Modern Power Inverters are reliable, quiet, and come in a variety of sizes. To help demystify the Power Inverter selection process so you can choose an Inverter that's appropriate for your needs, this article pertains to residential-sized Inverters that produce 1 to 6 kW. The same process applies to larger systems.
Modern off-grid Power Inverters are sold with two waveform options: sine wave and modified square wave (sometimes called “modified sine wave”). Sine wave output, which has low total harmonic distortion, will Power virtually any type of load, even sensitive audio electronics. Although almost all residential Power Inverters have sine wave output, a couple of modified square wave Power Inverters also work well. For instance, a typical 2,800-watt sine wave Power Inverter costs in the neighborhood of $2,000, while a modified square wave Power Inverter with the same output retails for about $1,500. However, modified square wave P0wer Inverters may not run some types of loads satisfactorily, and some loads may not run at all.
Total harmonic distortion (THD) is the measure of how closely the waveform matches a perfect sine wave. Glitches, transients, harmonics, spikes, and distortion all describe alterations to the waveform shape. Power Inverter electronics produce steps to approximate a true sine wave—the greater the number of steps, the less THD an Power Inverter will have.
A THD of 0% is a perfect sine wave, and the larger the percentage, the farther it deviates from a sinusoidal waveform. Sine wave Power Inverters typically show a THD of 5% or less, while the THD of modified square wave Power Inverters may range from 10% to 40%. Because THD for modified square wave Power Inverters varies and depends on the type of loads running, values given from manufacturers are hard to compare fairly.
It is important to note that grid electricity also can have waveform distortions due to activity from all the different loads on the grid (such as large motors starting), which can cause transients in the utility waveform. Because of this continual variation of grid activity, sine wave Power Inverters often have even less THD than grid electricity.
Rated Continuous Output Power. - An off-grid Power Inverter must supply enough power to meet the needs of all the appliances running simultaneously. Before selecting a Power Inverter, you must know the loads you will power—and their power and surge needs.
Sizing a Power Inverter for an off-grid system, which is based on instantaneous load, is very different from sizing a grid-direct Power Inverter, which is determined by the RE power source (i.e., PV array watts). A grid-direct Power Inverters job is simply to convert all the DC from the PV array into AC Power, which is fed back into the house electrical system—then onto the grid if production exceeds household energy consumption. In a grid-direct system, the Power Inverter is not responsible for meeting the AC loads, since practically unlimited utility Power is available. For example, a 2,000 W grid-direct PV system would require choosing a Poer Inverter that accepts 2,000 W of PV on its DC input.
In the case of an off-grid system, the Power Inverter is usually responsible for providing energy to all the AC loads. Say you need to simultaneously power 2,000 W of AC loads. For an off-grid system, you’d need a Power Inverter that could supply at least that amount. Note that the PV array size does not enter into this Power Inverter sizing.
Each Power Inverter has a nominal battery voltage that it can be connected to. Common off-grid Power Inverter battery voltage options are 12, 24, or 48 volts.
Smaller systems are typically matched with lower Power Inverters and lower battery voltages. The converse is true for bigger systems. For example, several 2,000 W Inverters have a 12 V nominal battery bank voltage; 4,000 W models generally have 24 or 48 V battery bank voltages; and 5,000 W units are typically matched to 48 V battery banks only.
For the same Power, higher nominal battery bank voltage means lower amps (watts ÷ volts = amps) in the battery cables—which translates to less energy loss for the same-sized cables, or smaller-diameter, less expensive cables and smaller over-current protection for those cables.
Power Inverters used in the United States typically Power 60 Hz 120 or 120/240 V loads. Most off-grid Power Inverters have 120 V output, although some have 120/240 V output, which allows the Inverter to power both 120 V and 240 V loads. Inverters with 120/240 V output cannot supply all their output on one leg. They are usually de-rated by 75% or so for single leg (120 V) output only. To maximize performance, be sure to balance the loads on both legs when running 120 V loads. Inverters with 120/240 V input also can accept both legs of a 240 VAC generator, enabling you to get maximum capacity for battery charging with a single Inverter.
Some loads (like motors) require significantly more Power during startup than they need to run. To start these loads, inverters will briefly “surge” or run at higher than their continuous Power rating. Surge ratings include the maximum amperage and a time period that the inverter can run at that high power level without sustaining damage or turning off to protect itself.
Inverters may have several surge ratings (stated in either AC amps or watts), each corresponding to a specified time period. Most load surge happens in the first few milliseconds of start-up. For Power Inverters with several surge ratings, generally it is fine to consider the shortest one. For instance, OutBack Power Systems’ VFX3648 can surge to 70 amps for 1 ms. Typically this is the rating you would use to determine if the Inverter can supply enough Power for your surges.
Loads with induction motors, like washing machines, pumps, and power tools, can have large start-up surges up to seven times the running wattage. Look for “VA” on the nameplate, “locked-rotor amps,” or “surge rating” for clues that the load may have a high surge. To determine the surge of a particular appliance, either measure the load’s maximum amps with a recording clamp-on ammeter, look for “start amps” in the specification sheet.
Some off-grid Power Inverters include the capability to connect several units together to operate as a single, larger unit. Various stacking options allow 120 V inverters to work together to Power 240 V loads (such as well pumps). These Power Inverter configurations can accept both 120 V legs of a 240 VAC generator, allowing for full usage and balancing of the AC generator output, just like a single inverter with a 120/240 V output is capable of.
