When you are designing the electrical system of your camper van, how much solar and how many batteries to get is a common and reasonable question to ask. There are several schools of thought on this topic depending on who you talk to. The reality is that there is no one absolute right answer to this. A lot of it has to do with your budget, how you plan on using your camper and when/where will you be traveling. In this article, I will lay out multiple scenarios for you so you can find the solution that is best for you. If you are just looking for a quick rule of thumb and do not want to read the rest of this article, just scroll down and take a glance at the first section.
Table of Contents
- 3 Watts to 1 Amp Hour Rule
- What Is Your Budget?
- Basic Solar System and Battery ($400)
- Solar System and Battery System for Digital Nomads ($1,500)
- I Want To Run My Air Conditioner. Sure, But..
- Advanced Solar System and Batteries ($5,100)
3 Watts to 1 Amp Hour Rule
There you go. To be clear, we are talking about 3 watts of solar to 1 Ah of usable battery capacity in a 12-volt battery. Another way to look at it in absolute terms is 1 watt of solar to 4 Wh of usable battery capacity. They are identical numbers expressed in different units.
I would say that this rule can be safely broken if you know why and what you are doing. Some good reasons to break this rule are where on the earth’s latitude you normally travel and how often you drive to enable alternator charging. But for someone looking for build guidance, it is a good place to start. I would err on the side of having more solar than batteries if that is an option.
- Base your solar output capacity on the season with the least amount of sun – Considering solar panel only work when the sun is out, that eliminates 50% of the day when you average the amount of day and nights you have throughout the year. But to realistically size your solar panels for the amount of sun, you have to factor in the season with the least daylight. Without being too extreme, we can assume about 8 hours of usable solar charging sunlight during those days.
- Solar output will vary throughout the day – From these 8 hours of active charging, we are only realistically going to get about 3 hours of 80% (because 100% is very rare) average power output (240Wh) from your solar panels assuming they are angled appropriately. The remaining 5 hours would average at closer to 33% (165Wh) of its rated output.
- How I arrived at the ‘3-watts per 1-Ah’ ratio- That puts a 100-watt panel at a daily output rate at just about 405Wh per day. You won’t get this much every day and some days you might get more. That means on the average you can expect about 33 Ah of 12-volt battery output which gives us the 3-to-1 watts to Ah ratio.
You can stop here and go size your panels and batteries with just this information and be totally fine. But if you want to know more and are wondering exactly how that factors into your build, continue reading. In this article, I am presuming the end-user is living full-time in their camper and spending a majority of their time not connected to shore power.
What Is Your Budget?
Before we get into how to size your panels and batteries, let’s talk about your budget. Some of you can only afford the bare minimum while others might end up with a rolling mansion. Regardless of what your budget is, planning and designing the right electrical system is critical to make your life on the road more enjoyable.
On a slim budget, you might be limited to just using a single solar panel and a single battery. Try to arrange your budget so you can have at a very minimum one 100-watt panel paired a 100Ah of AGM battery with 50Ah of usable capacity. The ratios are not an ideal 3-to-1 but since they are the most popular sizes, their prices have been commoditized to represent best value in a budget build. With conservation and smart use of your electrical appliances, you can get by with these basics. It is wise to consider buying components that will allow you expandability when your budget allows in the future. Here are some products that I would recommend for this basic system:
Basic Solar System and Battery ($400)
This is a system for someone on a bare minimum budget and does not need a computer to work, as running even a small laptop can cause your electrical demands to rise significantly.
- 100-Watt Solar Panel: These panels can be found for under $100 brand new. They are reasonable in quality and should last many years with care.
- The next component is a charge controller. If you plan to expand your system in the future, buy at least a 30-Amp charge controller. If you can afford it, try to buy one that uses MPPT technology instead of PWM. They will be more efficient and allow you to run multiple panels in the future with less wires. However PWM charge controllers will be significantly cheaper as a start.
- Finally you need to run that power from the sun into a battery that can store it all for later use. That is how you have to think about it. Rarely will you be running any appliances directly from the sun. The battery is like a water bottle that you drink from and the sun is that fresh mountain spring where you go to fill it up. When you are on the tight budget, lithium batteries might not be a possibility yet. The good news is that when you are able to upgrade, your existing solar components should all still work. If you are wondering if now is a good time to look into lithium batteries for your camper, we went into a lot of details in this article. Here is a basic sealed lead-acid battery you might consider.
These components will make up a large part of your solar electrical system. You will need to choose the rest of the peripheral components discussed in this article. As of October 2020, you will spend roughly $350 to $400 depending on which charge controller you choose.
So just what will you be able to operate with this simple system? Well, a 100-Amp AGM battery will have about 50 amperes of usable electricity at 12 volts. I would recommend that in this case you do not use a permanently installed power inverter for any AC appliances. They are generally very power hungry and will drain this simple system quickly. You can however use a portable lighter-socket style inverter as needed like this one.
Let’s put together a scenario of your daily consumption in this system:
- LED lights: 6 hours of use at 4 watts will require 24Wh of power or 2Ah out of your 50Ah capacity
- Water pump: 4 minutes of cumulative daily use at 180 watts (15 amps at 12 volts) will require 9 Wh or 1Ah
- Compressor fridge: Average hour draw of 0.8 amps. That is roughly 20Ah per day.
