It’s fairly easy to reduce the energy load in a typical North American home by 15% to 20% using common energy-efficiency measures. More radical efficiency work can reduce the load up to 50% or more. On-grid, reducing your energy demand not only saves you money, but also reduces demand for energy created by nonrenewable sources. Off-grid, this strategy is especially profitable, since every kWh comes at a cost in generating capacity, battery bank size, and the need for a backup generator.
Most off-grid systems will need to find non-electric ways to provide for:
Options include passive solar design; solar hot water; solar cooking; wood heat and cooking; and propane. Larger RE systems on homes that include ultra-efficient appliances (such as minisplit heat pumps) may be able to handle some of these loads. But these will come at a high cost in RE generating capacity, depending on the need and the resource.
The second step in off-grid system design is resource assessment. How much sun, wind, and/or falling water do you have?
Most off-grid properties rely on solar electricity, since sunshine is the most common resource. How much you have—measured in peak sun-hours—and when you get it is crucial to successful system design. Using measured data to find out your regional resource, and then a shade-analysis tool, such as the Solar Pathfinder, will give you a good idea of what a solar-electric system will do for you.
Wind resource assessment is more complicated, as are wind-electric systems in general. Your goal is to measure or predict the average wind speed at your proposed wind turbine location and height. With this hard-to-get information, you can make an accurate prediction of how many kWh your chosen wind turbine could produce in a month or year.
Hydro resource assessment is fairly straightforward, and very specific to your stream. Measuring the “head” (vertical drop) and flow allows you to calculate the power and energy potential. If you have a decent hydro resource, it may be all you need.
System design also includes battery bank and backup generator sizing. Both are influenced by your loads—in the amount of energy they use and when they’re needed. RE capacity, battery bank, and generator sizing need to take into account your weak season. Solar-only systems have a season of fewer sun-hours (usually, during the winter), and this needs to be planned for. Hybrid (two or more sources) systems may find a balance that reduces the need for a larger battery bank and backup generator.
On-grid RE system owners have a great deal. When their resource—sun, wind, or water—is available, they use it. When they make too much energy, the grid takes the surplus and gives credit. And when it’s dark, calm, or the creek is dry, the utility is there to provide the needed energy. Off-grid system owners have to take all the responsibility of generating all of their energy, all of the time.
The most challenging part of off-grid living is dealing with the variable resource. Raising a bunch of kids off-grid taught me a lot of lessons. One is that folks usually assume that electricity will be constant and abundant. This is part of our culture, and off-grid folks are not immune, since they interact with the on-grid culture on a regular basis. While there are many times when RE is very abundant—most every sunny day and whenever there’s a windstorm, for example—there are other times when it’s scarce. Surfing this wave of abundance and scarcity can be satisfying to some of us, but it’s challenging to others.
Systems can easily be designed to overcome the variation, and to allow use of any load at any time. But this will come at the increased expense of needing more RE generation capacity, a larger energy storage system, and a larger backup generator. This also means a less efficient and less environmentally friendly system.
Your solar-electric array will give you 30 to 50 years of trouble-free service. Meanwhile, your battery bank—even if well cared-for—will need to be replaced multiple times. And if you don’t treat it well, you may learn a hard lesson of having to replace this expensive component in just a few years. Some RE professionals suggest that new off-grid systems use a less costly battery bank initially—since there’s a learning curve with battery care and it’s best not to risk ruining an expensive bank while you learn.