Photovoltaic Tutorial:

The Solar Inverter

In order to use electricity from a photovoltaic array in your home, it must first be converted from direct to alternating current. That’s because most electric loads (i.e. lights, appliances, tools, etc.) operate on AC. In a solar power system, an inverter performs this vital function. A circuit of on/off switches manipulates the current polarity so that it changes back and forth, just like the current delivered by the utility grid. Then the electricity passes through a transformer coil, which adjusts the voltage to 240/120 volts at 60 cycles (or Hertz) per second.

Technically, a solar inverter is known as a power conditioning unit, or PCU, since it does other jobs besides changing DC to AC. But that name sounds a little creepy, so most people just call it an inverter. And just in case you’re wondering, a rectifier converts AC to DC, while an inverter does the opposite, or inverse. Hence, its name.

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The Germany company Fronius, in the solar energy business since 1992, makes lightweight, high-tech central inverters (left photo) are popular in the United States. At right, a diagram of a grid-tied inverter manufactured by another company. (Not all inverters use transformers.)

Together, the inverter and PV modules comprise the big-ticket items of every home solar electric system. In a utility interactive (aka grid-tied) system, any surplus energy not needed in your home when it spurts out of the inverter flows into the grid itself, something electricians refer to as backfeeding. As a result, your electric meter may spin or count backwards, and you credit for the power.

Two principle measurements go into choosing a grid-tied inverter. The first is how much wattage your PV array will produce. The second is the total voltage configured in the array when modules are wired in series strings. For example, if you have two strings of 10 modules, each rated at 235 watts and 30 volts, that would give you an array of 4,700 watts and 300 volts.

Most designers configure an array's modules to meet the specs of an inverter they'd like to use. Inverter manufacturers provide sizing tools and calculators to help you find the right-sized model, and to size your array strings to fit the inverter's parameters. However, it's a good idea to perform the sizing calculations on your own first. (Configuring an array to match the inverter's specs is discussed at length in "Steps to Going Solar".)

In addition to conditioning and moving electricity where it needs to go, an inverter/PCU monitors and adjusts the current in the solar array. The circuit that performs this function is called a Maximum Power Point Tracker. The MPPT monitors the changing voltage and current in the PV array to insure optimum power output. While it may seem like a minor technical spec not worth fretting over, power tracking is actually pretty important to the bottom line, since they extract as many kilowatt hours of energy as possible from your array.

An MPPT tracking screen.

As mentioned earlier, an array's power output is ever-changing, due to the sun's movement across the sky, passing clouds, occasional shading and the effects of temperature on module voltage. Thus, the MPPT acts like a traffic cop in adjusting voltage and current. Research has demonstrated that solar panels produce more energy when the entire circuit is maintained near the peak of its volt-amp curve (aka an I-V curve). To do this requires variable loading, which the MPPT provides in the face of changing conditions.

Inverter Specs and Features

When you start shopping for an inverter, the primary features to consider include:

Type of inverter (grid-tied, standalone, bimodal, microinverter)
AC output capacity in watts
DC input voltage range
Minimum starting DC voltage (a hard-to-find, but important spec)
MPPT range or capability
Ambient temperature operating range (if you live in a place that gets really hot or cold)
Warranty

Here's a closer look at each of the product specs:

Wattage rating - Inverter models are designated by watts they output, just like PV modules. As explained earlier, the size you pick depends on the size of the array (in watts). However, if you’re buying a standalone inverter, it's size will depend on your maximum AC load and battery bank.

Input Voltage - Inverters offer a range (or window) of voltages to accommodate different designs and sizes of PV arrays, as well as the constantly changing irradiance through the course of the day (and the year). A PV system designer must make sure the maximum array voltage possible is less than the high number in the range. At the same time, the starting voltage of the inverter shouldn’t be so high that the equipment won’t kick on during periods of low irradiance, or on really hot days when wire voltage drops regardless of irradiance. Ideally, your array’s maximum open-circuit voltage should be about two-thirds to three-quarters of the high end on the inverter’s operating range. For instance, an inverter range of 150-450 volts would be right for an array configured for 300 to 400 DC volts.

