Inverters buying guide

What is a solar inverter?

Most home loads and appliances use AC power. However, solar panels output DC power. The inverter takes in DC power that is produced by solar panels or stored in a battery and converts it into AC power in order to run your loads.

Inverter Guide Contents

Common Types of Solar Inverters

Grid-Tied

A straight grid-tied system is one of the simplest and most cost-effective ways to incorporate solar into a home that is already connected to utility electric power. A basic grid-tied system only requires two main components: the solar panels and the inverter. A grid-tied inverter will use available solar energy to power loads in the home and then export excess solar production to the grid/utility whenever net metering is available.

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Off-Grid

Most off-grid inverters require a connection to a battery bank to operate. These do not accept solar power directly like a grid-tied inverter but instead rely on an external charge controller (separate component) to regulate energy flow from the solar panels to the battery bank. The inverter then discharges the battery bank to power your loads. An off-grid inverter provides maximum independence from the grid or utility company and allows a solar system to be active regardless of the presence or status of the grid.

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Hybrid Inverters

A hybrid inverter, also referred to as grid-tied with battery backup or ESS (energy storage system), incorporates the best of both grid-tied and o-grid type inverters. A hybrid inverter is capable of selling excess solar production back to the grid like a grid-tied inverter but can also easily be connected to a backup battery bank like an o-grid inverter.

Some hybrid inverter systems are all-in-one, while others are configured with individual components (i.e., separate inverter, charge controller, battery monitor, etc.). There are some hybrid inverter options which do not require batteries to operate, meaning one can often start with a grid-tied system and then add batteries later. This is usually the preferred inverter type for anyone that wants battery backup during a grid outage.

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Mobile Inverters

Inverters used on mobile applications perform the same function as the inverters described previously, just on a smaller scale. Most of the time a mobile inverter will have a lower output power rating than a grid-tied or o-grid inverter. This is because the power and energy needs for most mobile systems are usually much lower than a typical residential home.

Mobile inverters are not designed to sell excess solar production to the grid, but some can pull power from the grid (shore power) to run loads and charge batteries in the vehicle. Mobile-specific inverters that are designed to accept shore or generator power also have a switching feature between AC neutral and ground which is not found in the other inverter types.

SHOP RV/MOBILE INVERTERS

What should I consider when choosing a grid-tied inverter?

String Inverters

Typically the easiest and most cost-effective effective option for grid-tied solar systems.
A string of solar panels wired together is fed into the string inverter.
Not much additional equipment beyond panels, mounting, and inverter.
Most modern string-inverters have a built-in DC disconnect and multiple PV inputs.
Examples: SMA Sunny Boy, Fronius Primo

Optimizer Inverters

Module-level electronic devices installed on every solar panel in the system.
Helps mitigate shading losses.
Allows for module-level monitoring.
Most satisfy module-level rapid shutdown requirements.
Some are required for particular inverters.
Examples: SolarEdge, SMA + Tigo TS4-A-O.

Microinverters

Small inverters designed to work with a single solar panel at a time.
Installed on every solar panel in the array with no central string inverter downstream.
Produce AC power right at the solar array, so there is no high voltage DC present.
Provide integrated rapid shutdown, shade mitigation, and module-level monitoring.
Good for future expansion.
Examples: Enphase.

Rapid Shutdown (RSD)

Can be required for some grid-tied systems as an additional safety measure for first responders.
Reduces the solar array voltage to a safe level within a defined array boundary.
Must meet the national electric code (NEC) and RSD requirements for the location.
Basic string-inverter systems may require added module-level electronics to comply with RSD requirements.
Examples: OBFRS, TS4, SolarEdge optimizers, Enphase micros, MNSOB.

How do I know what size inverter to get?

Sizing a grid-tied inverter or microinverter system ultimately depends on the amount of solar panels required for the project. The size of the solar array is determined by energy requirements at the site, percentage of energy offset desired, and the available solar potential at the location. Other details such as available roof space and local code requirements may also determine PV array size.

If you’re just starting out, you can learn all about how to size your solar system in this handy overview:

Once the solar array size is determined, it’s time to consider inverter input specifications. With string inverters, there are minimum and maximum input requirements for both solar voltage and current. DC optimizers and microinverters also have minimum and maximum input specs, but these are on a module level for an individual solar panel, rather than string level. Most DC optimizers and microinverters also have limitations regarding how many can be connected together in a string, as well as how many can be used in a single system without requiring additional equipment.

How do I choose an o-grid inverter?

Inverters used for o-grid or standalone applications are designed to work independently of the electric grid. Since these types of inverters don’t require a connection to the grid, most require a connection to a battery bank. For o-grid applications, things like output wattage, surge capacity, and sine wave type are factored in when choosing an inverter, as this is often the primary source of AC power for that system.

