How to calculate the perfect battery bank size for your campervan

Welcome to this comprehensive guide to calculating the perfect sized battery bank for your campervan, written by our friends at nohma.com.

Anyone who's spent an extended period in a campervan will know that having a well-designed electrical system is the backbone of a smooth and enjoyable experience. And, at the centre of this, is choosing the correct battery bank. 

On one hand, an undersized battery bank can lead to insufficient power supply, forcing you to compromise on your energy usage and often leaving you in the dark (quite literally). Conversely, oversizing your battery bank can add unnecessary cost and weight to your van build. Since it's tricky to upgrade your battery bank further along the line, as used batteries become incompatible with fresh ones, it's essential to size your battery bank right the first time. 

This guide will simplify this daunting process with a practical, step-step method.  Engineers at Nomadic Energy will walk you through the essentials of calculating the perfect battery bank size for your campervan, explaining key concepts, tackling common pitfalls, and offering practical examples to help you nail this aspect of your electrical system.

If you want to learn a bit more about LiFePO4 batteries before sizing your battery bank, have a look at our recent blog post, What is a LiFePO4 battery? 

Understanding your campervan's energy requirements

What affects your power consumption?

Depending on whether you’re a full-time van lifer, a weekend warrior, or somewhere in-between, your power requirements will be different. Put simply, your power usage is directly determined by the type, and number of appliances you have, and how often you use them. 

For instance, to frequently use power hungry appliances such as induction hobs and air conditioning units, you’ll either need a serious power bank, or regular shore power connection. On the other hand, occasional use of smaller devices such as LED lights and USB chargers can be supported by a relatively small battery, enabling you to go off-grid for an extended period.

What are amp-hours, and why are they important?

To quantify the amount of power a battery will supply, battery manufacturers give a capacity rating in amp-hours. Amp-hours, often abbreviated as Ah, are a unit of electrical charge and are a measure of the amount of a current a battery can deliver over a certain time. One amp-hour indicates that one amp of current can be delivered for one hour. So, for example, a 100Ah battery could deliver a steady current of 5A for 20 hours.

Why is this important? Well, for one, it gives you an idea of how long a battery could power your electrics. After doing a few calculations to find your daily energy usage in amp-hours (read on for these!), you can easily choose a campervan leisure battery bank that fits your usage needs. 

How to calculate the battery bank size for your campervan?

To accurately size a leisure battery bank, make sure you consider three main factors.

Firstly, your daily power usage. As mentioned above, this depends on the appliances you use and how long you use them each day.

Secondly, decide how long you want your battery bank to last between charges. For instance, if you're spending a lot of time off-grid, you’ll need a larger battery than someone who spends most of their time on a campsite.

Finally, consider the usable capacity of your batteries, i.e. depth of discharge (DoD). This factor is affected by the type of battery you pick, and how frequently you’ll use them.

Step 1: Work out your power requirements

To begin with, work out your power requirements. To figure out the appropriate battery size for your campervan, you need to first calculate your daily energy consumption in ampere-hours (Ah). Begin by listing all the appliances you plan to use in your campervan along with their power consumption. 

To give you a general idea, we've provided an estimate of power usage for some common appliances below. However, since this can vary significantly between appliances, we recommend consulting a product label or manual for a more precise figure.

Set out as icons with wattage labelled 

• LED Spotlight - 3W
• Phone charger - 10W
• DAB radio - 10W
• iPad charger - 20W
• Laptop - 30W
• Handheld games console - 40W
• Projector - 45W
• Games console - 150W 
• Blender - 600W
• Toaster - 800W
• Low power kettle - 1200W
• Hair Dryer - 1600W
• Induction hob - 1600W

Once you’ve got the power consumption for each appliance, you need to convert from watts to amps. Simply divide each power consumption figure by your system voltage, i.e. (12V, 24V or 48V) to find the current draw.

Current (Amps) = Power (Watts) / Voltage (Volts)

Then, multiply this current draw by the hours per day that you’ll use each product.

Daily power usage (Ah / Day) = Current (Amps) x Usage (Hours)

If you do this like we have in the table below, you can easily total up the amp hour per day usage of each appliance to find your total daily power usage.

Example

Step 2: Work out how frequently you can charge your batteries

The next step involves working out how frequently you can charge your batteries. Multiply your daily amp hour usage by the number of days that you need your batteries to last. For full-time, off-grid vanlife, we recommend a three day power reserve, to account for cloudy days without driving. Or, if you often find yourself at a campsite or you’ll mainly use your van for weekend trips, then one to two days should be sufficient.

Multiplying your total daily power usage by the ‘days off-grid’ figure gives the total usable capacity that you need from your battery bank.

