My Off-Grid Power System
This article describes the off-grid power system that I have implemented on the boat. I believe it’s a good template for those wanting to build their own simple off-grid power system for a cabin, tiny-house, van, RV, or boat.
I used to work as an electrical engineer planning out twelve volt electrical systems for semi-trucks, so I’ve spent a great deal of time thinking about how to build a robust, but simple, electrical system. I participate regularly on the Van-Dweller and Low-Cost-Voyaging mailing lists, and I see every week someone ask a seemingly simple electrical question that mushrooms into complex debate over electrical wiring best practices. The simplicity of my system is it’s greatest strength and why I think the diagram below represents an awesome template for others.
It’s taken me about two years to piece together my hybrid off-grid power system, but I finally saved up enough to purchase the last piece of equipment I need. Let me break it down for you:
Because this power system is on a boat, it’s common to have a shore-power system. This is what makes it a ‘hybrid‘ off-grid system, because it has the ability to charge the batteries when grid-power is available. Nothing too special here. I use a 6-amp marine battery charger from West Marine. It’s inexpensive, reliable, and has a multi-year warranty if it ever croaks on me.
There are some over-zealous people who are opposed to the idea of an off-grid system being able to be connected to the grid. If that’s your stance, then good luck to you. You’re going to need it! If it’s practical to install a grid-powered battery charging system on your vehicle, do it! You’ll thank yourself down the road.
Wind Turbine & Solar
I’ve had the wind turbine installed on my boat for over six months and in hindsight, I have to say that it’s one of the best investments I’ve made in my little floating home. The turbine and tower setup cost me about $1,000 for everything, but once setup, I haven’t had to touch them. I have never run out of power since installing it. The turbine and tower are made by SunForce and are rated for 500 watts, though I typically get (and only need) just a fraction of that. It provides reliable, quiet power for free.
I don’t have solar panels, though I may get some small, easily stow-able marine ones in the future. The problem with solar panels on a boat is that they take up valuable real-estate, or if permanently mounted on the rails, catch wind in inopportune ways. The other thing is that the sun only shines reliably for three months of the year here, for only a few hours a day. The wind blows all year, all day. It calms down in the summer, but a wind turbine in the Pacific Northwest makes much more sense on a boat than solar panels.
When sailing, I often turn the wind turbine off so that I don’t have to deal with the x-factor. It does add a significant amount of drag in heavy winds, and slows me down when sailing into the wind. It came with a big switch that electrically shorts out the wires. This forces the blades to turn very slowly, and in this state, it adds very little drag.
The generator and 40-amp battery charger are the newest additions to my complete power system. I really wanted to get a 1KW Honda Whisper as they are reliable and very quiet, but I picked up a Honeywell 800W generator for a third of the cost. I will probably upgrade when I start living on the hook full-time. For now, the wind turbine produces nearly all my power, so I plan to run the generator very infrequently.
I’ll talk more about the battery charger below, but the charger and generator are both ‘portable’, i.e. designed to be packed around easily and not permanently affixed to the boat. This was by intention. One advantage is that I can take them with me on camping trips, easily move them to a van or RV if I get one, or to take them with me if I upgrade to a new boat.
At this point, there is a non-functional sail-drive engine mounted inside the engine compartment of my boat. I plan to remove this in the coming year, at which point I will fabricate a mount and pipe in exhaust and vents for the generator. The idea is to have a semi-permanent mount that is easily removable.
Battery, Charger, and Generator Sizing
From the start, I planned to run my power system on two to three Group-31 deep-cycle lead acid batteries. These are slightly bigger than the ones used in cars. They are typically used on semi-trucks. From my experience, I know that one Group-31 battery stores enough power for a day of typical electrical use (laptop for a few hours, inside lights, navigation lights, etc). Each battery means I can go one day without needing to charge. This is based on my experience and your mileage may vary, but it’s a good place to start.
Because I knew how big my battery bank was going to be, I was able to design the generator and battery charger around them. While a lead-acid battery can be charged at higher charge rates, 20 amps is near the top limit of what many consider ‘healthy’. That’s how I chose the 40-amp charger, as that means each battery receives 20 to 15 amps of charging current, depending on weather I have two or three batteries.
Likewise, I also chose a 800 to 1000 watt generator because gas engines are most efficient between a 50% to 75% load. Charging batteries are going to vary between 12 and 15 volts. At 40 amps, that means I’ll be demanding between 480 and 600 watts from the generator (watts = amps X volts) or a 60% to 75% load. That means I’m converting gas into battery power as efficiently as possible, and will also minimize the amount of time I have to listen to the drone of the generator. In two hours I can get a day or more worth of battery power. Choosing the size of the batteries, the size of the charger, and the size of the generator to maximize efficiency is known as ‘sizing’.
I have proven to myself, yet again, that I am not as smart as I like to think. A reader pointed out that I was assuming 100% efficiency with the battery charger in my calculations. In reality, I was assuming that the battery charger would be at least 70% efficient or better. It turns out that my battery charger is only about 25% efficient.
While I’m shocked and disappointed, it does not affect the overall system much. Testing showed that I can charge my batteries at a rate of 16 amps off my little 800 watt generator (combining the two battery chargers on-board, one at 6 amps and the other at 10). That’s not as good, but it’s not too bad. The battery charger I got is programmable, so I always have the option of stepping up the charging amperage to 20 or 40 amps if I rendezvous with someone who has a bigger generator or if I need to poach some quick shore power.
No form of energy conversion is 100% efficient. In English, that means converting power from your 12 volt system to 120V AC through an inverter means you will always waste energy. For that reason, I try to use 12 volt electronics whenever possible. All my lighting is 12 volt LED. All the various electric motors and pumps on my boat run on 12 volts.
In the mailing lists, talking about inverters is a touchy subject. Sizing an inverter will depend largely on the size of your battery bank and what you need the power for. For me, the right size is 400 watts. The primary thing I use it for is powering a laptop. This inverter can power two or three at the same time. When it comes to inverters, keep in mind that anything you power with it is wasting energy, if it could be powered directly off the 12 volt system instead.
Electricity can do many wonderful things efficiently, but heating stuff is not one of them. Electricity is a bad choice for heating. Period.
Electricity is especially bad for heating when it’s supplied from batteries. It’s hard on the batteries and the entire process is terribly inefficient. If you need to heat stuff, use gas. Any gas. I don’t care if you use alcohol, propane, or diesel. They are all vastly more efficient than electricity, because the energy density of these fuels is orders of magnitude higher than the energy density of your batteries. If that sounds like a complicated answer, that’s because it is. Just trust me on this: never use your batteries to heat stuff. I will probably write a post in the future to expand on this idea.
FYI: That includes coffee makers. Use a Coleman propane coffee maker like I do. Don’t kill your batteries for coffee.