Designing an off-grid electrical system for a sailboat (camper van, RV, or tiny house)
Updated: Aug 21, 2020
I have gotten over a dozen requests from friends and viewers about our off-grid electrical systems and I wanted to put something together that would help with the planning and design process for your future build. I hope with the videos and this writeup that you will find this helpful and hopefully explain any of those complicated questions you may have. For anyone else reading this, please feel free to leave a comment and provide any feedback that can be helpful to all of us. I will make sure to add it to this. I would like this to be a collaborative writeup and collection of tips for all of us interested in sustainable off-grid electrical systems.
Our system, if you were to purchase it outright will cost over $8000 to build. But don’t let that price tag scare you. I know it scares me, but if you look at it in a different light, we spent less than roughly $1600 per year for 5 years building this system. The average person spends over $2000 a year paying for their utility bills in an average-sized house, so if you plan and build the system accordingly, this can be achieved with a very small investment.
Knowing how we ultimately wanted our solar setup took about 2 years before it became apparent what we needed to suit our energy demands. Being in Alaska and new to sailboat life back in 2015, we heard great reviews about the Silentwind Generator. Alaska is hit or miss when it comes to solar, so we thought it would be best to start with a wind generator. The Silent Wind Generator is a 400-watt wind turbine that, on average, creates 2-8 amps in light to moderate breeze (watch the install video here). The charge controller is designed to receive 40-amps max for wind and 20-amps maximum for solar. We thought this was the ideal charge controller for the solar and wind setup that we wanted. We later realized that we wanted to increase the amount of solar and that a second charge controller dedicated to solar would have to be added.
The boat came with 2 lead-acid deep cycle batteries on a 1-2 switch that allowed us to choose between one battery or the other. Each battery was roughly 125-amp hours (ah) totaling ~250ah. We later realized that we had to increase the amount of energy storage as our demand was more than what these two batteries could produce. This was also back in 2016 when lithium batteries were just making their way into the market and honestly, there wasn’t much information about them at the time. The battery of choice was the tried and true deep cycle lead-acid battery. Because of that, we resorted to buying 4 6-volt golf cart batteries which totaled over 400ah. We combined it with the other 2 batteries that came with the boat, totaling over 600ah of battery capacity. This also weighed in close to 400lbs of dead lead weight!
After upgrading the batteries, we also decided to purchase 250watts of solar. Not knowing much about solar, we went for the semi-flexible panels which ended up being a waste of money after 2 years. But for the first few months of having those new batteries, 250 watts of solar, and 400 watts of wind, we felt like we had things figured out!
Well, it turned out that it wasn’t enough once we left the docks for over a month and lived on anchor for the summer. That is when we realized that this setup was good for weekend and weeklong trips but just didn’t hold up much longer after that. That is because we couldn’t bring in enough solar or wind to keep the lead-acid batteries topped off. We were then required to run our engine every 2-3 days in order to bring some sort of charge back into our batteries.
This is about the time when I started the long quest of designing the ultimate off-grid system for our sailboat (and van). Lead-acid battery technologies are very archaic, at least in the sense of an energy storage unit. My best analogy is to compare it to a 5-gallon bucket with a small hole at the bottom. You can fill it up with water, and it will hold 5 gallons of water, but it drains out over time requiring the need to be refilled in order to stay full. Lead-acid batteries only hold a charge for up to a few weeks and require a consistent charge to stay “topped off”.
Let’s compare lead-acid batteries with lithium battery technology.
For the use in our boat (and van), when I mention lithium, I’m referring to the lithium iron phosphate (LiFePO4) technology. This technology is a 12volt drop-in replacement for deep cycle batteries used in marine and RV applications. This is the better technology for 12-volt applications as other lithium chemistries are better for 24 and 48 volt systems. I will not be referring to any other lithium technologies as my experience only includes 12 volt and LiFePO4 applications.
I would also like to state that I am not a trained professional, but do have over 5 years’ experience designing, building, and living off-grid with these systems. What works for me, may not work for others but I hope this can bring some clarity to those interested in outfitting their boat, van, RV, or cabin. I will also continue to update this page with relevant information as I continue to learn and improve upon my off-grid solar systems.
