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How to Choose an Inverter for an Independent Energy System
he inverter is one of the most important and most complex components in an independent
energy system. To choose an inverter, you don’t have to understand its inner workings, but you should know some basic functions, capabilities, and limitations. This article gives you some of the information you’ll need to choose the right inverter and use it wisely.
Why You Need an Inverter
Independent electric energy systems are untethered from the electrical utility grid. They vary in size from tiny yard lights to remote homes, villages, parks, and medical and military facilities. They also include mobile, portable, and emergency backup systems. Their common bond is the storage battery, which absorbs and releases energy in the form of direct current (DC) electricity.
In contrast, the utility grid supplies you with alternating current (AC) electricity. AC is the standard form of electricity for anything that “plugs in” to utility power. DC flows in a single direction. AC alternates its direction many times per second. AC is used for grid service because it is more practical for long distance transmission. (See articles on basic electricity in HP52 & 53.)
An inverter converts DC to AC, and also changes the voltage. In other words, it is a power adapter. It allows a battery-based system to run conventional appliances through conventional home wiring. There are ways to use DC directly, but for a modern lifestyle, you will need an inverter for the vast majority, if not all, of your loads (loads are devices that use energy).
Incidentally, there is another type of inverter called grid- interactive. It is used to feed solar (or other renewable) energy into a grid-connected home and to feed excess energy back onto the utility grid. If such a system does not use batteries for backup storage, it is not independent from the grid, and is not within the scope of this article.
74 Home Power #82 • April / May 2001
Windy Dankoff ©2001 Windy Dankoff
Not a Simple Device
Outwardly, an inverter looks like a box with one or two switches on it, but inside there is a small universe of dynamic activity. A modern home inverter must cope with a wide range of loads, from a single nightlight to the big surge required to start a well pump or a power tool. The battery voltage of a solar or wind system can vary as much as 35 percent (with varying state of charge and activity).
Through all of this, the inverter must regulate the quality of its output within narrow constraints, with a minimum of power loss. This is no simple task. Additionally, some inverters provide battery backup charging, and can even feed excess power onto the grid.
Define Your Needs
To choose an inverter, you should first define your needs. Then you need to learn about the inverters that are available. Inverter manufacturers print everything you need to know on their specification sheets (commonly called “spec sheets”). Here is a list of the factors that you should consider.
Where is the inverter to be used? Inverters are available for use in buildings (including homes), for recreational vehicles, boats, and portable applications. Will it be connected to the utility grid in some way? Electrical conventions and safety standards differ for various applications, so don’t improvise.
The DC input voltage must conform to that of the electrical system and battery bank. 12 volts is no longer the dominant standard for home energy systems, except for very small, simple systems. 24 and 48 volts are the common standards now. A higher voltage system carries less current, which makes system wiring cheaper and easier.
The inverter’s AC output must conform to the conventional power in the region in order to run locally available appliances. The standard for AC utility service in North America is 115 and 230 volts at a frequency of 60 Hertz (cycles per second, abbreviated “Hz”). In Europe, South America, and most other places, it’s 220 volts at 50 Hz.
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