Solar Starter Frequently Asked Questions
Q1: How do solar cells generate electricity?
A1: Photovoltaics or PV for short can be thought of as a direct current (DC) generator powered by the sun. When light photons of sufficient energy strike a solar cell, they knock electrons free in the silicon crystal structure forcing them through an external circuit (battery or direct DC load), and then returning them to the other side of the solar cell to start the process all over again. The voltage output from a single crystalline solar cell is about 0.5V with an amperage output that is directly proportional to cell's surface area (approximately 7A for a 6 inch square multicrystalline solar cell). Typically 30-36 cells are wired in series (+ to -) in each solar module. This produces a solar module with a 12V nominal output (~17V at peak power) that can then be wired in series and/or parallel with other solar modules to form a complete solar array to charge a 12, 24 or 48 volt battery bank.
Q2: Will solar work in my location?
A2: Solar is universal and will work virtually anywhere, however some
locations are better than others. Irradiance is a measure of the sun's power available at the surface of the earth and it averages about 1000 watts per square meter. With typical crystalline solar cell efficiencies around 14-16%, that means we can expect to generate about 140-160W per square meter of solar cells placed in full sun. Insolation is a measure of the available energy from the sun and is expressed in terms of "full sun hours" (i.e. 4 full sun hours = 4 hours of sunlight at an irradiance level of 1000 watts per square meter). Obviously different parts of the world receive more sunlight from others, so they will have more "full sun hours" per day. The solar insolation zone map on the right will give you a general idea of the "full sun hours per day" for your location.
Q3: How much will a system cost for my 2000 square foot home?
A3: Unfortunately there is no per square foot "average" since the cost of a system actually depends on your daily energy usage and how many full sun hours you receive per day; And if you have other sources of electricity. To accurately size a system to meet your needs, we need to know how much energy you use per day. If your home is connected to the utility grid, simply look at your monthly electric bill.
Q4: Can I use all of my normal 120/240 VAC appliances?
A4: Maybe. Many older homes were not designed or built with energy efficiency in mind. When you purchase and install a renewable energy system for your home, you become your own power company so every kWh of energy you use means more equipment (and hence more money) is required to meet your energy needs. Any appliances that operate at 240 VAC (such as electric water heaters, cook-stoves, furnaces and air conditioners) ar
e impractical loads to run on solar. You should consider using alternatives such as LP or natural gas for water/space heating or cooking, evaporative cooling instead of compressor based
AC units and passive solar design in your new home construction if possible. Refrigeration and lighting are typically the largest 120 VAC energy consumers in a home (after electric heating loads) and these two areas should be looked at very carefully in terms of getting the most energy efficient units available. Great strides have been made in the past 5 years towards improving the efficiency of electric refrigerators/freezers. Compact fluorescent lights use a quarter to a third of the power of an incandescent light for the same lumen output and they last ten times longer. These fluorescent lights are now readily available at your local hardware or discount store. The rule of thumb in the renewable energy industry is that for every dollar you spend replacing your inefficient appliances, you will save three dollars in the cost of a renewable energy system to run them. So you can see that energy conservation is crucial and can really pay off when considering a renewable energy system.
Q5: What components do I need for a grid-tie system?
A5: Grid-tie systems are inherently simpler than either grid-tie with battery back-up or stand-alone solar systems. In fact, other than safety disconnects, mounting structures and wiring a grid-tie system is just solar modules and a grid-tie inverter! Today's sophisticated grid-tie inverters incorporate most of the components needed to convert the direct current form the modules to alternating current, track the maximum power point of the modules to operate the system at peak efficiencies and terminate the grid connection if grid power is interrupted form the utility.
Q6: What components do I need?
A6: There are many components that make up a complete solar system, but the 4 main items are: solar modules, charge controller(s), batteries and inverter(s). The solar modules are physic
ally mounted on a mount structure (see question 7) and the DC power they produce is wired through a charge controller before it goes on to the battery bank where it is stored. The two main functions of a charge controller are to prevent the battery from being overcharged and eliminate any reverse current flow from the batteries back to the solar modules at night. The battery bank stores the energy produced by the solar array during the day for use at anytime of day or night. Batteries come in many sizes and grades. The inverter takes the DC energy stored in the battery bank and inverts it to 120 VAC to run your AC appliances.
