Solar Inverter Buying 101: What to Look For
Inverters are an important part of any solar power system, as they are responsible for converting the DC power generated by the solar panels into AC power that can be used by households and businesses.
When selecting a solar inverter, it is important to consider a number of factors to ensure that you choose an inverter that is well-suited to your particular needs.
The following are some of the things that you should keep in mind when shopping for a solar inverter.
Wattage rating
Like PV modules inverter models are identified by their wattage output. The wattage output will be greater than or equal to the nameplate rating of your solar panel array.
You will want to make sure that your inverter has a high enough wattage rating to handle the output of your solar panel array.
It's important to remember that the size of the array (in watts) or the size of your maximum AC load (in watts) is what determines the size of the inverter you purchase.
If you plan on adding more modules in the future, you may want to purchase a slightly larger inverter to handle the additional power. Please note the MPPT circuit may not function properly if you purchase one that is too large.
Input Voltage
Inverters provide a range (or window) of voltages to accommodate varied PV array designs and sizes.
A solar firm or system designer must ensure that the maximum voltage going out of the array is less than what is specified on the spec sheet of the inverter.
At the same time, the inverter's starting voltage should not be so high that the equipment does not turn on during periods of low irradiance.
The maximum open-circuit voltage of your array should ideally be two-thirds to three-quarters of the inverter's operational range. For example, an inverter range of 150-450 volts would be good for a 300-400 volt array.
AC Output
This is the maximum load that the inverter can handle at one time for your household appliances and other electric equipment.
This specification is not a huge concern for regular grid-tied systems, because the grid is there to back up any electricity generated by a PV array.
Surge Power
This specification, like the previous one, is largely relevant for standalone and bimodal inverters. Many appliances and tools necessitate an additional burst of energy to start their motors.
The voltage spike that occurs as a result of these events may last one-tenth of a second or a few seconds, implying that an inverter must be capable of transferring the extra juice for only that short length of time.
Efficiency Rating
An inverter, like solar panels, is rated for its efficiency in processing energy. The rating considers current losses caused by circuit heating, low voltage levels, changes in how current flows or is stored, and the unit's internal energy usage.
However, be wary of a single efficiency rating stated on a product specification sheet.
The actual efficiency of an inverter changes depending on the loads it meets (or does not encounter), therefore an inverter with 90% “peak” efficiency may not be better than one with 85% peak efficiency.
A better specification (if you can get it) is “weighted efficiency.” On the spec sheet, you may also see a graph known as the efficiency curve.
Type of Sine Wave
This feature may sound a little geeky, but it's very crucial. Most efficient (but more expensive) inverters shift electricity to conform to what is known as a pure sine wave.
Many common off-grid inverters use a modified sine wave, which is significantly less expensive than a pure sine wave. With this lower-quality electric current, however, you run the danger of damaging some gadgets (particularly computers).
A modified sine wave can cause an appliance or tool to consume more power than it would normally, potentially damaging a motor or other circuitry. Square wave inverters are even less expensive, but should probably only be utilized in a barn or warehouse.
Grid-connected inverters should always produce a pure sine wave. Because you'll be using it for many years, the increased cost of this sort of inverter will be offset by the extra energy that comes from its far better efficiency.
Ambient Temperature Range
If you reside in an area where the temperature fluctuates, your inverter will be vulnerable to excessive heat or voltage spikes caused by cold weather.
Aside from carefully selecting a position for the unit (indoor or outdoors, shielded from the sun, away from snow, etc.), the inverter should have its own heat mitigation reaction, such as a fan (aka forced air).
In any event, a temperature range of -20°C to 50°C (-4°F to 122°F) should be adequate for all except the most extreme conditions.
Noise emissions
If you plan on putting the inverter indoors, noise may be a consideration. A grid-tied system will make some noise when it's first switched on, but that should be the extent of it.
For a standalone or bimodal system, however, the inverter may need to run constantly, making an incessant humming noise that some people find bothersome.
In this case, you'll want to make sure the inverter is located in an out-of-the-way location, such as a basement or utility room.
Some types of inverters are also available with sound-dampening features. These special units emit very little noise, making them ideal for locations where sound could be an issue, such as a hospital or library.