Choosing The Batteries
Lithium, or LiFePO4 batteries, are a much better technology and can hold a charge for months or even years (with minimal loss) compared to lead-acid technology. If I would have invested in lithium batteries back when we were upgrading our battery system for the first time, I probably would have saved over a thousand dollars with all the extra fuel and headaches involved with archaic lead-acid batteries. But LiFePo4 batteries were just entering the market and weren't even on our radar.
The biggest improvement to any off-grid system is the addition of lithium batteries. Lithium batteries compared to lead-acid batteries have up to 90% usable storage compared to 50%. What that means is if I have a 100ah lithium battery, I can use 90ahrs from that battery whereas, with a 100ah lead acid battery, I can only use 50ahrs. Sounds deceiving when you spend $200 for a 100ah lead-acid battery, but in reality, it only provides 50ah. Now you may be asking what happens if I use more than 50% of the usable power from a lead-acid battery? Well, this is where we wasted over $1k and killed 6 lead-acid batteries after only a few months. If you consistently use more than 50% of a lead-acid batteries capacity, like going from 100% charge to 0% charge, you can see the lifespan of that battery go from 10 years to 10 months.
But lead-acid batteries cost $200 whereas comparable lithium costs $1000!?
So, I know what you are thinking, the upfront cost of a new lithium battery is a lot! But if we break it down, they actually cost the same, though I will argue that lithium is actually cheaper. I believe they both cost roughly around $0.20 per kWh plus or minus 2 cents. But how? You have to look at the longevity or depth of discharge (DOD) cycles. A top of the line 100ah lead acid battery runs around $200 and provides about 500 recharge cycles. A comparable 100ah lithium battery costs about $1000 yet provides over 4000 recharge cycles! That is the equivalent to purchasing 8 lead-acid batteries over the course of the life of a lithium battery. So, if we factor in the longevity of the battery, it makes more sense to spend the upfront cost for a more efficient and longer-lasting power source.
Now let’s talk about the energy capacity and recharge rate. If that $200 100ah lead-acid battery only provides 50% of the power source, it now requires 2-$200 lead-acid batteries totaling 200ah to be equivalent to a 100ah lithium battery. So, I could actually argue that you will then spend twice as much with lead-acid batteries as it would require 16 batteries or $3200 over the lifetime to equal 4000 recharge cycles when you could have just spent the $1000 upfront for that lithium battery. I should also state that the recharge rate of lifepo4 is about twice as fast compared to lead-acid.
4000 cycles mean 80% depth of discharge (DOD) at 1C (1C means maximum amps for battery capacity in a 1-hour recharge or 100ah in 1 hour for a 100hr battery). And what happens after 4000 cycles? Well, the battery doesn’t produce 100ah anymore but reduces to about 75% of that capacity or instead of a 100ah battery, it becomes a 75ah battery, after 4000 cycles at 80% DOD with 1C. Now let’s say that 1 day equals 1 cycle. That means this battery will last over 10 years but the energy holding capacity will be reduced about 75% of its original capacity.
The graph above is great as it shows the capacity retention to the number of cycles. Let’s give these batteries a real-world scenario.
With our system, we have 2-200ah lithium batteries totaling 400ah. I never give the battery 1C charge and truthfully, I see our batteries hanging out in the 60% range each day (according to the Bluetooth BMS image below). We normally get topped off with electricity in the middle of the day (65%-90%) and wake up in the morning with our batteries at 45%-55% charge after running cooktop, fridge, lights, computers, and more at night. Our scenario isn’t depicted in the graph above, but you could say it lies in-between the blue and green data lines on the graph, conservatively. So, I could expect 25 years or more before seeing a noticeable drop in capacity retention, as long as 1 cycle equals 1 day (according to our BMS, I’m averaging a cycle every 5 days. New battery purchased in June, 11 cycles in 60 days = ~5 days per cycle). So, with moderate use following the green trend line, after roughly 10000 cycles or 25 years, I can estimate that our 400ah battery bank will have diminished to a 300ah battery bank. If you ask me, compared to our lead-acid battery set up, that is way more efficient, sustainable, and cost-effective.
