The idea of off-grid living is really what drove our desire to implement a DIY solar system that reduced our electricity bill to $0!
So, the tricky part: How do you calculate and determine how many batteries and solar panels you need to meet your energy needs for a DIY solar system setup? We’ve been there and hope this guide will help you get a better idea of what you need.
Let’s get at it.
Creating an Energy Budget for your DIY Solar System
We can’t emphasize enough – making a realistic energy budget is encouraged! It’ll help you figure out and calculate how big of a battery (or multiple) that you will need for your DIY solar system.
You’re going to want to make a list of all of the appliances and sources of energy that you will be using in your rig. Then, take note of how many watts of power they require. You’re also going to need to separate each power source based on AC or DC requirements.
Don’t know the difference between AC or DC electronics? You’re 100% not alone – so let me break it down for you.
Direct Current (DC) – DC is commonly used to charge batteries like rechargeable laptops and cell phones. These products come with an AC adapter that converts alternating current (AC) into direct current (DC). Because DC power does not fluctuate like AC current, it is preferred by electronics requiring a consistent source of energy. DC power is a stable current and cannot be transferred across long spans without experiencing a noticeable voltage drop. When supplying DC current, always check an ampacity chart to make sure you are not experiencing a voltage drop across a long span of wire. Long span = lower gauge (thicker) wire.
Alternating Current (AC) – AC is used to deliver larger power requirements such as to houses, offices, etc. compared to DC that is used for smaller electronics with smaller power requirements. AC power can be transferred long distances without experiencing voltage drops, which is why it is used to carry power from the utility power plant to your home (which can be many miles away).
If you’re a little confused and have no idea where to start? First, think about the lifestyle you are looking for. What are your off-grid needs? Let’s see where you fit in:
Weekend Warrior (AC/DC Power)
Maybe you’re looking to camp in comfort, but don’t plan on using your off-grid rig for full-time travel. A DIY solar system for this may include enough energy to run your TV, fridge, microwave, etc. However, with all the comforts of home comes a bigger price tag. You will need more solar and a larger battery for this kind of power draw. Chances are you will have enough power for a few recharges of your devices, but the solar panels are just to help top off the batteries and not fully charge. If you are looking for more charging capacity you will need more solar.
100-200watt solar panels
Full-Timer (AC/DC Power)
When you’re living in your rig full-time, you’re going to want enough energy…and then some in case of emergencies. Your battery, solar, and inverter should be chosen for long-term, generator-free use while powering all the comfort of home. It’s best to use larger DIY solar systems that charge your batteries efficiently for this lifestyle.
> 300-400watt solar panels
Other Things to Consider
Once you’ve made a decision as to what type of traveler you are, you can begin with making a list of where you want to be and what time of year. Using an isolation table, you can calculate the angle of the sun based on the direction you’re facing, etc. You’ll wind up with an approximation of how many hours of sunlight per day you’ll be getting in accordance with the month and time of year you’ll be in that location.
We input northern locations including Whistler, Canada to determine our solar panel usage based on the lower common denominator. Our batteries needed to get enough power with about 3 hours of sun. We also didn’t want to use a generator nor the bus alternator.
The next step is to calculate how many hours or minutes per day you will be using your energy including lights, water pumps, blenders, etc. In order to get an accurate calculation, it’s important to make sure you include everything imaginable on your list.
Next comes the calculations. We calculated our budget based on living on the bus during the wintertime. All the numbers below are based on our personal calculations and how we came to choosing our battery and panels.
1. You’re going to want to convert the number of hours of energy use into watts per hour. 50w light bulb for one hour uses 50-watt hours
watts x hr = watts per hour
(1500 watts x 1 hr = 1500 watts/hr)
2. Use watts per hour to calculate your amp-hour (Ah) usage.
note: volts is in pack voltage
watts x hr / V = amp-hours
(1500 watts / 12V = 125 amp-hours)
3. We’d recommend multiplying your amp-hour calculation by at least 3. This will ensure that you have enough solar energy in the event that you can’t recharge (due to poor weather, etc.) for at least 3 days.
