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Battery-Based & Hybrid Inverters

 

A battery-based inverter converts direct current (DC) power from batteries into alternating current (AC) power to operate lights, appliances or anything else that normally operates on electricity supplied by the utility grid. All battery-based inverters can be used in off-grid systems and some can also feed power back into the utility grid using net metering, similar to the more common grid-tie inverters. All of these battery-based inverters require a battery bank to function.

Grid Interactive Inverters for Backup Power Applications

Grid interactive inverters, also called dual-function or hybrid inverters, can export power to the utility grid, but can also supply emergency backup power for critical loads during a grid outage. These inverters use a battery bank for energy storage (they will not operate without batteries) and include an automatic transfer switch that enables them to safely operate off-grid during a blackout.

The grid interactive inverter is connected to the battery bank (usually 24 or 48 VDC), an AC sub-panel for critical loads, and the building’s utility entrance load center. The battery bank is charged by the PV array connected through a charge controller (see Charge Controllers). Under normal conditions, it operates like any grid-tie inverter and will export any excess power produced by the PV array. During a grid outage, the inverter will automatically disconnect from the grid and supply AC power to the critical load subpanel by drawing energy from the battery bank and solar array.

When the outage is over, the inverter will automatically switch back to grid-tie operation and recharge the batteries. The OutBack Radian GS and the Schneider XW grid interactive inverters have split-phase 120/240 VAC output from a single inverter. The OutBack GFX and SMA Sunny Island inverters have 120 VAC output, but two inverters can be “stacked” for 120/240 VAC output.

The SMA Sunny Island is designed to interact with a Sunny Boy grid-tie inverter to create an “AC Coupled” power system. See the Sunny Island listing for more information. It is important to note that a significant amount of energy is used to maintain the battery bank. For this reason, systems with battery backup typically provide 5 to 10% less energy (kWh) to the grid than equivalent grid-tie systems that don’t include batteries.

Off-Grid Inverters

Off-grid only battery-based inverters convert DC electricity from a battery bank to AC. In this case, the PV array and/or wind generator is used to charge the batteries via a charge controller (see our Charge Controllers section for more information) and only the power demanded by the loads is inverted to AC. Because these systems do not have access to the electrical grid, it is important to properly size the inverter and battery bank (see our System Design and Batteries sections for more information on sizing
inverters and battery banks).

The nameplate capacity of an inverter is measured by its maximum continuous output in watts. The inverter capacity limits the sum of all AC loads you can operate simultaneously. Most AC appliances list their consumption on a tag located near the power cord and/or in the owner’s manual. You will need to add up the consumption of all the appliances you may need to operate at once and that will represent your minimum inverter size. If your appliances include induction motors, like washers, dryers, dishwashers, furnace electronic controls, and large power tools, be sure to select an inverter with sufficient “surge” capability to accommodate the higher start-up loads.

Off-grid inverters will output either sine wave or modified sine wave (modified square wave) AC waveforms. Sine wave inverters can closely mimic utility grid power and will run virtually any AC appliance. Sine wave inverters with cleaner waveforms, such as the Exeltech XP line, are often desired for sensitive audio or telecommunications equipment.

Modified sine wave inverters are an economical choice when waveform is not critical. They often have a high surge capacity for motor starting and generally retain good efficiency when partially loaded. Unfortunately, this type of inverter may damage or fail to operate some sensitive appliances, such as rechargeable tools and flashlights, laser printers, copiers, variable speed drives, and any equipment with silicon controlled rectifiers (SCRs). Some audio equipment will have a background buzz when operated with a modified sine wave inverter.

Output Voltage

In the past, most battery-based inverters supplied only 120 VAC 60 Hz single-phase outputs. Now many of the more popular residential-sized inverters, like the OutBack Radian, Schneider XW, and Magnum MS-PAE inverters deliver 120/240 VAC power from one inverter. These inverters can also be wired in parallel for greater power output. Pairs of some 120 VAC output inverters like the OutBack FX series and Sunny Island inverters can also be wired in series for 120/240 VAC split-phase, or 120/208 VAC three-phase output.

Inverters that supply 50 Hz power are also available for most product lines. Contact us with any special requirements that you have.

Interference

Battery-based inverters may interfere with radio and television reception, causing noise on telephones or buzz in audio equipment. Interference can be minimized by using sine wave inverters and by locating the inverter as close to the batteries as practical, twisting together the cables that connect the inverter to the battery, running AC lines separate from other wiring (such as telephone wires), and locating the inverter away from appliances that are susceptible to interference. All inverters cause interference with AM radio reception.

Wiring Considerations

Battery-based inverters require high current from a battery bank to operate large loads. A 2 kW inverter will draw nearly 200 A from a 12 VDC battery bank. Large cables and good connections are required for safe operation. Use caution when plugging a small inverter into a lighter outlet in a vehicle, as these outlets are usually not robust enough to handle high current for long periods of time. All battery-based inverters require proper fusing between the battery and the inverter.

