1) WAVE FORM:
This refers to the shape of the electrical AC wave that is produced by the inverter. The ideal form of AC power is the pure, smooth sine wave; it is what the electrical grid is supposed to provide. Different inverters have different wave forms that range from rough approximations of a sine wave to pure sine waves that are smoother, "cleaner," than that of the grid.

Potential Problems with Modified-Sine Wave: radio hum lines on television screens, buzzing of fluorescent lights, dimmer switches, and ceiling fans, malfunctioning of electronic controls, increased heating of transformers and electric motors, lowered performance and efficiency and shortened life span of electric motors, etc.

Xantrex Stepped Sine Wave

Pure Sine Wave

Modified Sine Wave is the wave form most inverters produce. It is more accurately called Mod square wave, since it more closely resembles a square wave than a sine). Since this wave form isn't fluid in shape and has large "corners," it sometimes causes problems with certain appliances.

Ultramodern washing machines and a few other appliances with electronically controlled, variable-speed motors won't work at all on mod-sine power, and some people in the RE field come down hard on mod-sine inverters because of this. However, mod-sine isn't as bad as all this might make it seem.

Most people experience no problems whatsoever with their mod-sine system: We have found mod-sine inverters to be great for low cost, off-grid homes and grid backup systems. Despite the claims, mod-sine inverters work perfectly well with the vast majority of appliances including most stereos and computers. We ran our entire shop off a Trace DR 2424 (the earlier version of the Xantrex TR) for years, and many of our more affordable systems still use TR inverters.
The best part is: they're the most reliable. In our years of RE work, we haven't had a single DR orTR fail in the field.

The next step up from mod-sine is the wave form generated by the Xantrex/Trace SW and SW+ series. Trace calls it a sine wave, but we call it a stepped-sine wave, since it is composed of 34 to 52 little "steps", depending upon the amount of load placed on the inverter at the time. The stepped sine is utility grade and can be fed (sold) to the grid, although it may not be clean enough for some critical applications. Stepped-sine inverters can cause the same problems as mod-sine inverters, though less pronounced. (see side bar of Mod sine problems).

The OutBack inverters also produce a stepped sine wave, though they too are billed as sine wave inverters. Their wave-form is much smoother than a Xantrex SW or SW+, having around five times as many steps. For many applications the OutBacks seem have the best of both worlds: a very clean wave-form, with surge capacity and the features of a general use inverter, such as battery charger and transfer switch. Magnum sine wave inverters have a wave form that is roughly twice as smooth as the OutBack.

Pure sine wave inverters are made by a few manufacturers. As mentioned above, the power provided by them can be "cleaner" than grid power. Exeltechs are the cleanest of the clean, being the preferred inverter for off-grid recording studios. High-end recording studios with grid power are sometimes set up with pure sine inverters for the musical equipment and recording gear, so as to guard against noise and voltage dips and spikes from the grid. Sometimes people will dedicate a small pure sine inverter for their stereo, computer, or other critical load, while using a larger, less expensive mod-sine inverter for all the other house loads.

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2) GRID-INTERTIE vs. OFF-GRID:

Inverters for off-grid battery-based systems are designed for stand-alone power systems that can operate independently of the electric grid. Many of these inverters have AC inputs for grid power as well; so, if grid power is available to the site, it can be used for back-up power and emergency battery charging the way one would use a generator. In this way, an "off-grid" inverter can have the option of grid back-up. This is different than what is called grid-tie. Grid-tie systems can be of two types: batteryless and battery-based.

Batteryless grid-tie systems don't use batteries to store electricity, instead they use the electric grid for that purpose: electricity is drawn (bought) from the grid when energy use is higher than system production, and electricity is fed (sold) to the grid when more energy is produced than is used.

The biggest downside to a batteryless grid-tie system is that when the grid has a blackout, so do you. Batteryless grid-tie inverters require a signal voltage from the grid; without it they shut down. The only way to gain true independence from the grid is with a battery-based system.

