Battery-Based Inverters 101

A battery-based inverter converts direct current (DC) from batteries into alternating current (AC) at the appropriate voltage and frequency 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 backup power to protected loads during a grid outage. These inverters use a battery bank for energy storage, 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 protected loads, and the building’s utility entrance. The battery bank is charged by the PV array connected through a charge controller or from a grid tie inverter via AC coupling. Under normal conditions, it will export surplus power produced by the PV array. During a grid outage, the inverter will automatically disconnect from the grid and supply AC power to the protected load sub panel 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.

There is now a new version of the UL 1741-SA standard to meet the new grid interactive requirements of the utilities in California and Hawaii with more states to be added during the year.

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) per kW of PV array than equivalent grid-tie systems that don’t include batteries. Grid-Interactive Inverters for Self-Consumption Applications Many of the grid interactive inverters can also work in applications where there is grid power, but net metering is not available or it is better to self-consume the solar power instead of feeding it back to the utility at a reduced rate. Another use for these is for peak load shaving applications, where supplying the peak load from the battery instead of the utility prevents the high peak load rates charged by the utility. There is a new type of hybrid inverter that is primarily a grid direct inverter that will operate without a battery, but can also use a battery bank for self-consumption with backup applications.

Off-Grid Inverters
Off-grid 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 Charge Controllers 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.

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 – that will represent your minimum inverter size. If your appliances include induction motors, like washers, dryers, dishwashers, furnace blowers, heat pumps, 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. 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 FXR 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 for export to other countries. Please contact us with any special requirements you may have.

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 are likely to 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 overcurrent protection 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. Prewired power systems with Schneider Electric, Magnum Energy and SMA Sunny Island inverters are also available. Please contact us for additional information.

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