A Guide to Inverters

Going off-grid is a great choice for many individuals, from people living in RV’s and vans to those in remote areas. Off-grid solar systems utilize batteries to store energy produced from solar panels. Inverters play another key role in solar installations and enable you to safely and efficiently power your devices and appliances.What should you know about solar inverters to ensure you have a powerful, safe, and efficient solar installation?


What is off-grid solar?

Off-grid solar systems, or stand-alone power systems, produce enough energy through the usage of solar panels and battery storage without having to tap into the electric grid. In the past, off-grid systems were often out of reach for most people because of the high costs of inverters and batteries. However, battery and inverter prices continue to drop and technologies continue to improve, making off-grid solar financially feasible for more people.



What is a solar inverter?

Inverters serve as the gateway between the photovoltaic system and the devices drawing energy from your system. They turn the direct current (DC) output into alternating current (AC), which is the standard used by all commercial appliances.

There are two types of off-grid solar inverters: pure sine wave and modified sine wave inverters. Sine wave inverters are capable of producing cleaner, smoother, quieter, and more reliable electricity to operate fans, lights, and other electronics without interference. Renogy inverters also provide overload protection for both DC input and AC output to prevent damage to the components and the unit. Renogy sells pure sine wave inverters of varying capacities. 

You typically need an inverter with any solar panels larger than five watts.



How are inverters configured in off-grid systems?

In an off-grid system, a charge controller will send the power to a battery bank and then an inverter will convert the DC to AC for the home. Off-grid inverters, known as stand-alone inverters, need a battery bank to function. Many off-grid solar inverters will also include a  charger in order to replenish the battery.



Which is the best solar inverter for me?

If you have an off-grid system, you’ll most likely be choosing between a pure sine wave inverter and a modified sine wave inverter. 

•  Pure Sine Wave Inverters: Pure sine wave inverters are capable of producing smooth quiet, and reliable electricity to operate appliances and electronics without any interference. Like its name suggests, pure sine wave inverters produce current in a pure sine wave shape. Renogy sells a range of pure sine wave inverters of varying capacities to fit your solar installation and your energy needs. Renogy inverters also provide overload protection for both DC input and AC output to prevent damage to the components and the unit.

•  Modified Sine Wave Inverters: In modified sine wave inverters, the polarity abruptly switches from positive to negative versus a true sine wave. When looking at the wave, it has a stair-step, square pattern, where the polarity is flipped back and forth. That choppy wave can negatively affect more delicate, sensitive equipment. If you have medical equipment you need to power, such as a CPAP machine, you won’t be able to use a modified sine wave inverter. Additionally, in many cases, you’ll hear a hum with devices attached to a modified sine wave inverter. However, with simple devices and appliances, modified sine wave inverters typically do the job.


What can I run with a modified sine wave inverter?

If you’re looking to save some money, modified sine wave inverters can be purchased and used in simple systems without sensitive electronics. If the electronic doesn’t have an AC motor and isn’t a delicate piece of medical equipment, you may be fine. Old tube tvs, water pumps, and phone chargers usually operate ok with a modified sine wave inverter.

Appliances like refrigerators, microwaves, and compressors that use AC motors won’t run as efficiently on a modified sine wave inverter. Some fluorescent lights will also not operate quite as bright, and some may buzz or make humming noises. 



What do I need a pure sine wave inverter to run?

• Newer TV’s

• Sensitive electronics

• Appliances with AC motors: Microwaves and refrigerators

• Medical equipment, such as CPAP machines with humidifiers

• Laser printers

          • Appliances with electronic timers or digital clocks

Your laptop may be ok with a modified sine wave inverter, although some claim that not using a pure sine wave inverter will shorten the lifespan of your laptop’s battery.



What are pros and cons of using a modified sine wave inverter?


Pros:

•  Less Money upfront: Modified sine wave inverters are typically cheaper than pure sine wave inverters, so if you’re on a budget and you’re only powering simple appliances, modified sine wave inverters may be enough to meet your energy needs.


Cons:

•  Lower efficiency: Modified sine wave inverters are not nearly as efficient as pure sine wave inverters.

•  Will not work with many appliances: As mentioned above, there are a variety of appliances you need a pure sine wave inverter to run, as TV’s, microwaves, and inverters.


How much are solar inverters?

Modified sine wave inverters typically cost between $50 and $600. Pure sine wave inverters cost between $150 and $900.


Can I use a modified sine wave inverter in off-grid applications?

Yes! Grid-tied systems require you to use pure sine wave inverters because the utility delivers pure sine waves. However, because you are operating an independent system, you are free to use a modified sine wave inverter. Keep in mind that some appliances and devices will not work with modified sine wave inverters, such as medical equipment. Additionally, in many cases, you’ll hear a hum with devices attached to a modified sine wave inverter.