Q7: What type of solar module mounting structure should I use?
A7: There are four basic types of mount structures: roof/ground, top-of-pole, side-of-pole and tracking mounts, each having their own pros and cons. For example roof mount structures typically keep the wire run distances between the solar array and battery bank to a minimum, which is good. But they also require roof penetrations in multiple locations (a potential source of leakage) and they require an expensive ground fault protection (GFP- device to satisfy article 690-5 of the National Electrical Code- NEC). On the other hand, ground mounted solar arrays require fairly precise foundation setup, are more susceptible to theft/vandalism and excessive snow accumulation at the bottom of the array. Next are top-of-pole mounts which are relatively easy to install (you sink a 2-6 inch diameter SCH40 steel pole up to 4-6 feet in the ground with concrete). Make sure that the pole is plumb and mount the solar modules and rack on top of the pole. Top-of-pole mounts reduce the risk of
theft/vandalism (as compared to a ground mount). They are also a better choice for cold climates because snow slides off easily. Side of pole mounts are easy to install, but are typically used for small numbers of solar modules (1-4) for remote lighting systems where there already is an existing pole to attach them to. Last but not least are the trackers, which increase the daily number of full sun hours and
are used for solar water pumping applications. Trackers are extremely effective in the summer time when water is needed the most. In the northern
Q8: Where should I mount the solar modules and what direction should I face t
A8: If your site is in the Northern Hemisphere you need to aim your solar modules to the true south direction (the reverse is true for locations in the Southern Hemisphere) to maximize your daily energy output. For many locations there is quite a difference between magnetic south and true south, so please consult the declination map below before you setup your mount structure. The solar modules should be tilted up from horizontal to get a better angle at the sun and help keep the modules clean by shedding rain or snow. For best year round power output with the least amount of maintenance, you should set the solar array facing true south at a tilt angle equal to your latitude with respect to the horizontal position. If you plan to adjust your solar array tilt angle seasonally, a good rule of thumb to go by is lat
itude minus 15� in the summer, latitude in the spring/fall and latitude plus 15� in the winter. Most mount structures provide for a seasonal adjustment of the tilt angle from horizontal to 65�. To determine if your proposed array site will be shaded at any time of the day or year you should
consider using the Solar Pathfinder.
Q9: Should I set my system's battery bank up at 12, 24 or 48 VDC?
A9: The PV industry really began with the 12V recreational vehicle market. These systems were typically small (1-2 solar modules) and had all 12 VDC loads. As the solar industry matured and entered the home market, systems became much larger (16+ solar modules) and no longer used DC loads exclusively. Most home systems today are 24 or 48 VDC since the higher syste
m voltage gives you a lot more flexibility as to how far away you can place your solar modules from the battery bank as compared to a 12V system. For a given power output, a higher system voltage reduces your amperage flow (but not your power) which allows you to use a smaller and less expensive gauge wire for your solar to battery and battery to inverter wire runs. Of course, if you already have a lot of 12VDC loads, that may be your deciding factor as to what voltage you set your system up at. Most grid-tied systems operate at 48 volts or higher.
Q10: Should I wire my home for AC or DC loads?
A10: It depends on the size of the system and what type of loads you want to run. DC appliances are usually more efficient than AC since you don't have to worry about the
loss through the inverter, but DC loads are typically more expensive and harder to find than their AC counterparts. Small cabin and RV systems are typically wi
red DC while most home systems are wired for AC loads exclusively. With improvements in inverter efficiency and reliability in the last 5 years, AC is the way to go for a home system. Another advantage AC has over DC is that the voltage drop for a 120VAC circuit is much less than a 12VDC circuit carrying the same power, which allows you to use smaller gauge wire.
Q11: Can I use PV to heat water or for space heating?
A11: No. Photovoltaics converts the sun's energy into DC electricity at a relatively low efficiency level (14-16%), so trying to operate a high power electric heating element from PV would be very inefficient and expensive. Solar thermal (or passive solar) is the direct heating of air or water from the heat of the sun and is much more efficient for heating applications than photovoltaics
Solar Lighting System
The UNI-KIT is a durable lighting system providing extended lighting hours. Simple plug-in connections make it easy to install. A basic power controller manages the system making it easy to use. This versatile system can also power a 12 volt radio or TV.The pre-wired UNI-SOLAR solar electric module converts sunlight to electricity, charging batteries during the daytime for lighting use at night. It's lightweight and durable. There are no glass or cystalline components to break or shatter.The unique structure of the UNI-SOLAR module enables it to keep on charging even on overcast days or when partially covered by shade.
