The Best Off-Grid Power Systems Explained

The folks who built my house in the early ’70s must have been back-to-the-land warriors because it’s completely off-grid. When my partner and I bought it, the property had a functioning—although undersized—solar energy system, but that was destroyed by a lightning strike a few years ago, and we’ve been plugged into the neighbor’s house ever since while we figure out our options.

Needless to say, the situation has given me a personal interest in off-grid power systems. One thing I’ve learned is that the solar equipment that would fulfill our fairly moderate energy needs is far more expensive than I had previously thought, even with the 30 percent federal tax incentives. There’s no possibility of supplementing it with wind turbines because we don’t get enough wind. Other power options, such as micro-hydro, are off the table simply because of geography.

For this post, I consulted with Joseph Guido, a local contractor who has been installing solar equipment since the 1970s. “Some people around here have a good wind set-up,” he told me, “but the best off-grid system is solar.” He explained how to size a system to meet the household’s requirements, how to use solar water heaters to reduce electricity needs and how to plan for extended cloudy periods. He also provided pointers for property owners who have enough wind to run turbines and streams close enough to generate hydropower.

Whether you’re planning to build a cabin in the woods or a multifamily home on an isolated mountaintop, this post is for you.

Every Off-Grid System Has Batteries

When the sun goes down, clouds gather, and the wind stops blowing, you can’t rely on the utility company for backup if you’re off-grid. Unless you run a generator during those times, which requires fuel and isn’t something most off-grid homeowners would want, you need a battery bank to store energy to get you through the doldrums.

Guido recommends doing an energy analysis to size your battery bank (and electrical generation equipment). This means adding up the power draw of all the electrical equipment you plan to use and estimating the maximum number of consecutive days you expect to be without sun or wind in the winter. Online battery-bank calculators can help you size your system.

Lead-acid vs lithium batteries

Lead-acid batteries are cheaper than lithium ones, but they’re less efficient, so you’ll need more of them to get the same storage capacity. They can be an economical choice if you’re powering a small cabin. Still, for a single-family dwelling, Guido recommends lithium batteries, which are more compact, longer-lasting and require less maintenance. He advises choosing LiFePO₄ over lithium-ion batteries. They last four to five times longer, and the latter are a known fire hazard.

Power Generation Systems

The three most common ways to generate power for an off-grid residence are photovoltaic (PV) panels, wind turbines and micro-hydro generators.

• PV panels “are getting really cheap,” says Guido, but don’t be surprised if you need many more of them than you expected to keep your home electrified. Our residence consumes about 1,100 kWh per month, and two separate installers estimated we needed twenty-four 200W panels to meet our needs, which is about twice as many as I thought. Out here in sunny California, highly efficient monocrystalline panels work best, but in colder, cloudier regions, Guido recommends amorphous panels. Their output might be 70 percent as much on sunny days, but because they process infrared and UV radiation more efficiently, “they kick butt” on cloudy days, he says.
• Wind turbines, in the right location, can be a great addition to a solar array or even a standalone power source, but they have drawbacks. Unlike PV panels, wind turbines have moving parts that need maintenance and can break, so you take a big risk if you rely exclusively on them for power. Moreover, they can be noisy, and depending on how much wind you get, they may have to be mounted on towers, which is expensive and impacts the scenery.
• A micro-hydro generator could be an option if you’re in the rare position of having a stream nearby that flows swiftly enough all year to keep the turbine spinning. If the stream is big enough, a micro-hydro unit could possibly supply all your electrical needs. Guido has tried it, though, and was unimpressed. A hydraulic ram pump, he says, makes better use of a stream by delivering water that you can use for irrigation and cleaning to the house.

Inverters and Charge Controllers

Once you’ve got the equipment to generate and store renewable energy, you need an inverter to sync the electrical signals from the generator and battery to your home’s electrical system. You also need a charge controller to prevent the batteries from overcharging.

If you require more power in the future — perhaps to accommodate a new appliance — you can add more panels and batteries, but Guido warns that the inverter and charge controller create a choke point, so it’s wise to oversize them during the original installation. In general, the inverter should be able to handle 20 percent more than the maximum number of watts you expect to use (if you consume 3,000 watts at one time, you need a 3,600-watt inverter), but you might want to bump that up to 30 or even 40 percent to make sure you’re ready for expansion.

The best inverter is 98 to 99 percent efficient, which means it wastes little energy in converting DC to AC. It also has a long warranty. Most manufacturers offer a 12-year warranty, but some guarantee their products for as long as 20 or 25 years.

Solar Water Heaters

You definitely don’t want to use electricity to heat water when you’re off-grid because to do so, you’d need to generate and store 18 to 20 percent more electricity. A gas water heater is an option, but that requires burning fossil fuels and leaves you vulnerable to supply shortages. If you’re using the sun’s energy to generate electricity, using it also to heat water is a no-brainer.

One way to construct a solar water heater, according to Guido, is to simply lay a black pipe on the roof, but that wouldn’t generate very much hot water. He says most of the off-grid homes he has serviced cycle a heat-transfer fluid from a panel on the roof to a tank in the house, where a heat-exchange coil transfers heat to the water.

He recommends this type of water heater, but it requires a circulation pump that adds a small amount to the electrical load. One way around this is to install a solar-powered pump. Another option is to install a tank above the panel, let the warm water siphon into the tank by convection and pipe that water into the house.

About the Expert

• Joseph Guido is a licensed California contractor who has been researching, selling and installing solar equipment for almost fifty years.