- USB Phone Charging: A smartphone with a 3,200mAh battery charged daily requires on the average 12 Wh per full charge. 2 phones will use about 2Ah per day.
- Roof Fan: 2-ampere draw for 5 hours a day will cost you 10Ah. This will depend on where you are and how much air circulation you need. It is pretty typical that in warm weather, you are going to run your fan through the entire night to keep your camper comfortable.
As you can see in the above scenario where you are running a bare minimum of appliances for a life on the road you will be consuming 35Ah out of your 50Ah of total usable capacity. Because you can usually find 100-watt panels cheaper at a cost-per-watt basis, that is why it is my recommendation. However, to best utilize the extra power storage capacity in a 100Ah AGM, replacing that with a 150-watt panel would be a worthwhile upgrade if budget allows.
This will put you right at the “3-watts per 1-Ah rule”. This is not considering any other parasitic draw you may have in your system and definitely not using any power hungry devices like a laptop computer. Add in a small laptop you will be right at the limit of your 50Ah daily capacity.
Don’t Want A Permanent Solution? No Problem – It is important to note that whether you plan to install a permanent solar and electrical system or use a portable one, the concept is the same. If you want a simple off-grid power system and don’t have the expertise or desire to install it into your vehicle, a portable setup like this will run you about $600, gives you the same capabilities and will work just fine.
Solar System and Battery System for Digital Nomads ($1,500)
If you are a digital nomad but still on a relative budget, you need a bit more capability. In order for you to sustain a life on the road, you need to be able to charge your laptop and power your wireless connection, on top of all of the other daily requirements of a sustainable life on the road. Laptop computers have batteries varying from 50Wh or 100Wh of capacity that can run your computer between 2-6 hours on the average. Depending on the type of work you do and how hard your computer has to work, connecting your computer to power might burn through even more electricity. Now factor in about 10% of efficiency lost due using a power inverter, you are going to need an extra 100-watt solar panel for each computer in your camper. So if there are 2 of you needing to work as digital nomads, then we are talking about 2 more 100-watt panels which brings your solar system up to 300-watts. Let’s see what components are available here.
- 2 x 150 Watt Solar Panels – With a bigger budget you can get more panels to power more electronics. Generally speaking, monocrystalline panels like this one below are going to be more efficient than polycrystalline versions at the same watt rating. As long as the ratings are the same, one is not going to output more power than the other but the more efficient panels will be smaller in size. When adding more panels to your camper, you have to be more aware of the amount of roof space it will take. Getting 2 of these monocrystalline panels at 150-watts each is a good upgrade from a single 100-watt. But if you are upgrading from a single panel, make sure you are buying additional panels that are matched in cell type, output voltage and output amperage. Mismatch panels can cause problems and decrease your output efficiency.
- MPPT Solar Charge Controller – with multiple panels you are going to want to think about upgrading to a MPPT charge controller. But if you already have a PWM that can handle the capacity, you can decide if the added benefits and efficiency is worth the cost. A MPPT controller will let you do a couple of things. First it will allow you to simplify the wiring of the panels by running them in series. 2 panels connected in series will allow you to use the existing wiring without the need for thicker wires. They will also regulate the input voltage and convert excess power into usable charging amps into your battery. We are talking as much as 30-40% more power. If you choose this charge controller shown below which is a Victron MPPT 100/30 model, it can take in voltage as high as 100 volts from your panels and output as much as 30-amps of charge to your batteries. Another very nice benefit is being able to monitor it all from your iOS or Android smartphone via their mobile app.
- AGM or Lithium Battery Banks – If budget allows, a lithium battery might be the right move here. If you want to know if now is the right time to jump into the world of lithium camper battery, I have an entire post dedicated to that topic. This battery below will equate to doubling the capacity as the basic solution at half the weight. That means it is only 25% of the weight at the same usable amp-hour capacity. These will also give you as much as 10 times the cycle life as compared to AGM cells. What you decide to get ultimately comes down to your budget. To get 100Ah of usable capacity, you are going to pay roughly 2 times the price to use a LiFePO4 battery as the one shown here. For simplicity and the purpose of this article, we are going to use the full-rating of a LiFePO4 battery as usable capacity. Keep in mind that manufacturers will often limit 10% of that use by the programming of the built-in battery monitoring system.
So by adding up all of your power consumption as a couple of digital nomads, you will see that this system will put you once again at the “3-watts per 1-Ah rule”. You will have 300 watts of solar and nearly 100Ah of battery storage capacity.
I Want To Run My Air Conditioner. Sure, But…
When you don’t have budgetary constraints, then everything will be based on the amount of space and capabilities you wish to have. Some people have said that you can never have enough batteries but I don’t think it is as simple as that. As a full time traveler, following the “3-watts per 1-Ah rule” is important to me because of the style of traveling we do. We don’t drive every day and rarely have an outlet to plug in to charge. Having lots of batteries is great because it opens up the ability to run high energy appliances like induction stovetops, electric water heaters and hair dryers. But keep in mind that these appliances will drain your battery bank really quickly.
