Some people take their backup power systems more seriously than others. Generally, it's a matter of geography: the farther out you live in the boondocks the longer it will be until the lights come back on following a blizzard, ice storm or hurricane. Ironically, if you happen to live so far out that the power lines don't stretch all the way to your house, you're better prepared than anyone to deal with the circumstances that cause grid failures because you are, by definition, off-the-grid and not subject to the travails of power-grid calamities.

For you in the nether regions just a few miles short of off-grid territory, however, preparedness can mean the difference between life and death. A reliable, well-maintained generator is always your first line of defense, and in the last issue we explored the various options for safely grafting one into your home's electrical system. But even with a deluxe setup with automatic switching, startup and shutdown, any backup system that relies entirely on a generator has a built-in Achilles heel. A generator is going to use a baseline amount of fuel no matter how small of a load it's running, and the bigger the generator the more fuel it's going to burn. All day and all night.

What's the solution? Add a bank of batteries to your backup system to store up the extra energy your generator produces during the times it's running relatively light loads. In this way the batteries can supply the home's needs for hours at a time while the generator rests quiescently until called upon to refresh the batteries' charge. Many people go a step further with their systems, installing solar panels and/or a wind turbine to recharge the batteries before and during extended blackouts. A battery-based backup system will work equally well whether it is charged with a generator or a solar array. The only difference is that the generator is smelly and noisy, and it burns a type of fuel Mother Nature has made very little of in the last hundred million years.

However you charge the batteries, you will need a power inverter to run your house loads when the grid is down. An inverter will also double as a battery charger for systems without renewable inputs, or during times when the solar and/or wind chargers just can't keep up with demand. There are several models of inverters on the market suitable for grid-connected backup systems, but beware: all have their idiosyncrasies so do your homework before you buy.

Unless you have a very small house—or plan on installing a very large battery bank—you should not try to run the entire house on battery backup during power outages. Instead, determine which house loads are absolutely essential and redirect them from the main circuit panel to a critical-loads sub-panel. This sub-panel should include the furnace, well pump, refrigerator, a few lights and whatever else you deem necessary for survival during power outages. Obviously, the less power you can get by with the less your system will cost.

The system in action

In the simplest system, a sufficiently large circuit from the main electrical panel supplies grid power to the inverter. The inverter, in turn, passes this power through to the critical-loads sub-panel. When the grid is up and running, grid power is merely shunted through the inverter without being inverted, converted, subverted or perverted in any way. In fact, during normal times the inverter is really nothing more than a sophisticated battery charger; its only job is to occasionally redirect just enough grid power to keep the batteries topped off. Ho hum.

Once the grid goes south, however, the inverter becomes all business. In around 20 thousandths of a second the inverter kicks into action, drawing power from the batteries to run the critical loads. It's all very seamless and may in fact be so seamless you won't realize the grid is down until you go to turn on something not included in your critical loads, such as your 58-inch big-screen tv with Dolby surround sound.

Depending on how much power you use—and how many amp hours your batteries can store—your batteries may run your critical loads for anywhere from a few hours to a couple of days. For most blackouts this is plenty. But for the big ones you should make provisions to charge the batteries by generator and/or renewable sources. Since the generator hooks into the system on the AC side of things and solar and wind sources are wired into the DC side (the batteries, by way of a charge controller), you will be able to charge the batteries with any and all sources at the same time.

Choosing the right inverter

There are two basic types of inverters suitable for backup systems: those designed to interact with the grid, sending and receiving power, and those that can accept input from the grid but are not designed to send any power back into it. While the former is really a bit too talented for simple backup duty, it might be worth the money if you think you may eventually want to add a solar array and sell power back to the power company. Grid-interactive inverters all produce a true sine wave, "clean" enough to run anything in your house without damaging it.

For considerably less money, you can set yourself up with an off-grid-style modified-sine-wave inverter such as the Xantrex TR-series. These inverters produce a waveform that will power most, but not all, loads in your house. (Unacceptable loads include dimmer switches, many battery chargers, and plasma TVs). For an off-grid-style inverter that does produce a true sine wave, check out the OutBack FX-series.

Some inverters have dual AC inputs, other have only one. If the inverter you buy has but a single AC input, you will need to mount a generator transfer switch inline between the main breaker panel and the inverter to prevent generator and grid power from flowing into the inverter at the same time. (for more on this, please see the May/June 2009 issue.) The transfer switch can be either manual or automatic, but for safety's sake it can't be overlooked.

Sizing the system

The inverter should be powerful enough to run all the critical loads you might conceivably ever want to run simultaneously. For backup situations this should be far less than the amount of power you might normally use. Most people find they can get by fine with inverters in the 1,500- to 3,500-watt range. Plan on spending $1,000 to $4,000 for a reliable backup inverter.

Batteries are the second big-ticket expense. Sealed batteries perform best in backup systems because they don't have to be vented or watered, and they can be stored in any position. On the downside, they cost about twice as much as flooded lead-acid batteries and for this reason many folks endure a little extra inconvenience to save money.

A typical sealed battery for backup duty is the Concorde PVX-2120L 12-volt absorbed glass-mat (AGM) battery, rated at 212 amp hours. How much power is that? To calculate, take 212 (amp hours) x 12 (volts) = 2,544 watt hours, or 2.54 kilowatt hours. But since it's dangerous to take a battery below a 50 percent charge, the most you should ever draw from this battery is 1.27 kWh. Hook four of these workhorses together and you'll have a respectable 5.08 kWh at your disposal which should allow for plenty of quiet time between generator runs.

Or, if you'd rather to take the economy route, a bank of eight Trojan T-105 6-volt wet-cell batteries will deliver about the same amount of power for less than half the price for the total package (around $1,000, versus over $2,500 for the Concorde AGM batteries). With intermittent duty and proper care, a bank of T-105s will serve you faithfully for eight to 10 years. Just don't forget to check the water every month or so.

Excluding the cost of the generator and the electrician, the components for a no-frills battery-based backup system can be pieced together for under $2,500. Like everything, the price rises proportionately as you add sophistication and capacity. It's just a matter of how seriously you take power failures in your neck of woods. And how much comfort you require to "suffer" through a long one.