System monitoring features
Solar inverters usually come with some type of monitoring system that allows you to keep tabs on the amount of power being produced, as well as how much energy is being consumed.
This information can be very useful in helping you to optimize your solar array for maximum efficiency. Many grid-tied inverters also offer monitoring capabilities that allow you to see how much power is being exported to the grid.
Some standalone inverters come with an automatic generator start feature that kicks in when the batteries are running low. This can be a very handy feature if you live in an area where power outages are common.
Overcurrent protection device (OCPD)
An OCPD is a safety feature that protects the inverter from being overloaded. It's essentially a circuit breaker that trips when too much current is flowing through the system.
All inverters should have some type of OCPD, so be sure to check for this feature before making a purchase.
Ground fault interrupter circuit (GFIC)
A GFIC is another safety feature that helps to protect the inverter from electrical shocks. It's essentially a ground fault circuit breaker that trips when there is a difference in the current flowing through the hot and neutral wires.
Not all inverters have a GFIC, but it's definitely something to look for if you're concerned about safety.
Nighttime power consumption
Although inverters don't consume a lot of power, they do use some energy during the night to keep the batteries charged and the system running.
Ideally, an inverter should have a “sleep mode” or some other power-saving feature that kicks in when there is no sunlight available. This helps to reduce the overall power consumption of the system and can prolong the life of the batteries. Power consumption of 1 watt or less is considered to be very good.
Expansion slots
Expansion slots allow for the connection of external sensors or a remote display. This can be very handy if you want to monitor the system without having to open up the inverter.
Some expansion slots also allow for the connection of a data logger, which can be used to record information such as power production, battery voltage, and ambient temperature.
Weight and size
The weight and size of an inverter can be important considerations, especially if you're planning on mounting it on a roof or in another difficult-to-reach location.
In general, standalone inverters tend to be larger and heavier than grid-tied inverters. However, there are some very compact standalone units available that are easy to transport and install.
Type of enclosure
The type of enclosure is another important consideration. Most inverters come in a weather-resistant enclosure that is designed for outdoor use.
However, if you're planning on installing the inverter in an indoor location, such as a garage or utility room, you'll want to make sure it has an enclosure that is rated for indoor use.
NEMA 3R is the typical enclosure for most inverters. But it's always a good idea to check with the manufacturer to be sure what type of enclosure is best for your particular application.
Enclosures are usually made of plastic or metal. Metal enclosures tend to be more durable and weather-resistant, but they can also be quite heavy. Plastic enclosures are typically lighter in weight, but they may not be as durable as metal enclosures.
The reputation of the manufacturer
When choosing an inverter, it's always a good idea to select a model from a reputable manufacturer. There are many different brands of inverters on the market, but not all of them are created equal.
Do some research to find out which brands have a good reputation for quality and customer service. You may even want to read online reviews to get a better idea of which brands are most popular with consumers.
Tax credit/rebate compliance
If you're planning on claiming a tax credit or rebate for your solar installation, be sure to check that the inverter you select is compliant with the relevant program.
Each program has its own specific requirements, so it's important to do your research before making a purchase. Otherwise, you may end up being ineligible for the credit or rebate you were counting on.
Ease of installation
Installing a solar inverter is not a difficult task, but it's important to make sure that the unit you purchase is easy to install. Most grid-tie inverters come with everything you need for a quick and easy installation, including mounting brackets and all necessary wiring.
Standalone inverters may require a bit more work, but there are many compact units available that can be installed without too much difficulty.
Price
Of course, price is always a consideration when making any purchase. Solar inverters range in price from $200 up to $20,000 depending on the size and features of the unit.
Be sure to shop around to find the best deal, and don't forget to factor in any applicable tax credits or rebates.
Warranty
Just like any other piece of equipment, solar inverters come with a warranty. The length of the warranty will vary depending on the manufacturer, but you can typically expect a minimum of five years.
Some manufacturers offer extended warranties for an additional cost. These extended warranties can be very helpful in protecting your investment, so they are definitely worth considering.
Now that you know a bit more about solar inverters, it's time to start shopping around for the perfect one for your needs. With so many different models on the market, you're sure to find one that fits both your budget and your requirements.
