Inverters: How to Choose An Inverter For An Independent Power System
The inverter is one of the most important and most complex components in an independent energy system. To choose aninverter, 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, andemergency 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.
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 conventionalappliances 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 homeand to feed excess energy back into 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.
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 night light 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 into 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). In Europe, South America, and most other places, it’s 220 volts at 50 Hertz.
An inverter should be certified by an independent testing laboratory such as UL, ETL, CSA, etc., and be stamped accordingly. This is your assurance that it will be safe, will meet the manufacturer’s specifications, and will be approved in an electrical inspection. There are different design and rating standards for various application environments (buildings, vehicles, boats, etc.). These also vary from one country to another.
How much load can an inverter handle? Its power output is rated in watts (watts = amps x volts). There are three levels of power rating-a continuous rating, a limited-time rating, and a surge rating. Continuous means the amount of power the inverter can handle for an indefinite period of hours. When an inverter is rated at a certain number of watts, that number generally refers to its continuous rating.
The limited-time rating is a higher number of watts that it can handle for a defined period of time, typically 10 or 20 minutes. The inverter specifications should define these ratings in relation to ambient temperature (the temperature of the surrounding atmosphere). When the inverter gets too hot, it will shut off. This will happen more quickly in a hot atmosphere. The third level of power rating, surge capacity, is critical to its ability to start motors, and is discussed below.
Some inverters are designed to be interconnected or expanded in a modular fashion, in order to increase their capacity. The most common scheme is to “stack” two inverters. A cable connects the two inverters to synchronize them so they perform as one unit.
Power Quality: Sine Wave vs. “Modified Sine Wave”
Some inverters produce “cleaner” power than others. Simply stated, “sine wave” is clean; anything else is dirty. A sine wave has a naturally smooth geometry, like the track of a swinging pendulum. It is the ideal form of AC power. The utility grid produces sine wave power in its generators and (normally) delivers it to the customer relatively free of distortion. A sine wave inverter can deliver cleaner, more stable power than most grid connections.
How clean is a “sine wave”? The manufacturer may use the terms “pure” or “true” to imply a low degree of distortion. The facts are included in the inverter’s specifications. Total harmonic distortion (THD) lower than 6 percent should satisfy normal home requirements. Look for less than 3 percent if you have unusually critical electronics, as in a recording studio for example.
Other specs are important too. RMS voltage regulation keeps your lights steady. It should be plus or minus 5 percent or less. Peak voltage (Vp) regulation needs to be plus or minus 10 percent or less.
A “modified sine wave” inverter is less expensive, but it produces a distorted square waveform that resembles the track of a pendulum being slammed back and forth by hammers. In truth, it isn’t a sine wave at all. The misleading term “modified sine wave” was invented by advertising people. Engineers prefer to call it “modified square wave.”
The “modified sine wave” has detrimental effects on many electrical loads. It reduces the energy efficiency of motors and transformers by 10 to 20 percent. The wasted energy causes abnormal heat which reduces the reliability and longevity of motors and transformers and other devices, including some appliances and computers. The choppy waveform confuses some digital timing devices.
About 5 percent of household appliances simply won’t work on modified sine wave power at all. A buzz will be heard from the speakers of nearly every audio device. An annoying buzz will also be emitted by some fluorescent lights, ceiling fans, and transformers. Some microwave ovens buzz or produce less heat. TVs and computers often show rolling lines on the screen. Surge protectors may overheat and should not be used.
Modified sine wave inverters were tolerated in the 1980s, but since then, true sine wave inverters have become more efficient and more affordable. Some people compromise by using a modified wave inverter to run their larger power tools or other occasional heavy loads, and a small sine wave inverter to run their smaller, more frequent, and more sensitive loads. Modified wave inverters in renewable energy systems have started fading into history.
It is not possible to convert power without losing some of it (it’s like friction). Power is lost in the form of heat. Efficiency is the ratio of power out to power in, expressed as a percentage. If the efficiency is 90 percent, 10 percent of the power is lost in the inverter. The efficiency of an inverter varies with the load. Typically, it will be highest at about two thirds of the inverter’s capacity. This is called its “peak efficiency.” The inverter requires some power just to run itself, so the efficiency of a large inverter will be low when running very small loads.
In a typical home, there are many hours of the day when the electrical load is very low. Under these conditions, an inverter’s efficiency may be around 50 percent or less. The full story is told by a graph of efficiency vs. load, as published by the inverter manufacturer. This is called the “efficiency curve.” Read these curves carefully. Some manufacturers cheat by starting the curve at 100 watts or so, not at zero!
Because the efficiency varies with load, don’t assume that an inverter with 93 percent peak efficiency is better than one with 85 percent peak efficiency. If the 85 percent efficient unit is more efficient at low power levels, it may waste less energy through the course of a typical day.
An inverter’s sensitive components must be well protected against surges from nearby lightning and static, and from surges that bounce back from motors under overload conditions. It must also be protected from overloads. Overloads can be caused by a faulty appliance, a wiring fault, or simply too much load running at one time.
An inverter must include several sensing circuits to shut itself off if it cannot properly serve the load. It also needs to shut off if the DC supply voltage is too low, due to a low battery state-of-charge or other weakness in the supply circuit. This protects the batteries from over-discharge damage, as well as protecting the inverter and the loads. These protective measures are all standard on inverters that are certified for use in buildings.