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Inverters: change DC current to AC.

The combination of a solar electric array, battery bank and charge controller can make and store a lot of energy in an orderly fashion, but there are only a limited number of ways to power anything directly from batteries. Gadgets called inverters convert this stored energy to a much more convenient and usable form: the standard 120-volt AC "house current" we are so familiar with in our homes and businesses.

Inverters change direct current (DC) into alternating current (AC). This means that no special electrical equipment or knowledge are needed within a structure to convert it to renewable energy supply--any electrician or skilled homeowner knows how to install the standard AC house wiring for outlets, switches and fixtures. The inverter output circuits connect to the main home breaker panel just like a feed from the utility grid.

Choosing an inverter

Pull into any truck stop in the USA and you'll likely see a variety of small inverters for sale in the display case, ranging in price from $50 to $500. Which one will work to provide power for your remote home or cabin? Unfortunately the answer is, none of them. These "portable" inverters are designed to run only items plugged into the 120-volt AC sockets on their front panel (Illustration 1). If you rig a 120-volt AC plug from the inverter that runs to a home breaker box as so many people try, you'll create a shock hazard and could ruin the inverter itself too. To run a home, you'll need an inverter specifically designed to be hard-wired into the breaker panel. It will be more expensive and much heavier in weight, but there are still many options that won't break the bank. You won't find any of them at a truck stop, though--you'll need to contact a renewable energy dealer, either locally or online.


Off-grid, on-grid or both?

If your plan is to connect to the grid and sell your extra generated energy back to the utility, you'll need a specific type of inverter and your electric utility must approve your install. They'll have to put in their own special two-way meter, you'll have to sign a pile of paperwork and the utility will likely require a licensed electrician to sign off on your system. There are two types of grid-tied inverter, often called "direct" and "islanding."

Direct grid-tie inverters don't use battery backup, so if there's a grid blackout you won't have power in your home even if the sun is shining on your solar array. That's a big surprise to some folks, but the main reason is simply safety for utility workers. During a grid blackout they are out there climbing poles and splicing wires, and don't want to get zapped by the power your solar array is trying to send out into the grid. There is one new direct grid-tie inverter out there that can provide a limited amount of power during blackouts when the sun is shining, without a battery bank, but for the most part when the grid is down, so is your house, unless you have a backup generator.

Islanding inverters are far more versatile--if the grid goes down into a blackout, they completely disconnect from it and supply power to your home from your battery bank, giving you a little "island" of light in midst of a sea of frustrated, blacked-out neighbors. They all will likely migrate to your house anyway to watch the big game or charge their mobile devices, so make sure your critical load subpanel is in order so they don't suck your system dry (Countryside Volume 97, Number 6, November/December 2013). All islanding inverters also have battery chargers built in, so you can also fill your battery bank via a backup generator when needed.


Off-grid inverters are just that; they can't sell power to the grid. But most do have battery chargers built in, which can be run via generator or a grid connection. There isn't much price difference between off-grid and islanding inverters until you get into smaller sizes, so many off-grid homes use the latter, with the option there to connect to the grid if it ever becomes available at a reasonable price.

120-volt or 240-volt?

Most off-grid homes don't have many 240-volt circuits, as appliances that use 240v are generally power hogs and usually avoided--for example electric ranges, clothes dryers, and the like. However, many off-grid dwellers require a few common 240v loads, such as a welder in the shop and a well pump. The cheapest option for these is to simply power them from a fossil fuel generator when needed. Some larger inverters supply 240v output, and with most brands two inverters can be "stacked" to provide 240v (Illustrations 2 and 5), with multiple stacks possible to increase maximum output wattage. Another option is a 240-volt "autotransformer" running from a 120v inverter, which is cheaper than stacked inverters but also less powerful.

A standard home breaker panel is wired for 240-volt input from the utility grid; each side of the panel is wired into one 120v "leg" of the grid. However the National Electrical Code allows you to install a jumper between the two sides on an off-grid system and power the panel from a single 120-volt inverter, as long as any existing 240v outlets are removed and the panel is clearly labeled "Single 120-volt supply. Do not connect multi-wire branch circuits."