Stacking setups can allow one Inverter to “sleep” when Power needs are low, which helps reduce standby energy loss. Series stacking 120 V Inverters means that the Inverters have a 120/240 V output from two Inverters. Parallel stacking two inverters means the Inverters will output 120 V at double the amps of a single inverter. Some Power Inverters can be stacked to supply three-phase Power, often used for heavier machinery.
Some Power Inverters have a 120/240 V output available from a single Inverter (discussed previously), so series stacking is not necessary. They can be stacked in parallel to offer more capacity (higher amps).
Inverter Peak Efficiency is measured as the ratio of the Inverters AC Power output to the DC Power input from the batteries. Higher efficiency means that the Power Inverter wastes less power while converting DC into AC.
Note that “peak” efficiency doesn’t necessarily represent actual operating efficiency, which changes with the size of the AC loading on the Inverter. Peak efficiency is typically reached at about two-thirds of the
Inverters continuous output rating, and decreases as the continuous output rating is approached. Most Power Inverter manufacturers publish efficiency curves in their documentation. It is wise to choose an Inverters that has high efficiency ratings across a wide range of output wattage.
No-Load Draw is the Power used by the Inverter just to keep running when there is no load. No-load draw can be surprisingly high in some models (up to 30 W). Since there may be long periods of time when no Power is required by the loads, this can add up to a substantial energy drain on the system. For instance, a Power Inverter with a 30 W no-load draw will consume a minimum of 720 Wh daily. On small systems, this load can have a significant impact, especially in the winter when solar-made energy is at a premium.
Search Power. - Most off-grid Power Inverters have a power-saving feature called “search” or “sleep” mode to power down the high-energy-use components of the Inverter when there are no loads on. Search mode also requires power, but much less than the no-load draw. In this mode, the Inverter periodically tests the circuit for active loads and powers up only if a load is detected. But homes that have continuously running AC loads (like a telephone answering machine) are unable to take advantage of this feature and are stuck with a minimum of the no-load draw. Some off-grid homeowners will strive for always-on loads to be DC-powered to allow their Inverters to spend more time in energy-conserving search mode.
Battery Charger. Many off-grid Inverters have an integrated battery charger that can be used to charge the batteries from an AC source, such as an engine generator. This feature negates the need for a separate external battery charger. Having an integrated charger is especially helpful during periods when an RE power source cannot keep up with household loads, such as during the short and often cloudy days of winter. The battery charger is also used to “equalize” batteries by giving them a controlled overcharge, making sure that even the weakest battery cells are occasionally brought up to full.
Chargers are usually rated in DC amps, but may be stated as AC amps, so read the documentation carefully. In the table, AC battery charger maximum current has been converted to DC amps.
Battery Temperature Sensor. The internal resistance of a battery increases as temperatures drop and decreases as temperatures rise, affecting battery voltage. At a given charge rate, at low temperatures batteries can get undercharged and at high temperatures they can get overcharged. To properly charge batteries where the temperature strays from the ideal 77°F, a temperature sensor provides data to the charger so it can adjust the voltage set points for higher and lower temperatures.
Generator Start. Some Power Inverters can start and stop a generator based on several criteria, such as battery voltage, battery state of charge (SOC), load draw, and time of day. Generators can have either a “two-wire” or “three-wire” start mechanism. A two-wire start refers to two positions—on and off—and requires only a simple relay and a signal from a controller in the Inverter/charger.
A three-wire start. (A crank position, run, and stop—is more complex.) There may also be pre-crank and other settings, as needed for diesel engines. Facilitating a three-wire start usually requires a separate controller from the generator manufacturer. Typically, Inverters that advertise automatic generator start can be assumed to provide only the signal for a two-wire start.
Some Power Inverters offer metering as an optional accessory. Metering can provide helpful information about the system, including battery voltage (lets you know if the battery is being charged or discharged), AC load amps (indicates the size of the AC loads), battery charging amps (from the AC power source), and even error codes (helpful for Inverter troubleshooting).
With programmable Power Inverters, the meter is often also a user interface for controlling other functions, such as turning the Inverter on/off, starting a generator, or adjusting battery charger settings.
Usually the Power Inverter is installed away from living spaces, and remote metering allows users to easily monitor their systems from a location away from the balance-of-system components. Often, remote displays show various other system metering details and have a switch to shut off the Inverter. Aftermarket meters are available.
Some Inverters can be part of packaged systems to ensure that individual parts—such as metering, charge controllers, and circuit breaker/disconnect boxes—work together and physically fit together.
Integrated system components offer a few advantages. First, the unit is engineered so the components fit together easily. Second, proper wire sizes are accommodated in appropriately sized boxes, and knockout holes that match up in the boxes and components. Often, a mounting plate that supports the whole system and provides the layout for the components is included. These systems can be pre-wired by the factory or distributor to meet the specific needs of an installation. All the installer needs to do to the integrated components is properly wire the inputs and outputs.
Electronic and communications integration can optimize operations such as battery charging and load support, eliminate the duplication of sensors (such as battery temperature sensors for both charge controllers and Inverters), and provide a means for external data collection. A central control/meter can display system settings and data values, and simplify the user interface. Operations like generator start-and-stop controls can easily access needed parameters and data values, such as PV input, loads, and battery SOC.
Most of the weight of an off-grid Power Inverter comes from the iron core transformer, which gives high surge capacity. An iron core can store energy for a few cycles, creating a flywheel effect to carry the Power Inverter through surges.
If you are running only small electronics that have negligible surges, a lightweight Power Inverter may serve you well. Many of these type of Inverters don’t provide much surge capacity, yet they have a very fine waveform for finicky electronics, such as audio or telecommunications equipment.
If you are powering loads with high surges, like induction motors, seek a heavier Power Inverter, and ensure that it is securely mounted to support its weight.