Some people just don't have the right stuff to live off the power grid. Whether the reason stems from a primal, unwavering revulsion to storage batteries, or a pathological need to have access to unlimited power at all times, it's a simple fact that some folks will never make the grade. Usually the forces of Social Darwinism sort out the misfits quickly and efficiently, sending them packing back to the waiting arms of civilization where hot tubs reside on every deck, electric towel racks and hair dryers are used with giddy abandon, and televisions and computers are left running all night.

But others persist against all odds. My wife and I know several of them; unlikely pioneers ill-favored for the wooly wilds where lurk temperamental wind turbines and un-tweakable photovoltaic modules, 200-pound storage batteries and persnickety power inverters. For these fish out of water, the mysteries of off-grid living will forever persist and multiply.

For the rest of us, however, life beyond the last power pole can be as rewarding as it is fulfilling, so long as the limitations of off-grid living are acknowledged and proper steps are taken to mitigate them. The tricks to living with solar and wind energy are endless—after 10 years spent off the grid we're still learning new ones—but if due consideration is given to properly sizing your renewable energy system, the path to energy independence will be less fraught with dead ends and bottomless pits.

Solar tax credits       As unpalatable as it might have been, the ITC bailout bill that was signed into law in early October did contain one tasty tidbit: the 30 percent tax credit for residential solar-electric and solar hot-water systems was extended for another eight years. Best of all, beginning in 2009 it will be uncapped from the previous limit of $2,000 per system. This means you can claim a tax credit of 30 percent of the cost of your entire system, no matter what it costs. Go wild. For details, see: www.dsireusa.org.

Whether for monetary concerns or a misplaced belief in their own abilities to conserve energy, lots of folks tend to undersize their photovoltaic systems. Besides making life an onerous exercise in frugality, it can also be hard on the batteries if they are routinely drained beyond their safe limit every time the demands of the house exceed the output of the renewable energy system. And, if (as is usually the case) a backup generator is used to make up the shortfall, the investment in fossil fuel soon exceeds the extra cost of a more robust system, with no equity to show for it.

Admittedly, sizing a home solar-electric system is not an exact science. It's easy enough to derive a set of numbers and run them through the proper manipulations, certainly, but how closely the bottom line agrees with reality is another matter. When it's all said and done, the system you end up with will be a reflection of your ability to examine your lifestyle and your needs objectively. So, as a failsafe mechanism, you should always leave room to add more batteries and solar panels at some point in the future.

Electrical usage

Your first step in sizing a solar-electric system is to determine how many kilowatt hours (kWh) of electricity your off-grid home will require, on average, each day (see the reference worksheet and energy consumption table). Most responsibly sized off-grid homes use less than 10 kWh/day. If you can squeeze that figure down to six or seven you'll save a bundle. As you go sleuthing for watts, you should take into account everything that will draw power in your new off-grid home, from the well pump to the furnace blower fan, from the clock on the microwave to the GFCI outlets in the kitchen and bathroom. And, of course, you should note how many minutes or hours per day each one is in use. This includes every plug-in appliance you own. For these things you can use a meter, such as a Watts Up? or a Kill a Watt meter, to measure either immediate usage or prolonged usage over time (as for a fridge or a freezer).

By the time you're done with this exercise the $10.00-per-watt figure you were probably quoted for your system will begin to loom large in your thinking. Practically overnight you will become a self-made efficiency expert, on a mission to replace every outdated appliance you own with a sleek, high-efficiency model. Do it; the planet will love you for your efforts.

Solar array sizing

Once you have a good idea of how much electrical energy you will need each day to run your off-grid home, the next step is to calculate how many solar panels it will take to provide it. There are several ways to do this, ranging from scientific to mystical (all else being equal, science usually works best). However you go about it, the first number you should be looking for is kilowatt hours of solar radiation per square meter per day (kWh/m2/day) in the area where you live, and the best place to find it is on the Web at: http://rredc.nrel.gov/solar/pubs/redbook/. This is a PDF file of the National Renewable Energy Laboratory's venerated Solar Radiation Data Manual for Flat-Plate and Concentrating Collectors, commonly known as the Redbook. In it you will find, for hundreds of U.S. cities, the average solar radiation for every month of the year at solar-array tilt angles ranging from flat to vertical. (For a thorough explanation of the Redbook data, read "Calculating Your Daily Solar Energy Harvest" in the May/June 2007 issue.)

Conveniently, "kWh/m2/day" translates nicely into "hours of noon-equivalent sunlight." To determine how much electricity your proposed solar array will produce, then, you merely have to multiply the kWh/m2/day for your location by the array's kilowatt rating, being sure to factor in the system efficiency (usually around 75 to 85 percent, depending largely on the type of charge controller you intend to use.) So, if your area boasts 4.3 kWh/m2/day of sunlight in December, your solar array is rated for 1. 82 kilowatts, and you plan on using a standard charge controller, your average daily production for December should be:

4.3 x 1.82 x 0.75 = 5.86 kWh

Or, by trading up to a Maximum Power Point Tracking (MPPT) charge controller that constantly looks for the "sweet spot" in the array's power curve, you might gain as much as an extra 0.79 kWh:

4.3 x 1.82 x 0.85 = 6.65 kWh

Likewise, by perusing the Redbook data, you will quickly discover how much you can improve your system's efficiency by adjusting the tilt angle of the array as the sun moves through the seasons.

Sizing the battery bank

After determining your off-grid home's approximate electrical usage and the size of the solar array needed to power it, the final step is to size the battery bank. This is a comparatively easy and straightforward undertaking, involving just a couple of variables. Specifically, all you will need to know is the home's baseline energy usage and how many (very cloudy) days you want your battery bank to be able to sustain that level of discharge before you finally relent and fire up the smelly old generator. For most of us, that's two or three days.

Suppose, for instance that you have calculated your off-grid home to use six kWh/day—at least on the days you don't wash clothes, sing lengthy ballads in the shower, or eat microwave popcorn—and you'd like to be able to run the house for three severely cloudy days before resorting to backup power. That means you'll need 18 kWh of available power which, practically speaking, is about half of the total battery capacity, since to drain the batteries any further may well cause irreparable damage to them.

Batteries, however, do not come with a kWh rating; they are instead rated in amp hours. The popular L-16 off-grid solar battery, for example, is rated at around 400 amp hours at six volts. To translate that into kWh you simply multiply: 6 (volts) x 400 (amp hours) = 2,400 watt-hours, or 2.4 kWh total capacity, of which you should never use more than half. That will leave you with 1.2 kWh per battery, meaning you will need fifteen L-16 batteries (18 kWh ÷ 1.2 kWh per battery = 15 batteries) to power your house for three consecutive cloudy days. Call it 16 batteries, since you will have to buy 6-volt batteries in multiples of four or eight for 24- or 48-volt systems.

Don't forget the meter!

Having successfully calculated the electrical usage of your off-grid home and the size of the solar system needed to run it, do yourself a favor: install a battery meter, such as the TriMetric Battery Monitor by Bogart Engineering. It will tell you at a glance how much charge remains in your batteries, plus the voltage, charging rate and a lot more. Call it a gas gauge for the system. After what you've just put yourself through, you and your family deserve it.