Usable capacity (Ah) = Daily power usage (Ah / day) x days off grid

Example

Let’s plug our example of 57.7Ah used each day into this equation, and say we require three days of power storage. This gives us:

57.7 Ah/day x 3 days = 173.1Ah usable capacity required

Step 3: Account for usable capacity

The final step is to account for the usable capacity of the battery. Completely discharging a leisure battery can permanently damage it, especially with lead acid. So, it's vital to only partially discharge your battery. The safe depth of discharge (DoD) varies between battery type and use case, and we should account for it when choosing a battery bank. 

If you're living in your van full-time and want to preserve your batteries for as long as possible, then we recommend regularly discharging lithium and AGM batteries by up to 90% and 60% respectively. If instead, your van sees a more part-time usage, and you’re happy to swap a lower cycle life for a larger capacity, then we recommend a 95% and 80% DoD for lithium and AGM.

 
To account for this DoD in your battery bank calculation, divide your required usable capacity by the depth of discharge.

Example

Let’s say that in our example, we want to use lithium batteries and discharge them by 90%. The necessary size for our battery bank would then be:

173.1Ah / 0.9 = 192.3Ah

Finally, you can look for batteries with the closest total capacity to your needs. In our example, the 230Ah Fogstar Drift would give us 3.6 full days off-grid without recharging.

It's clear that correctly sizing your campervan’s battery bank is critical for a hassle-free experience. Though it might seem a bit complicated at first, following Nohma's detailed, step-by-step guide can make the process more manageable and clear. You now have the necessary tools to effectively calculate your power needs, ensuring you never have to compromise on your energy usage or carry unnecessary weight. 

Still confused about how to size your campervan battery bank?

Nohma offer a free electrical design service which makes off-grid energy simple. They employ engineers who truly understand the systems they are designing. So, if you’re sure where to start with your campervan electrical system, get in touch and they'll design a system that’s perfect for your needs.

They supply every single component required for your system, and also provide a detailed wiring diagram and installation guide, so you can install your system easily. Their engineers also provide ongoing support and guidance throughout the install process.

Plus, they include the Fogstar Drift lithium leisure batteries in all of their lithium campervan electrical systems.

A campervan electrical system designed by Nohma

Battery bank sizing FAQs

What is a leisure battery?

Leisure batteries are designed specifically to power electrical appliances in campervans, boats and other off-grid electrical systems. Unlike car or van starter batteries, which deliver a high current burst of power over a short period, leisure batteries deliver a lower, steady current over an extended period of time. Several leisure battery types are available, but LiFePO4 and AGM are the most popular. 

What are amp-hours?

Amp-hours (Ah) are a measure of electrical charge that indicate the amount of current a battery can deliver over a certain time. It helps you understand how long a battery could power your electrics. A battery storing one amp-hour could deliver one amp of current for one hour. 

How do you calculate leisure battery size?

When sizing a leisure battery bank, you need to consider your power usage, the number of days power reserve required and your batteries usable capacity.

You can calculate your power usage by multiplying the current draw (which you find by dividing the power in watts by your system voltage) by the hours per day you use the product. Add up the daily usage for all your appliances to get your total daily power usage.

Next, multiply your daily usage by the number of days of power storage you require. This depends on the amount of time you spend away from a campsite. For full-time off grid, we recommend three to four days power reserve, to account for cloudy days with no solar charging. For more of a mixed use, one to two days can see you through between campsites. 

Finally, divide this required capacity by your intended depth of discharge. This depends on the type of leisure battery you choose, and how long you need it to last. However, a good rule of thumb is 80% DoD for lithium and 60% for AGM.

What type of batteries should I use for my leisure battery bank?

This depends on your power requirements, weight allowance and budget. If your budget allows, then LiFePO4 batteries are a great option, thanks to their long lifespan and light weight. Plus, they have a larger depth of discharge (DoD) compared to lead-acid alternatives, including AGM. 

Alternatively, if you're on a tighter budget, AGM batteries are your best option. They’re the best type of lead-acid leisure battery, with a longer lifespan and DoD than flooded lead-acid.

What is the depth of discharge (DoD)?

The depth of discharge (DoD) refers to the percentage of a batteries total capacity that has been discharged. Discharging a battery too much can damage it and reduce its lifespan, so it’s important to only partially discharge leisure batteries. An advantage of LiFePO4 batteries is that they have a much greater DoD than AGM batteries.

Can I upgrade my battery bank later?

One of the most common mistakes people make with their off-grid electrical system is trying to upgrade their battery bank after they’ve completed their van build. Unfortunately, you can’t just add another leisure battery to your battery bank. This is because after a few months of use, used batteries become incompatible with fresh ones, leading to a reduced lifespan. It's therefore important to size your battery bank correctly from the start. To learn a bit more about this, and other common mistakes, have a read of Nomadic Energy's great blog the top 15 mistakes to avoid with your campervan electrical system. 

Special thanks to our friends at Nohma

Their team of engineers design 2,000+ systems every year – that’s more than anyone else in the industry. Take a look at the Nohma website for more information.