How to calculate the solar array for your campervan
Solar power is a great source of free electricity, but it can be difficult to choose the right size for your campervan. In fact, a recent survey carried out by Nomadic Energy found that 82.5% of DIY van converters would fail to correctly size their solar array.
This part of your campervan’s electrical system is difficult to get right because there are so many factors to consider. The following article will explain how to take those important variables into account, then we’ll walk you through some basic calculations.
From the rust-bucket, surf-bum day-van, to the high-spec, luxury Winnebago - every campervan needs something different from its electrical system. The solar array is an important part of that system, and one that can be tailored to your needs.
To help you choose the right solar panel for your campervan, here’s a list of the most important considerations you’ll need to take into account.
How much time will you spend off-grid?
Whenever you’re not connected to mains hook-up, you’ll be relying on your leisure battery and solar panels to provide the power. As you draw power from the leisure battery, the solar panels will work to replace that charge. If you draw more power than the solar panels are producing, you will eventually drain your leisure battery and need to find an alternative source of charge.
For this reason, the size of your solar array will have an impact on how long you can last off-grid. If you want to last for one or two days without electric hook-up, a small solar panel will top up your leisure battery and give you a little extra power. If you want to last off-grid for much longer, you’ll need a solar array which can match your daily power requirements.
If you’ll spend your time fully off-grid with no access to hook-up, you’ll need a larger solar array to supply your power. (Photo: ClimbingVan)
How big is your battery bank?
To meet your daily power requirements, your battery bank and solar array need to work together. For a practical, well-balanced system, the size of the solar array should complement the leisure battery.
If you have a small battery bank and a relatively small daily power usage, you don’t need a large solar array. If your solar array is too big, you will be generating more solar power than you can store or use instantaneously.
Similarly, it would be impractical to install a large battery bank with only a small solar array. If you could be generating more solar power to keep your leisure battery charged, why bother with such a large battery bank? Batteries are heavy, expensive, and take up a lot of space inside your van. With more solar power, you can install a smaller battery bank, saving money and weight.
As a general rule, you should try to keep the size of your solar array and leisure battery well-balanced.
Where and when will you be using your van?
Harnessing power from the sun is a great concept in theory, but in reality, the amount of power you can generate is heavily reliant on when and where you’ll be using your campervan.
When - The time of year
In the winter months, the sun stays relatively low in the sky, and so hits the panel at an acute angle throughout the day. In the summer, the sun rises higher in the sky and hits the panel more directly.
When the sunlight hits the panel at an acute angle, much less energy is transferred into electricity. For this reason, your solar panel will be able to generate more electricity in summer than it can in winter.
As well as this change in angle, the number of daylight hours also changes throughout the year. In the summer, the longer days have the potential to generate more watt-hours to charge your leisure battery.
If you plan to use your campervan all year round, you’ll need a bigger solar array to keep you powered throughout the winter.
Where - Your latitude
Your latitude describes how far you are from the equator to the north or to the south. This will affect your daily solar yield because the sun rises higher in the sky in countries that are near the equator.
For example, with a 200W solar panel, the forecasted daily yield might be only 490Wh in Edinburgh, but 830Wh if it was in Barcelona.
Scotland (left) vs Spain (right) will give you a pretty different yield from your solar array! (Photos: ClimbingVan)
Finally, don’t forget that the weather will also have a big impact on your daily solar yield. For example, even a light cloud coverage could reduce your solar input by 50%. If you plan on spending most of your time in countries with unpredictable sunshine (like the UK!), you might want to slightly over-spec your solar array to make sure you’re covered, even on those bad weather days.
What other methods of charging do you have?
If you incorporate one (or both) of these methods into your electrical system, you won’t have to rely so heavily on your solar panels for charging power.
Installing a split-charging system is a really good backup option for those who want to travel off-grid. After a patch of poor weather and low solar production, you’ll have the reassurance that you can always top up your batteries as you drive.
How much space do you have on your roof?
This is ultimately the limiting factor when it comes to choosing the size of your solar array. If you have a large campervan, it might seem like you have loads of space on your roof, but once you’ve incorporated skylights, vents and roof racks, it can be difficult to fit everything in.