One of the most common questions people ask is ‘can I run my air conditioner on solar’. Well, the answer is yes but the real answer is a bit more complicated. I will explain.
- You are going to need a lot of roof space for a lot of solar panels
For example, on a hot day you decided to run your air conditioner for 6 hours at a rate of 1,200 watts per hour. It will deplete your battery by 600Ah. Maybe you have 10 x 100Ah lithium batteries which have plenty of capacity at 1,000Ah, but you will have a tough time charging them back to run it again the next day unless you have 3,000 watts of solar going by the 3-to-1 rule. That is pretty tough to do with today’s solar technology without a roof the size of a two-car garage. Unless you have other ways to replenish your batteries, you will spend the next several days catching up on the deficit of just running that appliance for 1 day.
Maybe you argue that well you can charge very quickly with your secondary alternator installed into your van. Well, that is certainly a nice thing to have. Let’s presume you can charge at a ridiculously fast rate of 200-amps while you drive. First of all, you will need at least a 000 gauge cable that is over 4/10 of an inch in diameter to handle that kind of current safely running from your alternator to your house battery. You will also need to drive for 3 hours to put 600 amperes of charge back into your lithium battery bank. If you are going to drive 3 hours a day anyways, you can just run your van’s AC during the drive and negate the need to run your house AC and the excessive battery capacity to begin with.
Another argument is that you could just find a place to plug in to charge. Well that also negates the need for running your AC off of your batteries if you are willing to pay or find an AC outlet on hot days.
A likely scenario where I see having more batteries than your solar can charge is for someone who regularly camps with electrical hookups or someone who does not full time travel. Having a huge battery bank with less than adequate solar is totally OK if you are comfortable running a daily power deficit and can just top off at the end of your trip when you arrive home.
Advanced Solar System and Batteries ($5,100)
So let’s say you want to have the ultimate electrical system so you can avoid using propane for cooking and heating water. This is a great application of an advanced solar and battery system because not only does it give you more capabilities, it also helps to eliminate the use of fossil fuel. It will of course come with some compromises. It will make it pretty difficult to use your roof for anything but solar panels. Some have built riser mounts for solar panels so items like surfboards or kayaks can still be stored below but that comes at a cost of higher center of gravity. If that is all ok with you, here is an estimate of what that system might look like.
- 1,200 watts of solar – You can install 6 of these on the roof of a long-wheelbase sprinter van giving you 1,200 watts of solar. when placed horizontally, these span just about the exact width of the roof of a Sprinter van.
- 400 Ah of LiFePO4 batteries – 2 of these batteries will give you 400Ah of total capacity. It matches your 1,200 watts of solar panels perfectly at a 3-to-1 ratio so you do not have to worry as much about running a daily charge deficit.
- You will need a big inverter to use all that power. The Victron MultiPlus is a good unit that will also work as a charge controller and shore power charge
With 400Ah of lithium, you will be able to run a lot more electrical appliances than in other simpler systems. If we save 100Ah of capacity to power you basic needs as in the moderate example, we will have 300Ah to power some of these items below:
- Induction Stovetop – An induction stove runs anywhere between 600 watts to 1,800 watts depending on the setting you are on. On the average of 1,200 watts, running one for an hour a day will cost you 100Ah of capacity.
- Electric Water Heater – Like any sort of electric heat, water heaters take a lot of power to run. Fortunately, they don’t need to be on for very long. Assuming you use it to heat water twice a day for washing dishes and once a day for showers, you will probably need it to be on for about an hour a day at 1,200 watts. That will cost you another 100Ah of capacity.
- The remaining 100Ah left of your 400Ah battery bank will handle the overhead of running your inverter more or less constantly as well as the general increase in consumption you are going to experience with a more complex system like this one. Some battery manufacturers might also limit your usable rating to between 80% to 90% for the longevity of its lifespan. It will also act as some excess capacity for those days when you need to cook a bigger meal or a longer hot shower.
As you can see, these systems are all based on the the “3-watts per 1-Ah rule”. We have traveled across every state in the United States, all provinces of Canada, most states in Mexico as well as 29 others countries across Europe, Africa and Asia. It is a travel-tested formula that has worked well for us across all of these geographical regions. As such, I will always err on the side of having more solar panels. Batteries by far are the most expensive components in these systems so making sure they are fully charged is the best way to keep their longevity. Although this rule is a useful starting point for your electrical system, your personal use case can certainly vary. If you live and travel primarily in the lower latitudes where sunny skies are the norm year round, you can bend that rule a bit and maybe get away with less solar. On the other hand, if your playground is the gloomy Pacific Northwest, the 3-to-1 ratio might not even be enough for you in those cloudy winter months.
This article should get you started on the right course of figuring out exactly what your needs are based on how you plan to live and where you plan to travel as you embark on this modern nomadic lifestyle. While I am a proponent of keeping things simple, I understand that many people would like to have some of the luxuries on the road as they did in a house. I hope it helped clear some things up and demystified some of the concepts behind building a solar system for your next adventure mobile.
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