- No Glass
- UL Listed
- Easy to Use
- Weather Resistant
- Silent, Safe, Dependable
- Reliable Lighting Source
- Radio/TV 12V Adapter Available
- Easy Plug & Play Connection System
- Tool Sheds
- Utility Sheds
- Remote Homes
- Surveying Sites
- Isolated Buildings
- Construction Sites
- Water Pumping
Solar Panel (Rated Power): 2 x 32 Watts
Fluorescent Lamps: 4 x 8 Watt
Performance (Lamps-Hours)*: 16-20
Warranty on Solar Panel only: 20 Years
Each unit comes complete with 8m (25ft) wire from modules to Power Control Center, 3m (10ft) wire for connecting lights, plus 5m (15ft) wire from Power Control Center to lights.
* Total lamp-hrs/day based on 5 hours of charging per day. Assumes optimal conditions (modules facing south, tilt-angle matches latitude, modules are clean and not shadowed). Calculated Performance (Lamp-Hours) is based on total system losses of approximately 35% and single lamp operation.
The Power Control Center maximizes battery life by controlling charging and load use and protecting against overcharging and excessive discharge. Easy to read indicators display current battery conditions (high, medium and low state change). The
e-year warranty.Fluorescent Lights
Easy mounting lights use efficient fluorescent lamps to provide long operation hours. Additional lights can be added to the system to provide more illumination. The lights carry a one-year warranty.
NOT INCLUDED in KIT:Mounting Structure
The UNI-SOLAR Lighting System has been designed to allow the installer to use locally available mounting materials which include wood, angle iron, etc. The modules can be mounted on poles, roof tops or other convenient locations.
12 volt deep-cycle battery required (90-120 Amp Hour) not included.
by Windy Dankoff
The 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.
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 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
Safety Certification 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.
INDUCTIVE LOADS and SURGE CAPACITY
Some loads absorb the AC wave's energy with a time delay (like towing a car with a rubber strap). These are called inductive loads. Motors are the most severely inductive loads. They are found in well pumps, washing machines, refrigerators, power tools, etc. TVs and microwave ovens are also inductive loads. Like motors, they draw a surge of power when they start.
If an inverter cannot efficiently feed an inductive load, it may simply shut down instead of starting the device. If the inverter's surge capacity is marginal, its output voltage will dip during the surge. This can cause a dimming of the lights in the house, and will sometimes crash a computer.
Any weakness in the battery and cabling to the inverter will further limit its ability to start a motor. A battery bank that is undersized, in poor condition, or has corroded connections, can be a weak link in the power chain. The inverter cables and the battery interconnect cables must be big, and I mean REALLY big, perhaps the size of a large thumb! The spike of DC current through these cables is many hundreds of amps at the instant of motor starting. Follow the inverter's instruction manual when sizing the cables, or you'll cheat yourself. Coat battery connections with a protective coating to reduce corrosion.
Idle power is the consumption of the inverter when it is on, but no loads are running. It is "wasted" power, so if you expect the inverter to be on for many hours during which there is very little load (as in most residential situations), you want this to be as low as possible. Typical idle power ranges from 15 watts to 50 watts for a home-size inverter. An inverter's spec sheet may describe the inverter's "idle current" in amps. To get watts, just multiply the amps times the DC voltage of the system.
LOW SWITCHING FREQUENCY vs. HIGH SWITCHING FREQUENCY
There are two ways to build an inverter. Without diving into theory, I'll simply say that there are differences in weight, cost, surge capacity, idle power, and noise.
A low switching frequency inverter is big and heavy (generally about 20 pounds (10 kg) per kilowatt), and more expensive. It has the high surge capacity (four to eight times the continuous capacity) needed to start large motors. Beware of the acoustical buzz that low switching frequency inverters make. If you install one near a living space, you may be unhappy with the noise.
