A residential battery-based inverter has a primary task of accepting DC electricity and changing it into standard, household AC electricity. This seems simple enough, but wading through the inverter specifications can be overwhelming if you don’t understand what they mean and how they pertain to your system.
Your system may have other requirements (such as battery charging from an engine generator or the grid), and figuring out which features will be advantageous often depends on the system type and specifics. With no access to utility power, an off-grid system has to supply all of the electricity at all times. A grid-tied battery backup system has access to utility power, can use the inverter to export excess electricity back to the grid, and usually supplies electricity only to specific “critical” loads during a utility outage. These are some of the differences that make certain inverter features desirable in one system type but not necessarily in the other. As you go through the specification descriptions, you will see how these differences influence the inverter selection process.
This guide includes a specifications table for available battery-based, sine-wave inverters that are listed to the Underwriters Laboratories 1741 standard and commonly used in residential applications (2 to 8 kW). The compiled data is from manufacturers and their specifications sheets.
The Specs
Off-Grid, Grid-Tied, or Both tells us what system type(s) this inverter is built for.
Rated Continuous Output Power represents the inverter’s capacity. For example, a 2,000 W inverter is rated to supply 2 kW of AC power continuously. In an off-grid system, this value determines the total wattage limit of AC loads that can be run simultaneously. You must specify an inverter with an output power rating large enough to handle all of your simultaneous AC loads.
Let’s say we want to power the following at the same time:
- a 1,400 W microwave
- six 15 W lights
- a 100 W refrigerator
- a 120 W TV
In this case, an inverter with a continuous output power rating exceeding 1,710 W would suffice (1,400 + 90 + 100 + 120). Surge ratings are discussed separately.
For grid-tied battery-based inverters, the power rating is examined under two scenarios—when the grid is available and when there is an outage. When the grid is up, the inverter’s job is to convert all available DC power from the renewable energy system to AC, which is used in the home. If the array output exceeds household demand, the excess is sent to the utility. The inverter capacity must be large enough to accommodate the RE system size. For instance, an inverter for a 4,000 W PV array will generally be sized at that same power rating. (However, because climate factors such as warm temperatures will limit PV array output, the array-to-inverter ratio may vary.)
When the utility is down, the inverter’s job is to supply power to all the AC loads connected to it. Most of these systems include a “critical load subpanel” so that not all of a home’s loads have to be energized, which keeps battery and system costs down. The inverter capacity must be large enough to meet the total requirement of all connected AC loads that might be run simultaneously, and large enough to handle the RE output. (See “Sizing a Battery-Based Inverter” in the Circuit: Methods in this issue.)
Nominal Battery Voltage dictates the battery bank configuration. Only a few inverters (such as those from Exeltech) can accommodate multiple battery bank voltages. Most household-sized inverters require either 24 or 48 V battery banks. There are a few 12 V inverters on our list, but these are usually for smaller systems (think cabin-sized) that serve only a few AC loads.
AC Output Voltage has been limited to 120 VAC for a single inverter until the last few years. Now, several inverters have split-phase 120/240 VAC to power both standard 120 VAC loads and 240 VAC loads (such as a well pump). These inverters also have battery chargers to charge the battery bank using both legs of a 240 VAC generator. This saves generator run time and avoids having to include a 120/240 step-up/down autotransformer. Split-phase inverters also negate problems with wiring a single inverter to a load center that has multi-wire branch circuits, since this can possibly overload the neutral conductor. Additionally, several inverters on our list can also be connected in groups of three to supply three-phase 208 VAC output, commonly used in small commercial systems. (To see which models have this functionality, see the “Stackability” column in the table and look for “3Ph.”)
Peak Surge ratings reflect the inverter’s capability to supply significantly more than its continuous power rating for short periods of time. Certain appliances (i.e., those that have motors, like washing machines, refrigerators, and well pumps) will briefly draw more power upon initial startup. To find the surge requirement for a particular appliance, check the appliance spec sheet for the “start amps,” or contact the appliance manufacturer. Alternatively, you can measure it with a recording clamp-on ammeter.
