Selling the Sun
Solar and wind options for grid-tied homes

Rex A. Ewing - COLORADO

Personally, I'm an off-grid guy. This is not because I have any particular gripe with the ugly, pollution-belching, coal-devouring power plant down the road; it's just that my wife and I happen to have built our house two miles from the closest power line. So for us anyway, the above-mentioned power plant might as well be on the moon for all the good it does us.

But I'm also a solar-and-wind guy, and I realize that off-grid living is not a practical alternative for most folks, since the vast majority of people live within spitting distance of that ubiquitous umbilical connecting your home to a huge nuclear or fossil-fuel-fired electrical generating plant, churning out gigawatts in some dark and distant location. For you, it makes more sense to go with the flow, and engage in a little give-and-take with the power company.

But how should you go about it? There are three basic ways to connect solar or wind systems to the power grid. The one you choose will be largely determined by where you live and how reliable your local utility is. If, for instance, you're prone to extended blackouts-from, say, power-line-rending blizzards or hurricanes-you will definitely want a system with backup batteries. On the other hand, if you live in a place with reliable power, lots of financial incentives and rebates, and net metering (where the power company is legally bound to consider your watts to be worth as much as theirs), you may choose the easy route and go with a battery-free, direct-tie installation. Either way, here are your options. Please note that any of these systems will work equally well with a wind turbine, although separate charge controllers and, in the case of direct-tie systems, special inverters will be needed.

Direct-tie installations
(no batteries)

Direct-tie systems are utter simplicity: no batteries, no charge controllers, no need to run your house from two separate breaker panels. In fact, there is no additional equipment whatever to clutter up the inside of your house. Your entire system will comprise a solar array, wired to run at between 48 to over 500 volts; a direct-tie inverter (one that wouldn't have the slightest idea how to charge a battery or handle the current from a backup generator); and a couple of extra disconnects, all mounted outside.

When the sun shines, the inverter will use the power from the solar array to run the house. If any extra power is needed, it will be drawn from the grid. When extra power is produced, it will be fed back into the grid. This surplus wattage makes your meter run backward and goes on down the line to run your neighbor's Jacuzzi. If you produce more energy than you use in any given month, your power company statement will show a credit, rather than an amount due. Pat yourself on the back. Not only have you saved some money, you've offset the burning of several hundred pounds of coal.

What's the downside? Whenever the power grid goes down, you go down with it, even if you're producing enough energy to power your house. Why? A safety feature, built into all direct-tie inverters, keeps your high-voltage, multi-kilowatt array from shocking some unsuspecting utility worker into the next world. It makes sense, if you think about it.

Battery-based systems

Systems with batteries are appreciably more complex and, because of this complexity, are generally around three to five percent less efficient than direct-tie systems. Battery-based systems almost always operate at a lower voltage (12, 24 or 48 volts) on the DC side than direct-tie systems, and require the addition of charge controllers, extra breakers and disconnects and, of course, batteries. But if you live in a place where normal grid power is prone to failure, the extra expense is more than worth it.

Grid parallel systems

There are two basic ways to go here. The least common-often called a grid-parallel system-only uses the utility grid as a means to recharge the batteries when they get too low. This system uses the grid in exactly the same way that off-griders use a backup generator. The only difference lies in the fact that you will probably only choose to operate certain critical loads, leaving the rest of the house subject to the inconveniences of periodic grid failure. You may, for instance, install an AC sub-panel tied to the inverter that runs the refrigerator and the home's heating system, along with a few lights and wall outlets. Then, when the grid goes down, you can at least enjoy fresh food in a warm, partially-lit house.

These systems are most often used in areas where there are no net-metering laws, since it makes no sense to sell your power to the utility at, say, $0.04 per kilowatt hour, just so you can buy it back later for $0.08.

Since these systems are basically off-grid, you will require enough batteries to get you through a reasonable stretch of cloudy weather, and the batteries will be asked to do quite a bit of work. For this reason, you should install standard deep-cycle, flooded lead-acid batteries (the kind you add water to, and have to vent to the outside) since these batteries are designed for front line of duty.

Grid-tie with battery backup

The final type of system is a grid-tied system with a battery backup. In this system the batteries are demoted to a secondary role, since they are never called upon to deliver any of their stored power until the utility grid goes south.

Here's how it works: when the sun is shining and the solar array is doing what it does best, the inverter will use the incoming current first to power the house, then to charge the batteries. If the batteries are charged and the array is still producing surplus power, this power is sent into the electrical grid and your electrical meter runs backward. When the sun sets, or the clouds roll in, the inverter will begin to draw power back from the grid to maintain the house systems.

And, like a backup quarterback, the batteries just sit out the game, waiting for the starting quarterback (i.e., the grid) to get clobbered. It's the only way the batteries ever get into the action.

Such systems are a satisfying compromise between direct-tie systems (no batteries) and grid-parallel systems, since you will be able to sell your power back to the utility (presumably at the same rate they charge you), while still being able to power at least part of your house when the grid goes down.

How can you be tied directly to the grid and still be able to power your house when the grid fails? After all, just as with a grid-parallel system, your inverter is in two-way communication with the power grid and should shut down when the grid shuts down. And in fact it does. Just not completely. For, while the circuit shuttling wattage to and from the grid becomes inoperative, a second AC circuit powering a critical-loads sub-panel remains functional. So, just as with a grid-parallel system, you won't have to eat rotting food in a cold, dark room waiting for the power to come back on. Which is nice.

Another nice aspect to grid-tied battery systems is that you can (and should) use sealed batteries, since they are designed to remain inactive for long periods of time. No need to add water or sequester them away in a separate, vented room.

These, then, are the three basic ways to connect a solar or wind system to the utility grid. Many states offer rebates and incentives for those willing to add solar and wind to their energy options. You can find out what programs are current in your state by visiting the Database of State Incentives for Renewable Energy (www.dsireusa.org). Don't know if your local utility offers net metering? Call them. Then get ready to be your own power company.

Rex Ewing, author of three books on renewable energy (Logs, Wind and Sun; and Power With Nature; Got Sun? Go Solar-See the Countryside Bookstore for details), lives with his wife, LaVonne, in a handcrafted log home powered solely by the sun and wind.