An inverter is the device that takes the low voltage DC current from your battery and changes it into 120 volt AC power. An invertor is essentially the exact opposite of a battery charger. They come in a wide variety of sizes and prices. You will need one that is larger than the largest load you will ever expect to draw from the inverter (and remember to calculate the surge or start-up load requirements, especially for items with motors such as refrigerator compressors). Inverter technology has improved a lot in the past decade, and today's investors are about 80 - 90% efficient. As with the other components of the system, size it generously.

Invertors are a source of AC power, but unlike generators, they do not always make a "clean" AC output.

Utility power is a perfect, 60 cycle sine-wave electrical power source. Invertors produce one of three types of power:

1. A pure sine-wave power output like utility power, (the more expensive choice).

2. Square-wave power. If at all possible, do not use a square-wave output invertor. These are generally good only for powering resistive loads such as lights and are not good for sensitive electronics or motors.

3. Modified sine-wave power form output. The modified sine-wave is the more typical invertor output type. These are better than the square wave output types but not as good a pure sine-wave output like the most expensive invertors and power generators. Because of the lower power handling ability of the modified sine wave produced by most of today's invertors, more current will be required to start some appliances as compared to what would be required from a pure sine wave source like a generator or utility power company. Make sure you figure this into your energy power requirement calculations. A refrigerator that requires 1200 watts on startup with utility power might need 1600 watts or more from an invertor with a modified sine wave output. The same refrigerator may refuse to start at all with a square wave output invertor that could produce 1600 watts!

Invertors with automatic charging circuits and power transfer circuits can provide uninterrupted electrical service. Within a fraction of a second of loss of AC service, the inverter is supplying AC to the home from battery. Even if you are not home or power loss happens in the middle of the night, everything keeps working. The un-interruptible power supplies used on computers are essentially a miniature inverter and battery charger system. Instead of supplying about 300 or 400 watts to your computer, the house inverter system might supply 2k or 3k watts to the house but the idea is the same.

If the inverter system is not large enough to supply all loads, some loads must be taken off-line when switching from the generator to the invertor.