To make the most of your roof space, try to plan everything out before you start cutting holes! This way, you won’t have to compromise on your solar array because you’ve run out of space.
You now have a good base of understanding for calculating your solar panel array, but before you go any further, it might be useful to know about some common mistakes.
Over-speccing or under-speccing your solar array
Over or under-speccing your solar array is easily done and is a really common mistake to make.
In 2021, Nomadic Energy, a company that designs electrical systems for campervans, conducted a survey to gather information from DIY van converters. In this survey, customers were asked to estimate how many watts of solar they thought they would need. Nomadic Energy found that 82.5% of customers failed to correctly size a solar array that would meet their needs, even with a 15% margin for error!
Most of the customers underestimated how much solar power they would need. Without Nomadic Energy’s help, they would have run out of power much sooner than they wanted and would have spent more time on campsites to recharge.
On the other hand, many of the customers over-specced their solar array. In this case, they would have wasted money on a system which would produce more energy than they would use when they were off-grid.
Not planning your roof layout
Through their work with DIY van converters, Nomadic Energy also found that poor planning was a common bump in the road. Before calculating their solar needs, some customers had already installed their roof vents or fans which left limited space for their solar panels.
To avoid this problem, try to plan everything you’re going to need on your roof before starting any installation. This way, you’ll make the most of your roof space and be able to prioritise your solar panels.
Using poor-quality cable seals
The most stressful part of a van conversion comes when you need to cut holes in your van and hope against hope that you’ve sealed them up securely! A suspicious ‘drip, drip, drip’ is not a sound you want to hear after you’ve installed your solar panels, packed your walls full of insulation, then carefully sealed it all up with cladding!
Using the right cable entry housing is a really important step in getting a waterproof seal around your newly cut hole. Scanstrut has a variety of cable seals on offer, and they have designed one which is perfect for solar panel installations. The DS-HD6-BLK has two water-tight cable ports, perfect for the positive and negative cables of a solar panel. The waterproof 3M gasket makes it easy to install and you won’t have to faff around with messy sealant.
The low-profile shape makes the connection look really sleek and the high-impact plastic is hard-wearing and UV-stable, so it won’t become brittle and faded in the sun.
The basic calculations
Now you know all the important considerations, here are the basic steps to calculate the solar array for your campervan.
Step 1. Calculate how many amp-hours you will use per day
You need to work out how much energy you think you’ll use per day. For each device you need to power, find the current rating and multiply that by the number of hours you’ll use it each day.
Number of amp-hours = Amps x hours
Repeat this step for every component in your campervan, then add them up to find your daily Ah usage.
Daily amp-hour usage = Sum of all components
Step 2. Decide how many days you want to last off-grid
The more solar power you have, the longer you will be able to last off-grid without having to find an electric hook-up. Once you’ve decided how long you need to last off-grid, you’ll know how many amp-hours you need to keep you powered. Simply multiply the number of days with your daily usage.
Required off-grid amp-hours = No. of days x Daily Ah usage
Step 3. Spec your leisure battery and solar array to match you’re required amp-hours
You can meet your daily power requirements with a combination of stored energy in your batteries and energy from your solar panels. The available charge in your batteries will keep you powered for a limited amount of time, but your solar panels will replace that charge whenever the sun is shining.
Solar panels are sized by their rated wattage: the more watts you have, the more amp-hours you’ll be adding into your battery bank every day. However, to estimate the daily power production from your panels, you can’t simply use the rated wattage in your calculations. The wattage of the panel is a theoretical, best-case-scenario number that will vastly overestimate the capabilities of the panel.
For a better estimation, you can use the Victron calculator. For any given combination of solar panels, Victron will provide a forecasted daily yield for the country you’ll be travelling in. Try it out with one solar panel, then play around with the settings to find the combination you’ll need to meet your daily requirements.
Written by Charlie Low & Dale Comley
Charlie & Dale (aka ClimbingVan) are authors of The Van Conversion Bible, the bestselling van conversion book. They also run Nomadic Energy, a free electrical design service for campervans, boats, and off-grid houses. Their team of engineers use a bespoke algorithm to carefully calculate exactly what’s needed for any off-grid electrical system.