What is the best off-grid inverter?

If your budget allows, pure sine wave inverters are typically the best choice because they can be used to power nearly all appliances, are efficient, quiet, and safe. Renogy’s pure sine wave inverters are equipped to meet the needs of your off-grid system.



How do you connect an inverter to a battery bank?

Inverters larger than 500 watts must be hard-wired directly to the battery bank. The owner's manual of your inverter will specify the cable size you should use. The cable size you’ll need also depends on the distance between the inverter and the battery. A good rule of thumb is to use the shortest length of cable that is practical. When connecting an inverter to a battery, we recommend using an overcurrent protection device, such as a fuse or circuit breaker, between the two devices.

AC and DC safety disconnects are also a requirement in all systems. In off-grid solar systems, a DC disconnect is installed between the battery bank and the inverter. This is used to switch off the current flowing between the two components and is important for maintenance, troubleshooting, and protection against electrical fires.


How many batteries do I need for a 3,000 watt inverter?

Inverters are sized in watts, just like solar panels. So if you have a 3,000 watt solar panel system,you’ll need at least a 3,000 watt inverter.

To figure out how many batteries you need for your 3,000 watt system, you can use the following equation: expected run timed needed * wattage of your inverter = total/voltage.

So if you have a 3,000 watt inverter and you need to have 3 hours of run time, you get 3 hours of runtime * 3,000 watts=9,000 watts total/12 volts DC = 750 amps total. You’ll need a total of 750 amps of stored power in the batteries. Remember you won’t want to fully deplete your batteries, so keep that in mind when calculating your needs. In this example, you’d need 250 amps per hour. So, you’ll need at least six 50aH or three 100aH batteries for a 3,000 watt system.

To size your system, we recommend using the  Renogy solar panel calculator.


What’s the difference between grid-tied inverters and off-grid inverters (standalone inverters)?

A grid-tied inverter for on-grid systems takes power from the solar panels, converts it from DC to AC, and pushes it into the grid. Typically, grid-tied inverters are easier to wire because there are fewer components to work with. Keep in mind, many jurisdictions require a system to be installed by a licensed contractor if connected to the grid. Grid tie solar inverters are the main gateway between the solar panels and the utility grid. It typically adapts the voltage and frequency of the alternating current to the grid. They also include various protections, such as anti-islanding protection, overvoltage protection, and insulation monitoring.

In off-grid systems, a charge controller will send the power to a battery bank and then an inverter will convert the DC to AC for the home. Off-grid inverters, known as stand-alone inverters, need a battery bank to function. When selecting off-grid solar inverters, it is essential that the output power of the inverter is large enough to support the loads of the system. Many off-grid solar inverters include an inverter charger in order to replenish the battery. Off-grid inverters will typically not communicate correctly with the utility grid and do not include the necessary protections present in grid-tied inverters, such as anti-islanding protection. This is when an inverter will shut down if it senses an issue with the grid, such as a power outage.



Can you use standalone inverters in grid-tie applications?

Inverters designed for off-grid systems should not be used if a system is going to be connected to the grid. This is because if there was a power outage, the PV system would still push energy into the grid, which would be a safety hazard for people repairing the grid. Grid tied inverters sense when there is a failure of the grid and shuts down the system.



Do I need a solar inverter charger?

Solar inverter chargers make a great addition to solar installations. As the name suggests, solar inverter chargers fulfill both an inverter and charging role. Inverter chargers are great for RV’s, boats, and other off-grid applications because the inverter charger can charge the battery bank from shore power, and the inverter will then convert the DC power to run AC loads in the space. They come in handy when you’re in areas where you may not be getting enough sunlight alone to charge your battery bank. The Renogy 3000W pure sine wave inverter charger can charge and maintain a battery bank when connected to shore power, as well as convert DC to AC for your appliances, making it a powerful addition to many setups.



How do you connect an inverter to a battery bank?

Inverters larger than 500 watts must be hard-wired directly to the battery bank. The owner's manual of your inverter will specify the cable size you should use. Cable size also depends on the distance between the inverter and the battery. It’s always good to use the shortest length of cable that is practical. When connecting an inverter to a battery, we recommend using an overcurrent protection device, such as a fuse or circuit breaker, between the two devices.

AC and DC safety disconnects are also a requirement in all systems. In off-grid solar systems, a DC disconnect is installed between the battery bank and the inverter. This is used to switch off the current flowing between the two components and is important for maintenance, troubleshooting, and protection against electrical fires.


Conclusion

Solar inverters are a crucial part of every solar installation, even though they are often overlooked. However, in order to protect your appliances and provide the proper output to function, a high quality solar inverter is an essential component worth investing in.