A high switching frequency inverter is much smaller and lighter (generally about 5 pounds (2.5 kg) per kilowatt), and also less expensive. It has less surge capacity, typically about two times the continuous capacity. It produces little or no audible noise. The idle power is generally higher. If the inverter is oversized for motor starting, its idle power will be higher yet, and may be prohibitive. Most homes that have a well pump or other motors greater than 1 HP will find a low switching frequency inverter to be more economical.
Both types of inverter have their virtues. Some people "divide and conquer" by splitting their loads and using two inverters. This adds a measure of redundancy. If one ever fails, the other one can serve as backup.
Inverter idling can be a substantial load on a small power system. Most inverters made for home power systems have automatic load-sensing. The inverter puts out a brief pulse of power about every second (more or less). When you switch on an AC load, it senses the current draw and turns itself on. Manufacturers have various names for this feature, including "load demand," "sleep mode," "power saver," "autostart," and "standby."
Automatic on/off can make life awkward because a tiny load may not trigger the inverter to turn on or stay on. For example, a washing machine may pause between cycles, with only the timer running. The timer draws less than 10 watts. The inverter's turn-on "threshold" may be 10 or 15 watts. The inverter shuts off and doesn't come back on until it sees an additional load from some other appliance. You may have to leave a light on while running the washer.
Some people can't adapt to such situations. Therefore, inverters with automatic on/off also have an always-on setting. With it, you can run your low-power night lights, your clocks, fax, answering machine and other tiny loads, without losing continuity. In that case, a good system designer will add the inverter's idle power into the load calculation (24 hours a day). The cost of the power system will be higher, but it will meet the expectations of modern living.
PHANTOM LOADS and IDLING LOADS
High tech consumers (most of us Americans) are stuck with gadgets that draw power whenever they are plugged in. Some of them use power to do nothing at all. An example is a TV with a remote control. Its electric eye system is on day and night, watching for your signal to turn the screen on. Every appliance with an external wall-plug transformer uses power even when the appliance is turned off. These little demons are called "phantom loads" because their power draw is unexpected, unseen, and easily forgotten.
A similar concern is "idling loads." These are devices that must be on all the time in order to function when needed. These include smoke detectors, alarm systems, motion detector lights, fax machines, and answering machines. Central heating systems have a transformer in their thermostat circuit that stays on all the time. Cordless (rechargeable) appliances draw power even after their batteries reach a full charge. If in doubt, feel the device. If it's warm, that indicates wasted energy. How many phantom or idling loads do you have?
There are several ways to cope with phantom and idling loads:
* You may be able to avoid them (in a small cabin or simple-living situation).
* You can minimize their use and disconnect them when not needed, using external switches (such as switched plug-in strips or receptacles).
* You can work around them by modifying certain equipment to shut off completely (central heating thermostat circuits, for example).
* You can use some DC appliances.
* You can pay the additional cost for a large enough power system to handle the extra loads plus the inverter's idle current.
Be careful and honest if you contemplate avoiding all phantom and idling loads. You cannot always anticipate future needs or human behavior.
POWERING A WATER SUPPLY PUMP
At a remote site, a water well or pressure pump often places the greatest demand on the inverter. It warrants special consideration. Most pumps draw a very high surge of current during startup. The inverter must have sufficient surge capacity to handle it while running any other loads that may be on. It is important to size an inverter sufficiently, especially to handle the starting surge. Oversize it still further if you want it to start the pump without causing lights to dim or blink. Ask your supplier for help doing this because inverter manufacturers have not been supplying sufficient data for sizing in relation to pumps.
If you do not already have a pump installed, you can get a 115 volt pump if you don't need more than 1/2 HP. A water pump contractor will often supply a higher power pump than is needed for a resource-conserving household. You can request a smaller pump, or it may be feasible (and economical) to replace an existing pump with a smaller one. You can also consider one of a growing number of high-effiency DC pumps that are available, to eliminate the load from your inverter.
Backup battery charging is essential to most renewable energy systems because there are likely to be occasions when the natural energy supply is insufficient. Some inverters have a built-in battery charger that will recharge the battery bank whenever power is applied from an AC generator or from the utility grid (if the batteries are not already charged). This also means that an inverter can be a complete emergency backup system for on-grid power needs (just add batteries).
A backup battery charger doesn't have to be built into the inverter. Separate chargers are, in some cases, superior to those built into inverters. This is especially true in the case of low switching frequency inverters, which tend to require an oversized generator to produce the full rated charge current.
