Inverter Pricing and Information

Inverters change direct current (DC) to alternating current (AC). Stand alone inverters can be used to convert DC, from a battery, to AC to run electronic equipment, motors, appliances, etc. Grid intertie inverters are used to convert the DC output of a photovoltaic module, a wind generator or a fuel cell to AC power that has the same phase angle as your electrical supplier. The energy goes back into your utility grid and your meter credits you with the amount of electricity you produce. Multifunction inverters perform both functions.

Stand alone Inverters

Stand alone inverters convert DC power stored in batteries to AC power that can be used as needed. Select an inverter for your power system based on the maximum load you will be powering, the maximum surge required, input battery voltage and optional features needed. High quality stand alone inverters are available in sizes from 100 watts, for powering notebook computers and fax machines from your car, to 60 kilowatts, for powering a commercial operation.

The size of an inverter is measured by its maximum continuous output in watts. This rating must be larger than the total wattage of all of the AC loads you plan to run at one time. Wattage of most AC loads can be determined from a tag or label on the appliance, usually located near where the power cord enters, or from the owner’s manual. (For help calculating the AC loads on your system, see our System Loads Worksheet.)  If the inverter is expected to run induction motors, like the ones found in top-loading washers, dryers, dishwashers and large power tools, it must be designed to surge, or deliver power higher than its rating for short periods of time while these motors start.

Every inverter gives you a digital representation of a sine wave.  A true sine wave is developed when the armature of a generator passes by its field coils.  When the speed of the generator is carefully controlled, you get 60 cycles per second. Good inverters produce a sine wave with as many as 380 steps.  Less expensive inverters use fewer steps.  Some are as low as 54.

So stand alone inverters are available with two basic power output waveforms: modified sine wave (often called modified square wave) and sine wave. Intertie multifunction inverters and utility companies deliver a sine wave. Xantrex UX series, DR series and TS series inverters and Samlex PSE inverters have modified sine wave output with harmonic distortion of around 40%. They are an economical choice in power systems where waveform is not critical. Some have a high surge capacity to allow them to start large motors while their high efficiency makes them economical with power when running small loads like a stereo or a small light. They can power most lighting, televisions, appliances and computers very well. Unfortunately, these types of inverters may destroy some rechargeable tools and flashlights, laser printers and copiers. They may not allow many laser printers, copiers, light dimmers and some variable speed tools to operate. Equipment with silicon controlled rectifiers or (SCRs) will also not operate. Some audio equipment will have a background buzz that may be annoying to music connoisseurs.  We also do not recommend them for computer systems with laser printers.  Some inverters have a sleep mode so they use very little power when there is no AC demand.  This can be a very important feature as you don't want to waste energy on an inverter that is taking power with no loads.

Exeltech,  Outback FX inverters and the new Xantrex XW are sine wave inverters. The older Xantrex SW series is a modified sine wave inverter.  Sine wave inverters have a higher cost, but they can operate almost anything that can be operated on utility power. Exeltech sine wave inverters are an excellent choice for power systems running audio or telecommunications equipment and other electronics that are wave form sensitive. The Outback and Xantrex XW series inverters can be ganged together for up to 18kw of output and can operate off grid or intertied. The Samlex sinewave PST inverters are a lower cost small system sine wave alternative.

Utility Intertie Inverters

Intertie inverters convert DC power from PV modules into AC power to be fed into the utility grid.

There are two major types of utility inverters, string inverters and low voltage input inverters.

The SMA Sunny Boy inverters are an example of a string inverters. The name "string" comes from the way the PV modules are wired together in series to achieve a higher voltage (+ to – and + to -). SMA inverters are designed to run at voltages up to 600 VDC. String wiring is faster to install, efficient and allows the use of small gauge wire. This makes for a very efficient system but DC voltages this high voltage can be deadly so string inverters should be installed and serviced with great care.

We are now happy to offer the Fronius IG series inverters which in some situations are more versatile than the SMA inverters because they will work between 150 and 450 volts. Hence, if the voltage of your solar array is too low for an SMA Sunny Boy, the Fronius inverter would be a good choice.

We can also offer the StarInverters from PV Powered in Oregon, highly efficient inverters, 96% for the 5200 W, that come with a 10-year warranty and use the fewest number of moving parts and are designed with MPPT to convert the most power available from your PV system, even on cloudy days.

A utility-tie PV system uses the utility company as a storage battery. When the sun is shining, your electricity comes from the PV array, via the inverter. If the PV array is making more power than you are using, the excess can be sold to the utility (power company) through your electric meter. If you use more power than the PV array can supply, the utility makes up the difference. This type of system makes the most sense if you have utility power, because there are no batteries to maintain or replace. Unfortunately, if the utility power goes down, this type of inverter will also go down.

Multifunction Inverters

Using a multi-function inverter allows you to sell excess power to the utility and also maintain a battery bank for standby power in the event of a utility power failure. The Outback FX and Xantrex XW Series are primarily stand alone inverters that can function as intertie inverters/battery chargers, at the same time but with a slightly lower efficiency than an inverter designed for intertie purposes only. (A charge controller with Maximum Power Point Tracking (MPPT), can make up for some of the lower efficiencies of the battery supported inverters.) The Beacon Power M5 is another intertie inverter that is designed to provide battery backup when the utility fails. SMA and Fonius are also working on  similar inverters.

In a typical installation, the inverter is connected to a battery bank, the utility power lines, a standby generator and the house load center. If the utility is available, the inverter will supply power to the house loads from the utility while keeping the batteries fully charged. If the utility fails, the inverter will supply power within 600 milli seconds to the loads from the battery while accepting power from the inputs from solar or wind or a standby generator. When the utility is available again, the inverter will switch the loads back to the utility and recharge the batteries. If the batteries become fully charged the excess power would be put back into the grid while crediting the electric meter.

Wiring Considerations

Stand alone inverters require high current from a battery to operate large loads. A 2000 watt inverter running at full power in a 12 volt system will draw nearly 200 amps of power from the battery. Large cables and good connections are required for proper operation. Use caution when plugging a small inverter into a lighter outlet located far from the battery. Typical DC house wiring may have insufficient wire sizes and too much voltage drop to supply the current required by these inverters. All battery based inverters require proper fusing between the battery and the inverter.  If there is ever a catastrophic failure of the inverter, this fuse or circuit breaker would prevent it from getting excessive power from the battery.  This power could do excessive damage.

Interference

The electronic inverters may cause problems with radio and television reception, noise on telephones and buzz in audio equipment. Sine wave inverters cause the least amount of interference. Interference can be minimized by locating the inverter very close to the batteries, twisting together the cables  that connect the inverter to the battery bank and by running AC lines separate from other wiring (such as phone lines). Also locate the inverter away from appliances that are susceptible to interference. All inverters cause interference on  lower channels of AM radios!  Mu metal can be used to shield the wires from the battery bank to the inverter.  Mu metal has high permeability and is used in shielding for transformers.

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