(125 Ah x 3 days = 375 amp-hours)
Since it’s not good practice to drain our lithium-ion battery (we’ll get more into that later) to over 75% of its capacity, we chose a 233 amp-hour battery at 24 V (466 amp-hours at 12 V) or 5.3kw pack of which we use 3.8kw useable energy.
What Battery Will Meet My Energy Needs?
Now that you have a better idea of the load profile and energy capabilities for your DIY solar system, you will need to choose a battery to store the energy from your setup. Batteries directly define how long you can provide power without sunshine available. After selecting a battery, you’d move onto selecting the type of solar modules, charge controllers, inverters, and balance of system components.
The most common types of batteries are lead-acid. Lithium-Ion batteries, on the other hand, are popular in small products and electric vehicles. As technology becomes more researched, we can definitely predict the use of Lithium-Ion batteries in off-grid DIY solar systems to become more popular.
Some other less common battery types include Sodium-Ion, Nickel-Iron (NiFe), Nickel-Cadmium (NiCd), and Nickel-Metal-Hydride (NiMH) batteries.
Lead Acid Batteries
Lead-Acid batteries are common and not generally expensive, making them a great option for an off-grid DIY solar system. They function at 80-90% efficiency. In order to extend battery life, it is highly recommended to store a full charge when possible. If not maintained regularly, the batteries can become damaged when overcharged or over-discharged.
Sealed or Valve-Regulated Lead-Acid (VRLA)
There are two main types of “sealed” batteries including Gelled and Absorbed Gas Mat (AGM). They are valve regulated to allow for off-gassing. This is a relatively low maintenance system that will not spill like flooded batteries. As a result, they can be mounted in various types of positions. Just note that they still require adequate ventilation.
Gel batteries are not usually recommended for off-grid solar since they recharge slowly. AGM batteries charge faster, are lighter, and less expensive per amp-hour compared to gel.
The main thing about AGM that will make you want them over their FLA counterparts, is that they do not need to be installed in a sealed compartment. They can be within your interior living space which will help maintain the batteries at a more desirable temperature than if mounted outside.
Flooded batteries are nearly half the price of CRLA, lighter per energy capacity, and come in larger capacities. However, flooded lead-acid batteries are by no means low in maintenance. They require consistent monitoring and measuring at least once every 3 months. If they are poorly maintained, you can destroy your battery. It’s important to not store these batteries in living spaces or areas where they could tip over. They have the potential to spill corrosive acid that could be dangerous.
With the growth of electric vehicles in the market since 2016, lithium-ion (Li-Ion) battery technology has become a great option for the off-grid DIY solar system.
Lithium-ion batteries have an enhanced energy density making them lightweight and easy to transport. As a result, we chose to add a Tesla lithium-ion battery to our conversion and haven’t regretted it. They are about a third of the weight and half of the volume when compared to lead-acid (flooded, AGM, gel). Additionally, our LFP battery has no cobalt making it a bit safer than some of the other lithium-ion battery options.
While these types of batteries have their advantages, it’s important to understand some of the downfalls. Most of the off-grid charge controllers and inverters on the market are designated specifically for lead-acid batteries. Attempting to use these electronics interchangeably with a Li-Ion battery can cause communication issues with the Battery Management System (BMS). Long story short, be sure to buy Li-Ion-specific charge controllers and inverters to prevent your battery from getting destroyed.
The internal chemistry of Li-Ion batteries allows them to last longer than lead-acid batteries in nearly all scenarios. Even if you discharged a Li-Ion battery to 50% and recharged it to 100% every day, the battery would last at least 3 times longer.
Lithium-ion batteries can be discharged up to 80% without damage to the battery’s capacity. Lead-acid batteries, on the other hand, shouldn’t be discharged any more than 50% of capacity.