Pre-wired power systems are available with most battery-based inverters to minimize design and wiring issues. Custom configurations are available for most OutBack FLEXware-based power systems.

 

More Information on Batteries and Grid Based Inverters

The weakest link in the chain with a battery-based inverter is the batteries. We are on the cusp of time when batteries are going to have a lot more power and the newer batteries would be similar to the lithium-based batteries used in electric cars.

Ultracapacitors

Another new technology are ultra capacitors, or super capacitors.  A good analogy for a battery and an ultracapacitor is holding water with a roll or toilet paper or a pitcher.  The roll of toilet paper releases water very slowly and has a limited life expectancy.  The pitcher of water can release water quickly or slowly and can be filled and emptied many many times. The cycle life of an ultra capacitor is often one million cycles, which is about 2,700 years.  This is a nice way to store electricity but right now the ultra capacitors don’t hold as much power as a battery.  This is rapidly changing and ultra capacitor banks are already used in some electric buses;  ultracapacitor banks are the perfect storage device for a vehicle that is always stopping and starting.

Battery Voltages

Most people don’t realize that a higher voltage battery bank works much more efficiently than one that is 12 or even 24 volts.  Charge controllers are rated by watts so if you have a 60 amp charge controller, as an example, and you multiply it X 12 volts you would get a much lower number than multiplying it X 48 volts and the charge controller can handle a lot more power when used with a 48 volt system, as opposed to 12 or 24 volts.

Battery Banks wired in Parallel or Series?

Another problem we have encountered is how many batteries are wired in parallel in a battery bank.  This is particularly frequent on 12 volt systems when as many as 8 strings of batteries are installed in parallel. In parallel setups the batteries will not get charged evenly — some will invariably have a higher charge than others.  This doesn’t happen when a string of batteries is connected in series, in which case the batteries work better, charge more evenly and will last a longer.  Three strings of batteries are the absolute max that you can use but it is better not to go over two strings.  If you need more power, it is better to have larger batteries in one string than smaller batteries in multiple strings.

Power when the Grid is Down

Grid intertie inverters can be battery based so that not only will you have a system that will give you power when the grid is down, but you will also have an inverter that can feed excess power into the grid.  This is called a syncronous inverter as it matches the phase angle of the electricity from the electricity supplier.  Grid tie inverters often are also battery chargers as well.  When the inverter sees 120 VAC on the input, it triggers a large relay inside the inverter which converts it into a multiple stage battery charger.  Some inverters have the battery charger using up to 5 stages of charging just like a charge controller.  This is good for the batteries and allows a gas or diesel generator to be an integral part of the solar system.
Some battery-based inverters use the input of the inverter to also feed electricity back to the grid.  The output connection of the inverter can be connected to one or several circuits that you would want powered by the inverter if the grid ever failed.  The inverter then acts like a large uninteruptible power supply and within milliseconds starts providing power to anything that is plugged into it–  good for items like computers or refrigerators.
Every inverter gives a digital representation of a sine wave and some inexpensive inverters give a signal that duplicates a sine wave very well.  This means it has many steps on each waveform.  Some inverters have as many as 375 and others have as little as 56.  This is the difference between a sine wave and a modified sine wave or a square wave and some things just don’t like modified sine waves.  A motor, for example, will produce a lot more heat when powered by an inverter with an inferior sine wave.
Some electric companies are giving customers a hard time with battery-based systems because of the fear that customers may be charging the batteries with solar and then selling the power back to the grid at peak demand for higher cost.  However, this is not a good use of renewable power resources and does not make economic sense.  There is no energy savings in this methodology since you will lose a  percentage of the power you have created as heat, which will make the batteries warm since a battery needs to go through a chemical change in order to charge. Also, keep in mind the higher cost of replacing batteries that have been frequently used and you can see the assumption that people will try and do this rather ridiculous.  Possibly this could be a viable option if one were using ultra capacitors instead of batteries for their inverter.

Transformerless Inverters from SMA

If you want  a really efficient inverter that can connect to the grid and allow power generation if the grid fails, consider the new SMA 3000 TL, 4000TL and 5000TL transformerless inverters from SMA.  These inverters are quiet, lightweight and easier to cool because they don’t have a transformer.  Also, being transformerless increases the efficiency of the inverter by about 2% and this 2% is manifested by producing extra heat in an inverter with a transformer.  These inverters also have an emergency plug which allows you to get 15 amps of 120 VAC power out of the inverter if the grid fails.  These SMA  inverters can remove the necessity of having a lot of batteries, as is the case with an Outback or Scheider (Xantrex) inverter,  as it can provide power if the grid fails.  Even though the transformerless  inverters are made to work by turning a meter backwards, they have the added benefit of an emergency power output plug,  a fabulous design.
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