Modified Sine Wave Inverters

O-grid inverters can be found with two types of output—pure sine wave and modified sine wave. Modified sine wave inverters are capable of powering a wide range of devices but are not the best choice for appliances with motors or high-end electronics. Sensitive equipment or newer electronics like computers, TVs and even some newer refrigerators and dishwashers are not compatible with modified sine wave inverters and may not function properly.

Pure Sine Wave Inverters

Pure sine wave inverters, on the other hand, have a smoother and more efficient output waveform. The electricity found in your home from the grid is pure sine wave. Inverters with this type of output will not have any issues running specific loads and are compatible with everything. All things being equal, pure sine wave inverters are usually more expensive than modified sine due to their increased efficiency.

Input and Output Voltage

O-grid inverters usually have fixed input and output voltages. Since nearly all o-grid inverters are battery based, they have a specific battery input voltage they are compatible with (most commonly 12V, 24V and 48V). A 12V inverter, for example, would not have a DC input voltage range wide enough to accommodate a 24V or 48V battery and could potentially be damaged by a higher voltage battery. A 48V inverter, on the other hand, might not even power up if connected to a 12V battery. Input voltage should be considered during the initial design, as most o-grid inverters cannot change or adjust their input voltage.

The output voltage of the inverter is just as important. In North America most household appliances use 120V AC. Some larger appliances like dryers, HVAC, water heaters, etc. require 240V AC. All o-grid inverters output 120V, but some can output 240V to power heavier loads. Also known as a split phase, there are o-grid inverter options capable of outputting 120/240V AC so that loads requiring either voltage can be operated, just like in most homes. Some inverters which are only capable of 120V output can be stacked with another unit to produce 120/240V. Inverter output voltage is also usually fixed, so if heavier loads are added later, a split phase inverter can make things easier.

Inverter/Charger vs. Inverter

Some o-grid inverters are actually inverter/chargers, meaning they can utilize an external AC power source for pass-through to loads and/or battery charging during times of high consumption or insufficient solar potential. In order words, an inverter/charger can pull power from the electric grid or a generator and use that to run the loads plus charge the batteries if they are low. That being said, inverter/chargers have both an AC input and AC output.

As soon as an inverter/charger qualifies incoming AC power from the grid or generator, an internal transfer switch within the unit engages. The inverter/charger switches to pass-through mode and lets power flow directly from its AC input to the loads on its AC output. In this state, the unit does not invert or discharge the batteries. Rather, the batteries are charged using the leftover incoming AC power which is not being occupied by the loads.

Most inverter/chargers allow you to adjust how much AC power they can pull from grid or generator, along with the charge settings for the batteries (charge voltages, timers, current, etc.). Some inverter/chargers even have two separate AC inputs, one for grid, the other for generator. An external transfer switch may be needed if multiple AC sources are to be used with an inverter/charger that has a single AC input. There are some inverter/charger options that will actually supplement incoming AC power with inverted power from the batteries to deliver more overall power to the load—such as in the case of an undersized generator.

High Frequency vs. Low Frequency

Within the world of battery-based inverters, there are two unique types based on the size and capabilities of their internal components. This and the speed at which the internal components operate during the inversion process are used to classify an inverter as high frequency or low frequency.

High-frequency Inverters

Make up a large portion of the retail market
Are usually smaller and less expensive than low-frequency inverters, however they can struggle with heavier loads like pumps, motors or other appliances with high startup surges.
Can be a cost effective and simple solution for many different types of applications, including light duty electronics and smaller appliances.

Low-frequency Inverters

More robust and heavy duty than high frequency types.
Are better suited for industrial loads and start up surges.
Are often more feature rich, including options for custom programming.
Often have higher quality components and longer warranties.

Rapid Shutdown

Rapid shutdown (RSD) is not as common in o-grid systems as it is with grid-tie, but it can still be required for some applications. RSD ensures that the solar array voltage reduces to a safe level within a defined PV array boundary in a set amount of time.

Ultimately, the local authority having jurisdiction (AHJ) will specify the correct National Electric Code (NEC) and RSD requirements for the site. There is not yet a uniform NEC for the entire country, so RSD requirements can vary by state and even by county. Inverters with integrated auxiliary outputs or relays can sometimes be used for power and/or control to the RSD components in an o-grid system.

Examples: OBFRS, MNLSOB, MNSOB

How do I size an o-grid inverter?

The most important metric in sizing an o-grid inverter is the unit’s output power rating. Calculating the maximum continuous AC load in the home or system is a good starting point in choosing the right size inverter.

