I’ve built a small off-grid cabin on a thickly forested property in central Ontario, Canada. The two-story 16-by-16-foot cabin has a 12:12 pitched roof. I work in town a lot of the time, and built this cabin for taking our kids there a maximum of two days a month. There are two upstairs bedrooms, each of which will have one main light and two outlets. The main floor will have a maximum of eight outlets and five lights. There is no water in the cabin. Space heat is fueled by propane; so is the cooktop and fridge.
I was planning to buy six Trojan deep-cycle 6-volt batteries and a Coleman 400-watt wind turbine and charge controller. For the record, I have zero experience with this off-grid stuff, but I’m a licensed electrician. All the lights and kids’ nightlights will be less than 8-watt LED bulbs.
Do you think this small turbine and batteries will be enough for two overnight visits a month and for a total run time of 5 hours each time? Any help would be greatly appreciated.
Jason Summerfield • Ontario, Canada
If you have such thick woods that you can’t use solar energy, you also will need a tall tower well above the trees to capture any wind energy. The standard rule for tower height is to get at least 30 feet above anything within 500 feet—an expensive proposition. If the mature tree height in your part of the world is, say, 60 feet, you’ll need at least a 90-foot tall tower. Do the math for your site, and round up. The photo above shows you what your wind turbine should see—lots of view.
Wind-electric systems also take care and maintenance that solar-electric systems do not need, and tend to have a higher failure rate. Putting a wind-turbine on a site that is largely unattended may not be the best idea.
Choosing your battery size and configuration at this point is premature. Your idea of six, 6-volt batteries would mean a 12-volt system, when you may actually want a 24- or 48-volt system, depending on your loads and overall system design. The total watt-hour capacity desired is the best place to start with battery design. And a “400-watt” wind turbine is not a very meaningful description. What you actually need to know is how much energy (kWh) a machine will produce in your wind resource (average wind speed).
You made a good start on the first step in any off-grid design process—load analysis. I’d encourage you to continue it beyond “XX lights and XX outlets” to actual watts, hours, and ultimately kilowatt-hours (kWh) per day. This will give you a real load target to design your generating system to handle. Using the most-efficient lights and appliances possible will shrink the size of the generating system needed.
Next, take a closer look at your resources. A solar site analysis might tell you that there’s more sun than you think, or you might find a “creative” way to get some PV modules up into the sun. In my neighborhood, savvy and handy folks sometimes put PV arrays in trees, on high roofs, or even on towers.
It’s wise to look at your wind resource more carefully, too. Not every location has a good wind resource, so find out how much your site has (the average wind speed) before installing a system.
You also might consider a fuel-fired generator, which may be needed as backup anyway. A battery bank with inverter/charger can give you AC, and the ability to charge from a generator. Adding PV and/or wind can make the generator a minor backup player, which is a good role for generators.
Your project may be quite attainable, especially because it’s only for occasional use. But careful analysis of your load and its timing; the needed storage capacity (battery size); and your charging resources are necessary to make it all work. You could take a “learn as you go” approach, and it’s a good adventure, but a bit more study and number-crunching could save you from expensive mistakes and disappointment.
Ian Woofenden • Home Power senior editor