The specifications that relate to battery charging systems include maximum charging rate (amps) and AC input power requirements. The best chargers have two or three-stage charge control, accommodation of different battery types (flooded or sealed), temperature compensation, and other refinements.
Be careful when sizing a generator to meet the requirements of an inverter/charger. Some inverters require that the generator be oversized (because of low power factor, which is beyond the scope of this article). Be sure to get experienced advice on this, or you may be disappointed by the results.
A good inverter is an industrial quality device that is proven reliable, certified for safety, and can last for decades. A cheap inverter may soon end up in the junk pile, and can even be a fire hazard. Consider your inverter to be a foundation component. Buy a good one that allows for future expansion of your needs.
Inverters: What is a Sine Wave?
by Windy Dankoff
Alternating current (AC) is electrical current that reverses its direction at a standard frequency of 60 Hz (cycles per second, or 50 Hz in South America and
An inverter is an electronic device that converts DC to AC through a switching process. Thus it produces a sort of "synthesized" AC. There are two types of waveforms available from high-quality inverters. These are the so-called "modified sine wave" and the "true sine wave".
The "modified sine wave" is not really a sine wave at all. It is a stepped wave, like a pendulum that is being hit back and forth by soft hammers. It achieves voltage regulation by varying in width according to the battery voltage and the load. Thus, the wave is not as smooth as a sine wave. The quality of "mod sine" inverters should not be underestimated, however. They are highly capable, and (by narrowing the waveform) they save energy when running only small loads, as happens during most of the day in a typical home. They also cost half the price of sine wave inverters!
The disadvantages of modified sine inverters are (1) additional electrical noise may be produced, showing up as a buzz in some audio equipment and from some transformers, (2) some electric motors and transformers run hotter and draw a bit more power, (3) digital clock and timing circuits can be fooled, sometimes counting double-time and (4) in rare cases, power supplies in sensitive electronic equipment can be damaged. In spite of these occasional problems, mod-sine inverters have been successful in many thousands of remote home, RV and marine systems since 1986.
True sine wave inverters are more efficient for running motors, including AC pumps. They are less likely to draw complaints from people who enjoy high quality audio, or who simply have lots of electronic gadgets. If a mod-sine user has a problem with one or two small applications, here is a solution. Add a second inverter to the system, a small sine wave unit, to handle the problem circuits. Sine wave inverters in the 125-1000 watt range are made by Exceltech and Statpower and are available from Affordable Solar.
YOUR FINAL CHOICE
Choosing an inverter is not a difficult task. Define where it is to be used. Define what type of loads (appliances) you will be powering. Determine the maximum power the inverter will need to handle. Is the quality of the power critical? Does size and weight matter? The inverter selection table will help you to determine what type of inverter is best for you.
Your next step is to learn what inverters are available on the market. Study advertisements and catalogs, or ask your favorite dealer. It is best to listen to professional advice, and to purchase your equipment from a trained and experienced dealer/installer. We hope this article helps you make the right choice.
Inverters: Electronic Loads, Surge Protectors and Radio Interference
by Windy Dankoff
Sizing an inverter for electronic loads
Most electronic devices (especially stereo and music amplifiers, computers and TVs) are labeled with power ratings that are based on absolute maximum or surge conditions, for the purpose of sizing power circuits. Their actual power draw may be HALF of that, or less. The best way to measure the peak and average power consumption is to use a Brand Power Meter or other kilowatt meter.
Surge protector warning
Do not use household or computer type surge protectors on circuits powered by a "modified sine wave" inverter. They may overheat. Inverters do not produce dangerous spikes or surges, so protectors are not necessary. EXCEPTIONS: Use a lightning arrestor on any long AC feed line to another building, for example, or to a well pump. Long lines can pick up induced surges from lightning, and feed them back to the inverter and to the AC circuits. The Delta LA-AC Lightning Arrestor is appropriate (available from Dankoff Solar). Place it at the beginning of the line, close to a main ground connection.
AM radio interference
ALL inverters produce radio interference in the AM and shortwave bands. It may be necessary to use a radio that is powered by DC or internal batteries, and is not located near the inverter. To hear distant stations, it may be necessary for the inverter to be off.