To determine your solar load, you can start with our Off-grid Solar Load Calculator

Once you’ve determined the continuous running wattage of all AC loads in the system which could be in use simultaneously, make sure your inverter can supply that amount of AC power continuously. Any potential startup surges from heavy loads like motors or pumps must also be considered. An unspoken rule in all o-grid systems is to manage your usage so all your appliances aren’t running at the same time. This can allow for a smaller and more cost-effective inverter to be used as the instantaneous power demand is lower.

Since almost all o-grid inverters are battery based, the voltage of the battery bank determines the input voltage of the inverter, most commonly 12, 24, or 48 volts DC. Most inverters have a fixed battery input voltage, so a 12V inverter is and forever will be a 12V inverter.

If a small 12V starter system is eventually upgraded to a 24V or 48V system, the inverter would also need to be changed as well to match the new system voltage. One very important note regarding o-grid inverters is that their power rating has nothing to do with the size of the solar array or battery bank. Most o-grid solar systems utilize charge controllers to manage energy flow from their solar panels to batteries, so the number of solar panels one can have in an o-grid system is not related to the size of the inverter, or vice versa. It is common to oversize an o-grid inverter past the initial demand to account for a future increase in power needs.

If you go the inverter/charger route, then it’s especially important that the unit be compatible with whatever type of battery is used in the system. Most battery manufactures have recommended charge settings, timers, low-voltage disconnect points, and more for their batteries. Ideally, the inverter/charger needs to be adjusted to match these parameters. This is critical with lithium batteries which rely on a battery management system for safe operation.

What is a hybrid inverter system?

A hybrid inverter system utilizes the best of both grid-tie and o-grid worlds for a custom and flexible solution. These types of systems are commonly referred to as grid-tied with battery backup or ESS (energy storage system). These inverters are used in applications where there is existing grid power.

Hybrid inverters can sell excess PV production back to the utility company, like a grid-tied inverter, but will also provide backup power from batteries during a grid outage, like an o-grid inverter.
Most will allow for the solar array to stay active during a grid outage as well.
The majority of applications using hybrid inverters will require installation of a separate subpanel to contain the critical loads which must have power during a grid outage.
Hybrid systems are often the best choice to maximize self-consumption or do things like peak lead shaving/time of use.
Some of the newer hybrid inverters are all-in-one units, meaning there are inputs for solar, grid, loads, generator and battery all built into one inverter—there are no separate charge controllers or other components.
Some hybrid inverters do not require batteries, making this a great option for anyone who wants to get started with grid-tied solar but have the capability to easily add batteries in the future.
Most will also allow for charging batteries from the grid in certain situations.

A big factor in selecting the right hybrid inverter is how you want to use (or not use) power from the grid.

Should I focus on grid up or grid down operation?

A stand-out feature for most hybrid inverters is their ability to sell excess PV production back to the grid while still being able to maintain critical loads during a power outage. However, their mode of operation changes depending on the condition of the grid.

When the grid is up, the hybrid works similar to a grid-tied inverter, in that it feeds loads (and potential battery bank) with available solar energy, and any excess produced is sold to the grid. Upon grid loss, the inverter instantly detects the outage and ceases to export power but keeps the critical loads running on batteries and/or available solar.

What is an AC pass-through limit?

The inverter’s maximum sell back rating and/or AC breaker size will ultimately determine how much power can be sold back to the grid. This also determines how much power the inverter can pass through from the grid to the loads or pull from the grid for battery charging. Even with stable grid, it may not be feasible to run some of the heaviest loads in the home on the load side of the inverter. Exceeding the inverter’s AC pass-through limitations would trip breakers or even potentially damage the unit. Best practice would be to keep heavy loads—like HVAC or hot tub—on the grid side of the system and set with solar production whenever available.

How do batteries “communicate” with equipment?

The advanced capabilities and flexibility of hybrid inverter systems can be optimized by allowing direct communication between the equipment and the batteries. Some hybrid inverter systems have a closed-loop communication profile with certain lithium battery manufacturers.

Closed loop allows the batteries in the system to provide real-time status to the equipment and dynamically control its operation. In other words, the batteries tell the inverter when and how hard to charge or discharge, depending on their status. This helps to get the most out of a lithium battery bank and eliminates the need for in-depth custom programming of the inverter. Closed loop is not a requirement for most hybrid inverters and lithium batteries, but it does take an already sophisticated, advanced solution to an even higher level.

RSD for Hybrid Inverters

RSD is required for most hybrid inverter systems, especially if configured to sell power back to the grid. Inverters with integrated auxiliary outputs or relays can sometimes be used to power and/or control the RSD components in an hybrid system. Often RSD transmitters or accessories can be used with a hybrid inverter to control the array located components.