Spec sheet data for several Xantrex GT (for Grid Tied) inverter models. "CEC" stands for California Energy Commission, which issues product quality standards used for their rebate program.

AC Output - This is the maximum load your home appliances and other electric devices can expect to get from the inverter at one time. For normal grid-tied systems, this spec is not a big deal, since the grid is always there to supplement any power generated by a PV array. For a standalone system, the AC output rating is a much bigger deal and usually requires that a load analysis be conducted. (Check the Sizing Worksheets page for a sample form.)

Surge power - Like the previous spec, this one is primarily important for standalone and bimodal inverters. Many appliances and tools require an extra burst of energy to start their motors. The voltage spike that accompanies these moments may last one-tenth of one second, or a few seconds. That means the inverter must be capable of sending the extra juice for that amount of time.

Efficiency Rating - Like solar modules, an inverter is rated for its efficiency in processing the available energy. The rating takes into account current losses due to circuits heating up, low voltage levels, changes in the way the current flows or gets stored, and the unit’s own internal energy consumption. Beware, however, of a single efficiency rating listed on a product spec sheet. The actual efficiency of an inverter should vary with the different loads it encounters (or even no loads at all). For this reason, an inverter advertised as having 90 percent “peak” efficiency may not be better than one with 85 percent peak efficiency. A better spec (if you can find it) is the “weighted efficiency”. You can sometimes find a graph on the spec sheet, known as the efficiency curve, which provides a more detailed picture of the inverter's capacity to produce power.

This is an efficiency curve for the Kaco Blue Planet 1500-watt inverter. The axis representing the percent of rated output power is based on how much voltage is being generated by the array. Notice that efficiency dips significantly when less than 20% of output power is used. However, the efficiency still remains well above 90%, as indicated on the Efficiency axis. Also, the variation between a high voltage (400VDC) and low voltage (125VDC) is not significant.

Type of Sine Wave - This feature may sound kind of geeky, but it’s still important. Most efficient inverters move electricity around to conform to what’s known as a pure sine wave. Many popular off-grid inverters use a modified sine wave, which is a lot cheaper to buy than an inverter with a pure sine wave. However, you run the risk of destroying your electrical devices (especially computers) with this lower-quality electric current. A modified sine wave may also cause an appliance or tool to use more power than it would otherwise, which can burn out a motor or other circuitry. Even less expensive are the square wave inverters, which should probably not be used outside a barn.

Grid-tied inverters should always feature a pure sine wave. In fact, utility companies are extremely picky about the characteristics of any electrical current being pumped into their grids. Since you’ll be using it over the course of many years, the higher expense for a pure sine wave inverter is offset by the additional power output that results from its much higher efficiency.

An oscilloscope is a measuring instrument that tracks electrical current in such a way that it produces a line moving horizontally across a screen. This is how scientists measure the amplitude and frequency (i.e. how frequently the polarity changes) in an AC current.

Ambient Temperature Range - If you live where the temperature dips or rises to extremes, then your inverter will be susceptible to excessive heat or voltage spikes due to cold weather. Besides carefully choosing a location for the unit (inside or outside, shaded from the sun, away from the snow, etc.), you’ll want the inverter to have its own heat mitigation feature, such as a fan. In any case, a temperature range of -20°C - 50°C (-4°F - 122°F) should work in all but the most extreme environments.

Depending on the design of its enclosure, an inverter can potentially generate enough heat to cause a significant power loss over time. A good-quality inverter includes a fan or other cooling mechanism.

