That's a mouthful, isn't it? What it means is taking the stored electrical energy out of a 12 volt DC storage battery, and inverting (really CONVERTING, I don't know why this term was used) it to 110 volt AC.
UPDATE: 092315: I noticed that the prices listed in this article were out of date, so I have revised them to their current status.
With the invention of heavy-duty power transistors several decades ago, the development of an efficient system for making AC power out of DC was developed. Prior to that, the motor-alternator was the only way to do it. A 12 volt DC motor was connected directly to an AC generator (alternator) and the output was AC power, but of inferior frequency and voltage control, because as the voltage input to the DC motor varied, so did it's speed, and therefore the frequency (60 cycles per second) of the AC. If AC frequency is not maintained exactly, bad things happen, such as AC motors overheat.
The solid-state inverter eliminates all these problems, and inverters built in the last 10 years even have the correct sine-wave pattern to the AC, along with exact frequency control. This means that you can use them for computers, which have to have a very stable source of power.
There are two means of making your own AC power: you can generate it with a factory-built generator set (genset), or you can make it from battery power via an inverter. Generator sets are very common, but they have restrictions on their use. If you live in an apartment block, you probably can't use one. They require constant supervision, since they consist of an engine running at high speed, and such troublesome things like being maintained in fuel and oil. Oh, did I mention that they are noisy? Yes they are. For most people this is their biggest drawback. They can also kill you, as their engines make carbon monoxide while running.
None of these disadvantages apply to battery-inverter power generation. It is quiet, and in a small room containing your equipment, all you will hear is the cooling fan running on your inverter, if you have a model with a fan. It requires little attention, the only two items of maintenance being to keep flooded-cell (lead-acid) batteries topped up with distilled water, and to occasionally brighten the battery connections to promote perfect contact. Other than that, when you need power, you switch on the inverter and plug your appliances and lights into it.
Here is a generic list of materials which you will need to put a set together: battery (batteries if you want a beefier set), battery boxes for the batteries, an inverter, cabling to connect the batteries to the inverters, a battery charger, and ordinary extension cords.
Like generators (to some degree), battery-inverter set-ups are scalable. That is, you can build them to suit the work. If you wanted to run a few necessities in an average house, you wouldn't need a very big set-up, but if you plan on running refrigerators and freezers, a large TV and most of your lights, you will need more batteries and a bigger inverter.
Let's take this one component at a time.
First the batteries. If you go to your local department or auto-supply store, you will find what are called "RV Deep-Cycle" storage batteries at $65-80. These are usually re-manufactured, some offshore. You don't really know what you are getting. A battery is a simple device, but, as with any device, the quality of construction usually means something. In a battery's case, it definitely does. That's why batteries are sold with guarantees by the month. Personally, I don't buy any sort of battery with less than a 72-month guarantee. Storage batteries are rated as to their total capacity in "amp-hours", but they are also rated in CCA (Cold Cranking Amps) and Reserve Minutes. You may ignore the CCA rating, you won't be starting any engines with these batteries, and if you were, all of them would start a gasoline V-8 with no difficulty. The Reserve Minutes rating means that the battery will be able to put out x-number of minutes of power (at 25 amps draw) until it gets down to 10.5 volts (dead).
Let's remember Ohm's Law: Volts times Amps Equals Watts. Amp-hours are a quantity of amps over time. 120 amp-hours means that your 12-volt battery will deliver 10 amps for 12 hours. Except that it won't really do that. A 120-A/H battery will give you 60 useful A/H. The reason is that you don't drain your batteries below 50% of capacity before recharging if you want them to last, and all inverters will cut out when their input voltage drops below 10.5 anyway. That means that a battery down to 12.06 volts will not operate an inverter, because when you load that partially-discharged battery with the inverter, it's output will drop about 1 1/2 volts. Use this table as your guide. You don't need a fancy battery-condition computer (they sell them for about $250) to tell you how your batteries are doing. Just switch off your inverter for a moment and use a cheapo digital voltmeter on it and compare the voltage reading to the chart above.
If you get a marine-rated battery (the best ones are all marine-rated), it WILL be a quality battery. The reason is that things move around in the marine environment, and these batteries are built to take some bumping around, so they cost more. They will also weigh more, because they have more lead plates in them which gives them their increased power. A cheapo RV battery may weigh about 60 pounds, but a Trojan of the same size weighs about 72.