Let’s dive into the design of our system.
After 3 years of living aboard our sailboat, we took a hiatus and bought an inexpensive Sprinter van. This is when I designed our second electrical system and improved upon the mistakes I made previously. We were also investing in equipment that we could utilize for the boat as we had plans of improving upon our sailboat electrical system. In the van, we had limited room but were able to install 350 watts of solar on the roof of the van. This worked excellent with 200ah of lithium and a 2000watt standalone inverter. It was a simple and perfect micro off-grid system for the van that gave us ample electricity to run our Vitamix, electric induction cooktop, lights, and computers.
After utilizing this system for 2 years, we knew we wanted a similar setup on the sailboat but wanted to double to size of it in order to have excess electricity and the same luxuries found in a house. We wanted to be able to run all of our lights, navigation equipment, computers, induction cooktop, Vitamix, juicer, and more while not having to rely on the engine to recharge our batteries even in the middle of a PNW (Seattle) winter. (Only time will tell how this system works in the winter and conservative measures will need to be made due to minimal solar).
When we built out our camper van, we didn’t include any shore power (120volt) plugin to the system as we wanted to be less reliant on grid-tied electricity found at RV parks and more sustained far away from busy campsites. That threw a wrench into the system as most sailboats are designed to be compatible with 120volt grid shore power systems. If we are to spend the winter in places like Alaska, we would have to be somewhat reliant on grid power during the darkest times of the year as no amount of wind or solar could be possible to keep up with our energy demands on a small sailboat. This meant we had to design the system with the correct shore power charge controller capable of charging lithium batteries. DO NOT USE a charge controller designated for lead-acid batteries with a lithium battery. You will destroy your lithium battery. This is because the charging rates are different. Lithium requires a higher voltage that shuts off as soon as the battery reaches 100% whereas lead-acid needs a consistent voltage to stay topped off (refilling the bucket with water).
Our sailboat came with a 3-phase charge controller designed for lead-acid batteries. Because lithium batteries can be destroyed if charged by a lead-acid battery charger, we decided to replace the original charge controller with a Victron Energy Multiplus inverter/charger. We went with this as it allows us to have the correct charge controller for lithium, but also eliminates having to have a separate inverter to run appliances. The installation of this unit was also extremely simple and allowed us to utilize all the power outlets throughout the boat just as if we were connected to shore power. It also has a great feature that when connected to shore power provides a 4amp trickle charge to a designated starting battery (AGM deep cycle dual-purpose battery).
The Silentwind Generator that we installed 5 years ago has proven to be a great addition. The only issue with the charge controller is that it is limited to 20amps of solar. Knowing that we wanted to have 600 watts of solar (theoretically 50amps, (600watts/12volts) we knew we had to add a 30amp designated charge controller to the system. This would allow us to connect 200 watts of solar to the Silentwind charge controller and 400 watts to a second charge controller.
When building out the van, I found the EPEVER 30amp MPPT charge controller to be the best suitable charge controller for the price on the market (but looking to upgrade as there are better and more efficient charge controllers on the market). So, since this was a controller that we had, I wanted to utilize it in the boat system. I was going to connect 4 solar panels into this charge controller and connect two panels to the Silentwind charge controller. I ended up connect 5 100-watt panels to the 30amp EPEVER charge controller and only 1 panel to the Silentwind charge controller.
We also found that switching to a high-quality glass monocrystalline solar panel was much more efficient than the semi-flexible panels that we had previously installed. For the van, we used Aleko solar panels that we got locally in Seattle, but for the boat, we decided to try out the $1 per watt Renogy solar panels that you can get inexpensively off of Amazon. Watch the and off-grid solar build video here.