Functionality and Other Specs - This refers to the selection of optional features an inverter model may offer, as well as the pertinent data needed to hook it up to other components. Some features to watch for:

noise emissions (hopefully not too loud)
system monitoring features (to track performance and troubleshoot)
overcurrent protection device (OCPD) (should be included)
ground fault interrupter circuit (GFIC) (should be included)
night power consumption (should be 1 watt or less)
expansion slots (allows plug-in of external sensors or a remote display)
weight and dimensions (to fit a desired location or mounting structure)
type of enclosure (NEMA 3R is good. See also NEMA ratings.)
warranty (At least 10 years is required for most rebates)
reputation of the manufacturer
tax credit/rebate compliance (Some incentive and rebate programs publish a list of eligible models.)

To explore inverter features and specs in more detail, check out this product spec sheet for the Fronius IG 4000-5100-watt series.

Transformerless Inverters

Popular in Europe, the transformerless inverter is a lightweight alternative on the market that dispenses with the heavy (and expensive) traditional copper coil that normally steps up or down the circuit voltage. A TL inverter offers two other distinct advantages over traditional inverters:

It provides a separate MPPT circuit on each input port or channel to regulate power. This is a godsend if you have uneven or partially shaded module strings, arrays with different tilts/orientations, or want to add a different power source altogether (e.g. a wind turbine).

It delivers energy to the home and utility grid more efficiently, due to the lack of a transformer (which generates heat) and the fact that neither current-carrying conductor has to be grounded.

Because there's no grounded conductor, the NEC sets out different wiring requirements for PV systems under Article 690.35. For one thing, PV wire must be used for any exposed wiring. (For most rooftop arrays, this is already standard practice, even for grounded circuits.) In addition, where overcurrent protection is required in the circuit, it must be placed on both conductors, not just the hot wire. That means using a larger box for the disconnects, combiner or any other enclosure that will hold the fuses or breakers. Also, when sizing the green equipment ground conductor (EGC), the ampacity rule requires a wire gauge that can handle double the calculation of the circuit ampacity.

Another downside of TL inverters: Many data monitoring systems on the market use the grounded wire in a PV circuit to communicate downstream. Now that the wire isn't ground, these devices won't work.

To learn more about ungrounded PV systems, download this in-depth article posted at SolarProfessional.com.

Bimodal Inverters

If you want to use your solar power system when the grid goes down, you have to buy a bimodal (aka bipolar) inverter. This unit functions as a normal central inverter most of the time, but when the inverter detects a utility power outage, it immediately disconnects from the grid and becomes a standalone unit.

A grid-tied inverter always shut downs in an outage and cannot be used in the home. This is required by law, since utility workers could be injured by backfed solar electricity pumped into distribution lines they're trying to repair. This dangerous scenario is known as islanding.

A bimodal inverter contains circuitry that allows it to transfer electricity away from the main service panel (which connects to the grid). The array's current is diverted through a subpanel that's set up to power critical loads in the home for the duration of the power outage.

To incorporate a bimodal inverter in your PV system, you must also set up a battery bank to store array electricity until it's needed.

Micro-Inverters

Using a single, central grid-tied inverter is the most uncomplicated approach when installing a home solar electric system. But it’s not the only game in town. Where shading is a significant problem, you may have better luck installing microinverters. One manufacturer, Enphase, has pioneered the development of this small, compact unit, which attaches underneath each PV module. Although this option costs more than a central inverter, it compensates for those extra dollars by:

Optimizing an array's power output by placing an MPPT at the site of each module.
Preventing shade across one module from lowering the voltage of others.
Allowing for easier expansion of your solar array in the future.
Immediately converting the DC electricity into AC at the array site.

Phillipsburg Electrical and Air
An Enphase microinverter mounted on racking. The PV panel to which it attaches will sit right on top when the array is installed

Enphase includes software that allows you to track your array output from a mobile phone or personal computer. The data can also be sent either through a hard wire or with a wireless transmitter. Not all brands of modules work with microinverters, so be sure to check the list Enphase has posted on its website.

Screenshot of Enphase’s data monitoring app, which reports the wattage produced by each module.