Stackability is the capability to connect multiple inverters together to create 120/240 VAC output (series stacking) or increase output current (parallel stacking). Historically, the ability to series stack was handy for systems that needed to power 240 VAC loads, using inverters with 120 VAC-only output. Now that more split-phase inverters are available, stacking is usually done to increase inverter output capacity (amps). Stacked inverters can be programmed to activate only if needed so that when there is low power demand, standby losses are reduced (see “No-Load Draw”).
Peak Efficiency is the ratio of AC power out of the inverter to power in from the DC power source. The higher the efficiency, the less energy that is wasted in the inversion. Actual operating efficiency will vary depending on how much power is being pulled through the inverter, so inverter efficiency curves can be more helpful than the peak efficiency specification, and are often available in inverter manuals. On-grid systems spend most of their time processing RE-generated power to send to the home/grid, so high efficiency at the RE system’s power output rating is best. Since off-grid systems can spend much of their time requiring significantly lower power (when only a few loads are running), consider an inverter that has high efficiency at lower power output.
No-Load Draw (aka “Idle,” “Standby,” or “Tare” Loss) tells how many watts each inverter consumes simply by being “on.” This power needs to be accounted for when performing a load analysis for an off-grid system. Multiply the no-load draw by 24 hours to calculate the daily energy (watt-hours) consumed by the inverter.
Search Power ratings reflect the energy-saving “search” or “sleep” mode available in most off-grid inverters. This mode allows the inverter to nearly shut off during times of no-load draw. While the inverter still consumes some power to monitor household loads, the search power consumption is commonly about 75% less than the no-load consumption. Depending on the inverter “wake-up” wattage threshold, there may be some small AC loads that will no longer work if nothing else is turned on. Small, always-on AC loads (security systems, clocks, answering machines, etc.) can keep the inverter awake, consuming energy all of the time. For small loads, one tactic that can work is to shift to a consumer battery (like AA rechargeable) counterpart. Another tactic is to include a small, always-on inverter that is dedicated to those household appliances.
Integrated Battery Charger/Maximum DC Amps—Most of the inverters in the table include battery chargers that work on an AC power source (see the “Integrated Battery Charger” sidebar). The battery charger has a maximum DC current rating that will limit how much from the available charging source can be used. While the generator may be adequately sized, a lower battery charger limit can increase generator run time. One strategy is to install multiple inverters/chargers, which increases battery charger capacity. Ideally, the generator will be sized according to charger capability (see “Engine Generator Basics” in HP131).
Generator Start enables inverters to remotely start and stop a generator. Users can select a low battery voltage value that triggers the inverter to initiate a generator start and run sequence to charge the batteries. Other parameters can also be set to run the generator during times of high power consumption and/or during specific times of the day. While this feature can be handy, there are drawbacks (see “Automatic Generator Start” sidebar).
Dual AC Inputs allow users to use more than one AC power source, such as the grid and a generator, for battery charging. This is useful in grid-tied systems with battery backup, since it allows charging batteries from the grid when it is available and from an engine generator during times of utility outages (and low RE-system output), offering another source of backup power.
Remote Display is useful for keeping tabs on the system from a convenient location (such as the kitchen). These displays usually include user buttons to turn the inverter on and off, and to adjust programmed settings.
Prepackaged with Balance of System Equipment can be a time-saver when it comes to installing a battery-based inverter, since these systems have many components that need to be wired and located in the vicinity of the inverter and battery bank. These additional components are required in battery-based systems because there are multiple power sources (such as a PV array, batteries, generator, and the utility grid), and it is required to have disconnects and overcurrent protection between each system component and each power source. Other components can include charge controllers, meters (and shunts), ground-fault protection devices, inverter bypass assemblies, and communications hubs. Additionally, all of these components need a backplate to be mounted on and neatly fitted and wired together, further increasing the time and hassle savings offered by optional prepackaged power-panel assemblies.
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Justine Sanchez is a Home Power technical editor and an instructor for Solar Energy International. She is certified by ISPQ as a PV Affiliated Master Trainer.
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