Now, theoretically, 500-watts of solar divided by 12-volts is 41-amps. Why did I connect 500-watts or 41-amps of solar to a 30amp charge controller? Well, I connected 4 and measured the amount of solar for a few weeks before adding the 5th panel. But due to efficiency loss from solar panels and the nominal voltage each panel makes, I found that I have never seen any more than 5.5-amps per panel or 28-amps for 500-watts. So, it was safe to add the 5 panels together. Plus I already had the 10-gauge wire installed from the previous solar installation and didn’t want to have to run another set of wires through the boat. (10-gauge wire is rated at 30-amps max)
Connecting the solar panels is rather simple. Since this is a 12-volt system, all that is required is to match the positive terminals with the negative terminals. There are a variety of different MC4 connectors on the market and with a simple search online you can find y connectors, 3 to 1 connectors, and a variety of other types. I had a half dozen y connectors on hand so I utilized the parts at my disposal. The MC4 connectors are foolproof and can only be connected positive to positive, negative to negative.
Once the solar panels were all connected in parallel, a positive and negative 10 gage wire ran from the solar array to the positive and negative ports in the EPEVER charge controller. This is rather self-explanatory, but please double-check you get the appropriate cables into the appropriate location. From the charge controller, a positive and negative 10 gauge cable connects to the positive and negative bus bar to supply electricity to the batteries. Make sure to add fuses accordingly to each of the positive wires before they connect to the bus bar. These fuses should correspond with the maximum amperage load. For instance, 30-amps is the fuse size recommended for the solar charge controller and a 40-amp fuse for the Silentwind charge controller. (For ease of understanding, I didn’t include any fuses into the wiring diagram as I wanted it to be as self-explanatory as possible. I have included a few other wiring diagrams to help with the design process)
The same process is repeated for the addition of the individual solar panel located on our dodger and connected into the Silentwind charge controller.
Connecting the charge controllers to the heart of the system.
This is where the fun part begins! Connecting the positive and negative cables to the positive and negative bus bars! This is rather self-explanatory but please use correct fuses on the positive wires for each controller. I've found this diagram by Victron Energy to be very helpful as a detailed diagram.
Our batteries are wired in parallel, meaning the two batteries are connected positive to positive, negative to negative allowing for the 12-volt 200ah batteries to act as 1 large 12-volt 400ah battery. If these were in series, it would create a 24-volt battery at 200ah. High voltage battery banks work better for off-grid systems that require more voltage demands such as in a house that runs 120-volt appliances more often. When connecting in parallel, I prefer to connect the bus bars and controllers to positive and negative terminals farthest away from each other. This helps with balancing the electrical demand between the two batteries and serves to pull or charge more evenly between them.
Lastly, I have a designated AGM starting battery connected to negative terminals of the house battery system. I joined the positives together using a Blue Sea isolator relay which allows the system to be recharged by the alternator once the starting battery has reached its target voltage. The isolator relay is designed to isolate the starting battery from the house bank so that it doesn’t accidentally get drained. The isolator relay “turns on” after the engine has been running for a period of time to ensure the starting battery gets the ample electricity for recharge. Once on, the starting battery is connected in parallel and recharges the house bank.
Since lithium batteries should not be used with a standard alternator or lead-acid battery charger, an alternator designed for lithium batteries is required. That is why lithium batteries are sold not to replace a starting battery, unless the alternator is replaced (but good luck getting your hands on a lithium specific alternator…)
There is a workaround and that is by using a designated lead-acid starting battery and connecting the lithium house bank batteries via an isolator relay. I also include an on/off switch and manually monitor the batteries when running the engine. Lithium can quickly be destroyed if overcharged and even though the isolator relay is designed to not overcharge the lithium batteries, I don’t want to take any chances. I currently have not used the engine to recharge our house bank battery on the boat and have only used renewable sources, though when the days get shorter this will be utilized when motoring to and from the dock when getting supplies every week or two.
I have gotten over a dozen requests from friends and viewers about our off-grid electrical systems and I wanted to put something together that would help with the planning and design process for your future build. I hope with the videos and this writeup that you will find this helpful and hopefully explain any of those complicated questions you may have. For anyone else reading this, please feel free to leave a comment and provide any feedback that can be helpful to all of us. I will make sure to add it to this. I would like this to be a collaborative writeup and collection of tips for all of us interested in sustainable off-grid electrical systems. I will continue to update this as I upgrade my system and learn more from you all!