As to brands, you can't go wrong with Trojan. They are on the upper end as to price, but they have more plates stuffed into their tough container, so they actually give a little more power (8% more) than a bargain-basement battery. Another good one is Exide Nautilus, specifically the Nautilus Gold. Note that the Trojan is an "Absorbed Gas Mat" type of battery, which is newer (and more expensive) technology than the usual lead-acid. It is not the same as a "gel-cell", which are perhaps the ideal battery, but they have their own issues, the first being that they are triple the cost, and the second being that it takes a special type of battery charger to charge them. They are made in all shapes and sizes, and may be used in any position, not really important to our consideration. They do have one huge advantage, in that they may be drawn down to zero, like a nickle-cadmium or lithium ion battery, without damage, whereas a lead-acid or AGM battery must not be taken below 50% of charge, except in emergency and with the knowledge that you have just seriously shortened it's life.
Battery boxes. The most popular is the Tempo brand, and their ordinary battery box, sized for either the smaller Group 24 or the bigger Group 27 size, is about $12-15 at your local RV or marine supply place. They have an upscale one also, which has a 12VDC outlet wired into the top, as well as a set of indicator lights which give you the state of charge while you are using the battery, or with a press-to-test function when you are not. It is about $50.
Inverters. Inverters are all over the place in size (and price). I have three small ones, a Xantrex 175 Micro, a West Marine 150 and a West Marine 250. I have two larger ones, both older superseded models but similar to this Xantrex 800 and 1500, except that the older models ran on 12 volts, whereas with these new ones you have to tie two 12 batteries together in series for 24 volts. That's probably a lawyer-driven modification, since there is zero functional difference between the two except that with 24 volts, there is half the amperage, therefore less chance of heat buildup in the cabling. You decide how much inverter to get by adding up the AC wattage of your loads. If you have 150 watts of lights (you don't NEED that much, but you might plan for it), a 400-watt fridge and a 400 watt freezer to run, you DON'T need a 1,000-watt inverter, because you aren't going to be running the reefer and freezer at the same time. There is an idle current with these inverters, and it gets bigger as the size of the inverter goes up. For the average user, I would get two inverters, one small one for low power applications and one 800-watt one to run the reefer or freezer. That makes your batteries last longer. If money is no object, and you are going to put together a large battery-inverter set, get one of these. It is a combination inverter and charger, and it has automatic changeover, so you could connect it directly to house power to make an Un-Interruptible Power Supply (UPS). You will need at least 4 batteries, and you'd be best to go to Golf Cart batteries. They are more expensive, but since they are meant to push a 400# golf cart with two 250# blutos around a hilly golf course, they have the ability to discharge more current safely than other types. They can be bought at Costco for $90 each, and since they are six-volt batteries, you need at least two, and preferably four.
Battery chargers. You need to be able to recharge the batteries. You do that under emergency conditions with either a generator set powering a battery charger, or with your automobile alternator. A couple of caveats here: I can hear your first squawk already. You are saying "I thought that the whole idea of this batter-inverter idea was to avoid generators". You're right, the concept avoids generators for the continuous-draw loads, but you have to recharge those batteries somehow. If you have a protracted outage like they have had in recent years in St. Louis, MO and Seattle, WA, you will have to recharge those batteries several times. If the city can't supply that power, you have to. The simplest solution is to have your vehicle outfitted with a 12-volt heavy-duty pigtail lead that you can plug the batteries into and let the car alternator charge them up, which it will do quite rapidly. Most cars have a 50-70 amp alternator, which will recharge your battery in about an hour, two batteries in about 2 1/2 hours (Group 27 storage batteries). You need some way to keep your car's engine revved up to about 2,000 rpm, where the alternator makes the best charge rate. A brick will do nicely, but you could have your vehicle fitted with a hand throttle as well. Any fab shop which services farm equipment can do this modification for you.