Examples: OBFRS,TS4, MNSOB

How can I size a hybrid inverter?

Finding the right size hybrid inverter requires some of the same tactics used in both grid-tie and o-grid inverter sizing. Once energy needs and potential system configuration have been determined and solar array size calculated, then the PV input of the hybrid inverter must be rated high enough to accept that number of solar panels.

The voltage and amperage input limitations of the hybrid inverter must also be considered. Maximum sell-back capability to the utility while the grid is up should be factored in as well, especially if that will be the primary operating mode. For instances of grid down, then sizing becomes most similar to that of an o-grid inverter. The maximum continuous running wattage of all critical loads which must operate during an outage is used to determine the output power of the inverter.

The inverter must also be able to handle any potential start up surges from heavy loads during grid down events as well. Some hybrid inverters have different wattage ratings for PV input and AC output power.

What should I think about when choosing a mobile inverter?

Inverters used in the mobile industry (RV, marine, vans) are extremely similar to o-grid inverters but on a smaller scale and with some other specific requirements.

Mobile inverters come in all different shapes and sizes, but their availability in pure sine vs. modified, high frequency vs. low frequency, and inverter-only vs. inverter/charger are just a few of the ways their features are similar to an o-grid inverter.

Inverter/chargers are extremely common for these types of applications, as well as inverters with AC pass-through. A mobile inverter with AC pass-through does not charge the batteries like an inverter/charger but lets external AC power go straight to the loads without discharging the batteries.

What’s an N/G switching requirement?

One of the standout features for most mobile inverters is that they automatically connect or disconnect their AC output neutral to ground. AC neutral to ground switching is a requirement for most mobile systems and prevents having multiple neutral-to-ground bonds within the AC side of a mobile system.

Generally, an entire electric system can only have a single instance where the AC neutral is bonded to ground. This is done for safety purposes, and in a normal home this bond is typically found in the main breaker panel. In mobile systems, the inverter creates this bond internally while it is inverting.

When a marine/RV system is connected to shore power or a gas generator is fired up, the mobile inverters break their internal neutral-to-ground bonds and rely on that same bond being made in the external AC source (shore or gen). This is done so that only one single instance of neutral-to-ground bonding is present in the system at all times. When the external AC source is disconnected, the inverter then re-establishes its own internal bond before it starts inverting again.

How can I determine my AC input limits?

Many mobile applications rely on occasional hookups to shore power or the use of a small generator for their power needs. If an inverter/charger or pass-through inverter is used, it’s important that the unit can properly utilize the external AC power from shore or generator. Common campsites and boat docks provide 30A or 50A shore power, but “moochdocking” with friends or family with just a 15A or 20A plug available can be a reality as well.

Most inverter/chargers and pass through units have an adjustable AC-input setting, so as not to overload the external source. The inverter would be adjusted to connect to a 30A service, and then adjusted again if the next campsite is 50A. The inverter’s transfer relay does need to be rated to match the size of any potential shore power or generator connection. Most inverter/chargers have just a single AC input, so an external transfer switch can be used to automatically select between available shore power or generator before connecting to the inverter.

What should I know about power distribution?

Distributing a mobile inverter’s AC power within a vehicle or boat is also part of the design process. There aren’t many cases where split phase 120/240VAC power is required for mobile systems, so often the AC output is 120VAC single phase for most of North America. Systems that plan to travel internationally may utilize a 230V 50Hz single phase rather than the 120V power used in the States.

For most mobile 120V applications, a single inverter can be used to power a 30A breaker panel. In systems with a 50A breaker panel, that run on two separate 120VAC legs, a single inverter typically only provides power to one side of the 50A panel. In that case, day-to-day or boon-docking loads would be moved to the inverter side of the breaker panel, while heavier loads reserved for shore or generator power remain on the other side.

Multiple inverters can be connected in parallel, or a transformer can be used with a single inverter to potentially power up both sides of a 50A RV breaker panel for instance.

How might size limit my inverter?

Perhaps one of the most unique constraints related to mobile inverters is their physical size and weight. Often there are times with mobile systems where equipment needs to be mounted in confined or difficult-to-access areas. Places like cabinets and storage compartments, under beds, in closets, etc. can all be called upon to house electrical equipment when free space is already a premium.

That being said, the weight and dimensions of the inverter should be verified ahead of time to make sure it will fit in the proposed space. The orientation of the inverter is also important, as some are designed to mount only one way, vertical on a wall for example, as opposed to sideways, horizontally or upside down. The inverter must have sufficient clearance when mounting and also some amount of airflow so that it does not overheat.

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