Which inverter waveform?

When you start shopping for inverters, you'll see the terms "sine wave" and "modified sine wave" prominently, with the modified version being quite a bit cheaper. In the world of inverter reality, though, both terms are inaccurate and somewhat misleading.

With only a couple rare (and expensive) inverter exceptions, the only place you'll find a "pure sine wave" is from the utility grid or a backup generator. Back in the day, inverters could only produce a "square wave." It was noisy and inefficient, and sensitive electronics didn't deal with it very well, either overheating or refusing to run at all. The problem is those quick voltage rises and drops on each wave cycle.


Modified sine wave inverters should really be called "modified square wave." They smooth the waveform out a bit, but are nowhere near a real sine wave. Illustration 3 compares all three waveforms. Some electronic devices may still have problems with modified square wave inverters (see the sidebar), and many items will see an efficiency loss of 10 to 20 percent. The waveform may also cause interference to radio and TV signals. Modified square wave inverters still have their place--they are very affordable for smaller systems, and are most often used in campers, RVs and cabins where a more expensive inverter doesn't make sense.

Modern "sine wave" inverters give a close approximation of a pure sine wave by using numerous small steps to simulate a smooth curve (Illustration 4). I have never received complaints from any clients about appliances refusing to run on a modern sine wave inverter, though interference in AM radio and sensitive recording studio equipment has been reported. In these cases, folks often choose to run the ham radio shack or recording studio from a separate (and expensive) "true" pure sine wave inverter.

Sizing an inverter

At first glance, sizing an inverter seems easy. Just add up the nameplate wattage rating of everything that might be running in your home at the same time, and select an inverter (or stack of inverters) with about 20 percent higher capacity to minimize wear and tear. For most efficient off-grid homes a maximum capacity of about 4000-watts is very typical, and many inverters are available in this size range. Inverters with less capacity are always a more affordable option, too, but may require some lifestyle changes on your part. My smallish house has a smallish solar and wind power system, so I went with a 2000-watt inverter. It's all I need--most of the time. I can run the rice cooker, microwave, lights and the television at the same time while making dinner, but there's not much capacity left over. If someone were to start a power tool or the vacuum cleaner during all that, the inverter could cope for a couple of minutes, but would then shut down and need to be reset (with no damage done).

You'll also have to consider the factor of "surge" or "start-up" current from different appliances and other loads. Well pumps and power tools are the worst, often drawing three to four times (or more) their continuous operating current to start spinning. Even household appliances like refrigerators can pull a significant startup surge; my fridge is rated at 350 watts maximum load on the nameplate, but a 1000 watt inverter is unable to get it started.


Most inverters do have a "surge rating," for example a 4000-watt inverter might be rated at 6000 watts for 5 minutes. But these ratings don't factor in how intense of a surge they can deal with. It's best to buy your inverter from a reputable renewable Energy dealer who has first-hand experience with what loads different inverters can handle. Also note that the higher your system battery bank voltage is, the better the inverter can deal with heavy surge loads. This is yet another reason 48-volt battery banks are by far the most common these days and 12-volt systems are now only used for very tiny systems (Countryside Volume 98, Number 3, May/June 2014 issue).

Power saving modes

Modern inverters are very efficient, in most cases over 90 percent when supplying loads. But they do draw power whenever they are connected to the system, even if no loads are running. So, most inverters have a power-saving "search" mode that uses only a minuscule amount of power until a load is sensed, and the inverter immediately comes up to full power. In larger off-grid homes that feature is often disabled, but with smaller systems where strict energy conservation is needed, it can be very handy.

Which brand of inverter is best?

I'm happy to say that if you buy a non-portable inverter from a reputable renewable energy equipment dealer instead of a truck stop, all of your choices are excellent, sturdy and long-lasting products with solid technical support, warranty and repair policies. Outback, Magnum, SMA and Xantrex (Schneider Electric) are the main players in the off-grid inverter market, with many other brands available for direct grid-tie systems without battery banks.

You may end up inverter shopping by price, but do try to seek some advice from a renewable energy professional. I have a particular brand of inverter I prefer simply because the remote display and programming unit is so simple and foolproof to work with--I can usually walk clients through diagnosing problems and changing settings right over the phone with no need for an expensive site visit.

