's unlikely you'll often be unable to charge your auxiliary battery from either solar or your vehicle's battery), it's still crucial to consider when sizing your battery. In the event of poor weather conditions or equipment failure, you'll want assurance that your auxiliary battery has enough capacity to keep your essential appliances running until you can access a power outlet. 

Days of autonomy is the number of days the battery can power essential appliances without receiving a charge.

What is energy load?

Energy load draws refers to the amount of power a device or system consumes while operating. It is typically measured in watts (W) or kilowatts (kW), depending on the scale. 

Load is the total power demand from all the devices and appliances used. Each device contributes to the overall load. 

Draw refers to consuming power from a source, such as a battery.

Therefore, the energy load draw is the total amount of electrical power required by all devices at any given moment. 

Energy load draw is the total amount of electrical power required by all devices at any given moment.

Step 1: Determine how many days of autonomy you want

Think about how remote you travel and how long you typically plan to stay off-grid. The more remote your location, the more likely you'll need a larger battery capacity to ensure you have enough power to sustain your trip until you can access a reliable power source. 

Days of autonomy are also dependent on the chemistry of the battery chosen, as different battery types have varying levels of maximum depth of discharge rules. For example, lithium has a higher depth of discharge compared to AGM, meaning that you can use a larger portion of the lithium battery's total capacity without affecting its performance or lifespan. 

As a general guide, we recommend the following days of autonomy:

• Lithium: A minimum of two days of autonomy is recommended, but three days is ideal.
• AGM/Gel: We recommend at least 3-4 days of autonomy.

Step 2: Figure out your daily load

Grab some scrap paper or a spreadsheet and list all of the devices and appliances in your vehicle that you plan to use while off-grid. Each device should have its load draw written on either the label or its packaging, and it will be in either Amps or Watts. 

This toaster's load is 1350 - 1600W. The range is indicative of how many pieces of bread you toast at the same time - if you use all four toaster slots at once, the toaster will draw 1600W. 

Amps are easy to determine as you can just multiply that number by the number of hours you will use it per day. The resulting figure is your amp-hour requirement. If all your devices are expressed in Amps, then all you need to do is add each device's daily Ah requirement together to calculate your total daily energy load. 

If you have devices that express load in Watts, follow the below equation to convert to Ah and calculate the total daily energy load. 

Watts / Voltage = Amps

Amps x (No. of hours used per day) = Ah

Add Ah of all devices together = total daily energy load

Write down your total daily energy load so you can remember it. 

Step 3: Calculate your battery bank size

This is the easiest step. Now that you know how many days of autonomy you need and what your daily load is, you just need to multiply these numbers together to calculate your battery size. 

The equation looks like this.

Load x Autonomy = Battery size

For example, if your total daily load is 100Ah and you want a lithium battery:

100Ah x 3 days = 300Ah lithium battery capacity

This is a completely bombproof way to size your batteries correctly and ensure you always have enough battery capacity to keep you powered wherever you go.

Springers Solar | Your Trusted 12V/24V Components & Accessories Supplier

We only stock the best brands in the extra-low voltage industry for the best prices - you can trust that when you buy gear from Springers, you are setting up your system to perform long-term. We don't waste your time or our time with low-quality products. 

With an in-house team of 12V/24V experts, we can offer advice and support as you set up your off-grid, off-road, or DIY battery system. We are Queensland-based and house our stock, so check out our online store here or visit us at Lawnton or Capalaba for in-store help.