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Backup Power for the Conventional Home
Inverter Selection
An inverter is a device that converts DC electricity stored in batteries to the AC electricity we use in our homes. The inverter provides the back up power most of the time. The generator is run only 2 to 3 hours a day to recharge the batteries. The charging process is performed basically by running the inverter backwards as a battery charger. Since selecting an inverter can involve many different variables, its best to discuss that topic in detail on an individual basis. The discussion below is meant to give you an idea of how this process might evolve.
There are two basic kinds of inverters and a full range of sizes. The AC loads you want to power--both their size and type--will determine which inverter you choose.
Typical inverters range in size from 1500 watts to 5500 watts. The need to power a water pump usually determines how large an inverter you would need. If powering a water pump is not necessary you may be able to provide backup power to your home with as little as a 1500 watt inverter. On the other hand if you have a one horse power submersible pump you'll need a 4000 watt (4kW) inverter. The most common household water pump is a one-half horse power submersible pump. This pump would require at least a 2400 watt inverter, or a larger one if you hope to run both a refrigerator and freezer as well as the pump.
The type of inverter you choose will depend on what kinds of loads you want to power. There are two types of inverters: a sine wave inverter and a modified sine wave inverter. Sine (pronounced 'sign') and modified sine refer to the type of wave form each inverter produces. Sine wave power is the type of power the grid supplies. Therefore a sine wave inverter is the 'best' type and is at least twice as expensive as a modified sine wave type.
Fortunately a modified sine wave inverter will power most necessary household loads including refrigerators, freezers, pumps and all motors, most TV's and stereos, PC computers and of course lighting. Central heating controls might be a problem when run on a modified sine wave inverter. Laser printers and personal copiers most likely will not work properly, and in some cases may even be destroyed by modified sine wave inverters! Other than central heating controls, some of the above equipment isn't really necessary in an emergency back up situation. And after talking with other vendors, it seems most Honeywell heating controls will work on modified sine wave inverters. A sine wave inverter produces electricity very close to power grid quality and so will power anything the grid is able to power.
We recommend using a sine wave inverter if you can (or want to) afford one. They have the advantage of being able to power anything, both now and in the future. If its just basic back up of a water pump, refrigerator, lighting and possibly a central heating system that you want to power, a modified sine wave should do just fine. They have the advantage of being much less expensive than a sine wave inverter. Consult the adjacent chart for other differences between the two inverter types. Also consult the Inverter section of this catalog on pages 50-53 and the Generator/Charger Kits on pages 46-49 for more detailed information.
Equipment Installation
Once the equipment is selected, it must be installed so it can safely provide back up power to your chosen loads. In almost all cases this installation will be performed to National Electrical Code Standards. It is important for you to understand what this entails. There are several methods possible for equipment installation, all of which will most likely require the services of an electrician. Diagrams are provided for the most common system designs. These should help both you and your electrician understand how an inverter is installed in a home back up system.
The first thing to consider is system voltage. An inverter is a 120 volt device. Most of the appliances and systems you want to provide emergency power for also require 120 volts. The one exception is the water pump. It is usually a 240 volt device. This problem can be solved either by using a transformer to convert the 120 volts to 240 volts or by purchasing a second inverter. Two inverters of the exact same model can be "stacked" together to provide 120/240 volt power. In some instances, depending on the size of the pump and the type of generator being used, this might be the preferred method. Using a transformer is a less expensive and more common solution.
It is very important when supplying emergency power to your home not to supply it back to the power grid! This is an obvious danger to any lineman working on the power lines. Needless to say, it is also illegal and violates the electrical code. Conversely, if grid power is directed to the inverter's output, the inverter will be permanently damaged.
Three devices, either alone or in combination with one another, may be used to integrate the inverter and generator into your electrical system. They are a generator switch, a sub-panel or a device known as a Gen-Tran. The generator switch disconnects your main AC distribution panel from the power lines and reconnects it to your inverter/generator power supply. The switch must be thrown manually when loss of grid power occurs. A Gen-Tran (stands for generator transfer) performs much the same function, only with a few selected circuits in your main AC distribution panel instead of switching the entire panel. The use of a sub-panel is in many ways the most elegant solution. All the device and system circuits to be backed up would be moved from the main AC distribution panel to the sub-panel. The inverter would literally be wired between the two panels getting its input from the main AC panel and feeding its output to the sub-panel. Under normal circumstances the inverter would simply transfer the grid power from the main panel to the sub-panel. If grid power was out for only a day or two, most likely you wouldn't need to run your generator--the battery bank and inverter would be able to provide the emergency power necessary. When the power grid was re-energized, it would automatically recharge your battery bank. If, however, the power was off for a longer period of time, the generator would be connected to the inverter. Then it would be run to recharge the battery bank as needed--normally only 2 to 3 hours a day.
There are different reasons to use different combinations of the generator switch, Gen-Tran, and sub-panel. The size of the generator, the number of inverters involved, and the type of loads to be emergency powered are some of the determining factors. The drawings below are meant to give some idea of the various combinations possible. You are certainly welcome to consult with us to help you decide which equipment to use and how to integrate it into your electrical system. We urge you to consult with an electrician before attempting any rewiring of your home.
Inverter Selection Chart
Note: Information on Backup Power is continued on the next web page.
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