As utility companies become more accepting of grid-tied RE systems with buyback or net metering capabilities, they are becoming more popular. Having a batteryless system not only does away with the considerable expense of buying and periodically replacing batteries; also avoided are the hassles of maintenance, as well as the need to monitor the batteries to make sure that the energy demands placed on them balance the energy production, lest they become too drained. Since excess demand will simply be handled by the grid, the result is a very self-regulating system that needs little attention.

A batteryless grid-tie inverter, like the SMA SunnyBoy, is able to accept a wide range of Solar module power that can be much higher than the voltages standard with battery-based inverters (up to 600V with the SunnyBoy 2500). Wiring a solar array for a higher voltage saves money on wire and reduces the power losses associated with lower voltages.

The biggest drawback to a batteryless grid-tie system is the fact that the system will shut down when the grid goes down and will not provide back-up power during a power outage. This is done in order to protect utility workers who will be trying to restore grid power.

Battery-based grid-tie systems combine the best of both worlds: surplus power can be sold to the utility company, and power can be bought from the utility's electric grid when needed. Plus, if grid power fails, the system does not shut down with it. Programming is still being developed to keep up with all the choices possible with this arrangement. When to sell, when to charge the battery, etc.

Note: the rest of this guide does not apply to batteryless inverters.

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3) PRICE & CAPACITY vs. NEED:
Price is obviously a big consideration. Buying too much inverter is a waste of money: getting an inverter with more power or more features than needed, or getting a pure-sine wave inverter when modified-sine wave power would run all your loads perfectly well. But not buying enough inverter can also be a waste: getting a cheapo that will be a door stop in a year, a quality one that is smaller than you need or might soon need, or a mod-sine to power sensitive electronics. The very first step in selecting an inverter is to look critically at the loads it will need to run. The cost of your inverter, and of your power system as a whole, is directly related to the amount of electricity you use.

4) SIZING THE INVERTER:
To do this, you must first figure out and add together the wattages of all the continuous loads that will likely run simultaneously, in a highest electrical use scenario. (Relax, absolute precision isn't important; it's best to overestimate a bit). These include lights, home entertainment, computer, microwave, etc. (To help you with this, see the Solar Sizer for a more complete listing of household loads.) This total must be the same or less than the continuous wattage rating of the inverter you buy.

LIST OF COMMON SURGE LOADS: Air Compressor (15A)-- 1100-9000 W Circular Saw (not worm drive)-- 2000- 4000W Refrigerator (Energy Star)-- 400 W Table Saw (1.5hp)-- 2000 -7000w Vacuum Cleaner-- 1200 W Washing Machine--700- 3000w Well Pump (1hp)--1000-3000w (These are average surges, actual wattages may vary.)

Next, to determine the surge capacity of the inverter you need, add up the surge watts of all the loads that might start at the same time. Then add this total to the continuous wattage total you calculated above. Think of the water pump and the washing machine turning on at the same time as the refrigerator compressor (three surge loads) while you microwave dinner with the TV and half the lights in the house turned on (continuous loads).

If your inverter is undersized, a combination of loads such as this will trip the inverter's output breaker; this is known as nuisance tripping, and it truly is a nuisance.

Inverter Power Rating to start an AC well pump (with no additional loads)

* Typical running power is 10-20% less, but add 10% for typical inverter loss. For "modified sine wave" add another 15%, due to loss of efficiency in the motor.

An inverter sized by these minimum guidelines will dip its voltage during the starting surge. This is not harmful, but it will cause lights to dim. Fluorescents may blink off, and desktop computers may crash. To eliminate voltage dips, oversize the inverter by an additional 50% minimum plus the watts capacity required to handle other household loads at the same time.

Inverter Sizing Chart Copyright ©2002 by Dankoff Solar Products, Inc
Link to Complete Dankoff Article