The other solution is a small generator powering a battery charger. Forget about the "battery charging" loop on the genset, none of them provide more than a 15-amp charge, and that's too slow (3-4 hours for ONE group-27 battery). The best battery chargers are made by Xantrex, the same people who make the inverters. I have a Tru-Charge 40. It's an expensive charger, but it will do all the charging functions. To charge a battery properly, first you need to stuff in some fast amperes. This is called the Bulk charge, and it recharges the battery up to about 85 or 90%. then you get to the Float charge, which tapers off and does the rest (but takes a lot of time, all day). The very best chargers also have a maintenance cycle, in which they regularily discharge the battery and recharge it though the three stages. This is called "de-sulphating", and it make a battery last years longer. A good battery charger does just that - it makes the batteries last much longer and provide better output when they are called on to do their job. You can get a useful charger for about a third of the Xantrex, though. This one will fill the bill nicely.
Cabling. You need cabling to ties the main components together. All connections should have lug-terminals on them, the kind that go on a bolt-type battery terminal. DO NOT use anything with spring clamps. The actual connection area of a spring clamp is so small that heat build-up is almost guaranteed, with it's attendant fire hazard and loss of efficiency. You could only be drawing 10 amps from the battery (giving you 100 watts of AC), but the clamp might be drawing another 3 amps in heating itself up. That 3-amp heat build-up WILL burn something up eventually. Your cables should be at least #4 size. You can go to a marine supply place or an automotive supply place and they will have a good selection. Use the correct color coding, red for positive and black for negative, and measure before you shop because the longer a cable you have, the more power you waste. You want the shortest cables that will tie your system together.
An alternative. There are ready-made systems available. The best one I have seen is this Xantrex, but they come smaller also, and are available everywhere in big-box stores like Home Depot or Lowe's. The Xantrex will power good-sized appliances with it's 1,500-watt inverter, but it only has a 60 A/H battery in it, so it won't run for long at high loads. It does have inputs, and might be capable of having additional storage batteries put in parallel with it, giving it the legs one needs on output, but I'll bet it's charger won't back-charge those other batteries, so you are back to dealing with battery charging anyway.
Let's put a set together. Let's build a basic battery-inverter set. We will need:
- One battery. Let's get a mid-range Exide Nautilus Gold 105 A/H. We'll pay $160 for it locally.
- One battery box, the Tempo Power Center box. That's about $62 with shipping.
- One inverter, an 800-watt Xantrex Micro, about $110 shipped.
- Cabling, two 14" battery cables, two 24" at $24, buy locally.
- A cheapo DIGITAL multimeter, about $8 locally.
- 2 size 14 extension cords. Price depends on length, about $25 locally.
- Battery charger, about $190 shipped
You are going to fabricate a hold-fast to fasten the inverter to the top of the battery box. Time, with pop-rivets and fender-washers, about 10 minutes with whatever strapping you have handy. While you don't have to fasten the battery charger to the battery box, I'm thinking that this whole lashup will fit nicely in a RubberMaid footlocker, about $40 at Ace Hardware. Just to be sure we can stretch out our recharge capability, we are going to add the pigtail-lead set in, another $90 or so shipped, and if you pay to have it hooked up, whatever those labor charges are.
Gensets? I would only recommend the Honda line, and they are not that cheap. An EU1000 with about the same power capacity as the battery-inverter set we just built will set you back about $650, while a beefier EU2000 will cost you $950. I have the EU2000, and it did just fine when we lost our power for a night during the recent Northwest storms, but I also used a small battery-inverter setup for my computer power and local lights in my computer room. Because I ran the genset continuously (save for refueling it), I didn't get a lot of sleep. With just 3 more batteries, I can lash together a 1500-watt set with my large inverter, so I'm going to do that soon.
Running all this? Easy as pulling the extension cords out, hooking up the things you are going to power, and throwing the switch on the inverter. Just a quiet hum tells you that it's running, plus the lights on the battery box come on. You can do the math if you have a steady draw, and calculate when you will be down to 50% on the battery, or you can hook up your multimeter and just eyeball the battery voltage occasionally. When it's time to recharge, you run out those 30 feet of jumper cables, plug into the pigtail lead, start your car and you're packing amps! Run the car for an hour, then shut it down and repeat as necessary. If the mains power comes back on, plug in the battery charger to get the battery back up, but don't leave it plugged in. For long-range maintenance, have a bottle of distilled water and a battery-bulb handy and refill the cells to the ring as necessary. Once a year or so, take apart all the connections, and using a small welder's steel-bristle brush, polish them bright and re-assemble.
You're prepared for a power outage now, my friends, but if you want to get educated in more depth, read this excellent 12VDC primer.