While both batteries have the issue of overheating and going into thermal runaway, it’s a more common problem for lithium-ion. This is because these types of batteries have more energy packed into a smaller volume. As a result, the cells may rapidly heat and can release electrolytes, flames, and dangerous fumes. Fortunately, most batteries come with a Battery Management System (BMS) that protects the battery from thermal runaway discrepancies.
Types of lithium-ion batteries including Lithium Iron Phosphate (LFP), Lithium Nickel Manganese Cobalt Oxide (NMC), and Lithium Nickel Cobalt Aluminum Oxide (NCA).
How Do I Know If I Need to Move Up to 24v or Even 48v?
12v is a great starting point. If you arent full-timing or don’t have any high drawing appliances, then you can skip this section.
When you are looking to protect your battery system and install wiring to connect everything together you will quickly realize these electronics add up quickly! As a general rule of thumb ~< 1000w of solar, 12v is fine. Anything over 1000w of solar, you should begin looking at bumping to the next voltage. Ok…great. So, what does that mean and why do you want a higher voltage?
House appliances run on 120v. Powering them with an inverter requires you to jump from your 12v nominal pack voltage to 120v. That’s a huge jump. Moving to 24v to 120 is a smaller jump, and 48v is even smaller. This means that the wires. required to run from the battery to the inverter can be smaller while still carrying the same amount of power. take a 1000w microwave at 120v ( 1000w/12v =n 83A) will require the wire to be able to handle 83 amp. At 24v you will only need a wire that can handle half that amp load (1000/24v = 42A).
**Make sure you use a wire that has at least a 15% overhead with all your appliances combined to prevent fires and overheating.**
Additionally, by moving up to the next voltage level you will save money by being able to use a smaller fuse in your system.
Battery C Rates: Used for overheating and the maximum draw of the battery when big appliances are running ( microwave, air conditioning, etc.)
How Many Solar Panels Will You Need for your DIY Solar System?
The size of your solar panels matters much more than how many you have. Once you have calculated how many amp-hours of batteries you need from your energy calculations, you work off that number to figure out how many panels you should have in your DIY solar system to charge that battery.
Your solar panels should be sized to meet the electrical demands of your battery. Useable amp-hours differ between battery types. For example, most AGM batteries only allow you to use 50% of the total capacity. However, you can use about 85-90% of a lithium-ion battery’s capacity. If your AGM battery is 100Ah, you only have 50Ah useable when charged. Let’s begin:
1. Convert the useable power from your battery into watts of power.
466 amp-hours x 12V = 5592 watts of power or 5.6kwh pack
(we use 85% of our total pack. We charge up the top 5% and drain it to the bottom 10%) We use 4.7kwh of our pack.
2. The average hours of sunlight in the US are 6 hours.
3. You’re going to use what you determined as your watts of power to calculate watts of solar.
5592 watts of power / 6 hours of sunlight = 932 watts of solar
Our solar needs are based on a far smaller amount of daily sun. In northern regions during the winter, we were expecting to see approximately 2-3 hours of sun. We also built a bare-bones energy budget that, worse case, we could “survive” for about 3 days using minimal lights, heat (YAY for incorporating 3 sources of heat into the bus), and our converted fridge which sips on power, especially during the winter. This allows us to get away with multi-day use even while not charging up completely over the period of a few days.
In order to charge your battery appropriately with only 6 hours of sunlight (using the above information), you will need at least 932 watts of solar. It’s always recommended to add a bit more to charge your battery more efficiently, especially during cloudy weather. For our solar array setup, we chose 1420 watts which include (4) 355-watt monocrystalline panels.
Confused what that term means? Don’t worry. We have another article that discusses How to Pick Solar Panels for Your Off-Grid Rig COMING SOON.
Here are some more AMAZING resources that we used throughout our build. Make sure to check them out!