Back in the day ...

Every time I install a new inverter, I think back to when they were so expensive, inefficient and recalcitrant that 12-volt DC house wiring and cigarette light plug sockets were the norm. Modern inverters are a marvel for off-grid and grid-tied homeowners alike, and make off-grid life "just like living in town"--as long as you pay attention to how much energy you generate versus how much you use.


Modified sine wave madness

It all started with my much-anticipated upgrade from a propane refrigerator to a brand new, high efficiency electric one. The addition of another 800 watts of solar electric to my off-grid system made it possible. Automatic defrost! A big freezer! And little lights that come on when I open the doors--no more fumbling around with a flashlight, and no more accidentally mistaking the carton of chicken broth for the carton of soy milk at 5:00 a.m., trying to make coffee.

The big burly fellows from Lowes who delivered the fridge weren't exactly happy about the long, bumpy drive up here to the mountain or having to take my front door off the hinges to bring in the fridge, and were even less happy when we plugged the thing in and ... it didn't work. One of them obviously had some experience, and asked "That isn't a GFCI outlet, is it? These new fridges don't work on a GFCI. Says so right in the owner's manual." That's short for Ground Fault Circuit Interrupter, and sure enough, though the outlet itself wasn't GFCI, it was downstream from one that was. I grabbed my portable generator, plugged the fridge into that, it worked fine, and those gentlemen left with a healthy tip.

My venerable 1500-watt modified sine wave inverter had no problem starting the fridge compressor, so I filled it with food and reveled in the luxury of electric refrigeration. After about a week, though, something seemed fishy. Literally, in fact ... those trout I had in there were starting to smell a bit "off" and the fridge didn't seem very cold. I procured a pair of thermometers and over the next couple days the freezer temperature got down to 35 below zero, while the fridge compartment came up to room temperature. Uh oh.

It turned out that the computer "brain" in the new fridge couldn't handle the modified square wave from my old inverter--the defrost cycle wouldn't turn on, and the plenum between the fridge and freezer had filled with ice crystals. Running the fridge off the portable generator for a few hours proved this.

My next step was to buy an inexpensive 1000-watt portable pure sine wave inverter; I figured to run the fridge via an extension cord until I could afford a bigger one that would run the whole house. Unfortunately, even though the nameplate wattage on the fridge was only 350 watts, the little inverter couldn't start the compressor. In desperation, I ended up breaking into the piggy bank and shelling out the money for the bigger, modern inverter I really needed--and I have no regrets. It's a huge improvement.

Because of the more friendly waveform, the fridge now runs fine, all my lights and appliances are now using about 10 to 15 percent less energy than before, and I have excellent power quality compared to the old inverter, which caused a buzz in my stereo and other issues. The portable sine wave inverter has been re-purposed to charge cordless tool batteries from my truck alternator at jobsites. And when I open the fridge for a midnight snack, a little light comes on!


Why is it called an "inverter?"

Because inverters convert direct current (DC) electricity from a battery into alternating current (AC), doesn't it seem more logical to call them "converters" instead? As it turns out, that name was already taken and the story is pretty interesting.

Back before the advent of semiconductors, the only way to convert electrical energy from AC to DC and back was through a complex, rotary mechanical switch called a "commutator." Big city trains, trolleys and trams ran on DC power because that way their speed could be controlled by adjusting the voltage fed to their motors, but the massive amounts of power needed to run them were produced by AC generation stations.

Big rotary "converters" with giant commutators (illustration 6) were used to accomplish this for the trains. As is turned out, converters could also be fed with DC power and produce AC on the other side by inverting the connections--an inverted converter, with the name now thankfully shortened to "inverter."

By Dan Fink


Dan Fink is the Executive Director of Otherpower, Buckville Energy Consulting, Buckville Publications, EEC, NABCEP/IREC accredited Continuing Education Providers. Phone 970-672-4342
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Title Annotation:Alternative energy
Author:Fink, Dan
Publication:Countryside & Small Stock Journal
Date:Jun 29, 2014
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