How to Choose the Right Solar Inverter for Your Solar Power System (Part 2)

Photo by William Mead from Pexels

There are three types off-grid (also known as stand-alone) solar inverters according to the output waveform of the voltage – pure sine wave, quasi (modified) sine wave, and square wave. 

Pure sine wave inverter has the output waveform of the voltage like a sine wave and this form of electrical output is the same as you will find coming from the utility grid. Modified sin wave inverter has the output waveform not exactly sine wave but it resembles the shape of a sine wave.

Pure sine wave inverter has many advantages over modified sine wave inverter. Pure sin wave inverters can run all electrical home appliances effectively. They are necessary for sophisticated electronics, like a fancy new television or gaming console, fluorescent lights and dimmers, inductive loads like microwave ovens. Pure sine wave inverter keeps appliances safe and more durable. 

Modified sine wave inverter is inexpensive but has a serious disadvantage: it is not compatible with all appliances and may create excessive heat in appliances which will reduce their lifetime. Also, modified sine wave inverters create more noise and a high electrical surge can lead to malfunctioning of home appliances. Modified sine wave inverter is suitable for simple systems or a DIY emergency solar kit, or a cabin with modest electrical needs. They work best with older TVs, incandescent lights, motors with brushes. Generally, they are not good with modern electronics, audio, induction motors, rechargeable batteries, or digital clocks.

The square wave inverter has the output waveform of the voltage like a square wave. This type of inverter is least used among all other types of inverter because all appliances are designed for sine wave supply. If we supply square wave to sine wave-based appliance, it may get damaged or losses are very high. The cost of this inverter is very low but the application is very rare. It can be used in simple tools with a universal motor.

Off-grid inverters are not connected to the electrical utility grid and they are designed to be integrated with a battery bank. The off-grid inverter draws power from the battery and converts it from DC power from the solar panels into AC power. The off-grid battery inverter only discharges the battery (unlike an inverter/charger). When the batteries’ charge level gets below a threshold, the inverter will shut down to protect both home appliances and the batteries.

In an off-grid system, PV panels charge the batteries via a charge controller, and only the power demanded by the loads is inverted to AC. These systems do not have access to the electrical utility grid so it is important to properly size the inverter as well as the battery bank.

The battery inverter/charger combines the functionality of a DC/AC battery inverter and a battery charger. Like any inverter, an inverter/charger converts DC (battery) power into AC power but it will also charge (converting AC into DC energy) the attached batteries when an AC power source is present. Generally, they are designed for applications in which AC power will be available from an outside source, such as a generator or shore power. Besides, in an off-grid solar power system, they are also used in a boat or a RV. 

Some advanced models of inverter/chargers (multi-mode, bi-directional) can charge a battery, converting AC power from the utility grid to DC power, and also feed power back into the grid using net metering, similar to the grid-tie inverters. If there is a power outage, the inverter/charger will automatically switch to battery power to provide power to connected equipment. 

A hybrid solar system is connected to the electrical utility grid and also has a battery bank. Hybrid inverters combine a standard solar inverter and a battery inverter/charger in one simple unit. Hybrid inverters convert the DC power your solar system generates into AC power, which can be used to power your home appliances or fed directly into the grid. The hybrid inverter can also manage inputs simultaneously from the solar panels and a battery bank. The batteries are charged with either solar panels or the electricity grid, depending on which is more economical or preferred.

Not all hybrid inverters provide battery backup in the event of a blackout. A basic hybrid inverter cannot supply power when there is a blackout (commonly known as an uninterruptible power supply or UPS function). There is another type of hybrid inverters so-called multi-mode inverters with backup power capability during a blackout, which are more advanced hybrid inverters designed to work in on-grid mode and off-grid mode for a long time.

A grid-tied solar power system will not power your home during a blackout, so in areas where blackouts and weather-related outages are common, a hybrid solar power system with a multi-mode hybrid inverter may be a solution. You can also buy a hybrid inverter for your grid-tied solar system if you plan to add batteries soon and this is the most cost-effective option. 

The modern hybrid inverters are more versatile than grid-tie and off-grid inverters, and they are also more expensive. The advantage of hybrid solar inverters is that they allow the centralised monitoring of the solar array. Thus, the homeowners only have to check on one portal to understand how much energy their solar panels are producing and how the batteries are performing. 

You can always add storage to an existing grid-tied solar power system later and it is still the common practice as the homeowner already has a grid-tie inverter installed. However, a disadvantage of this method is the higher cost. 

Another option is by adding an AC coupled battery system such as the Tesla Powerwall 2 and Sonnen ECO (they have built-in inverter) to your grid-tied solar system. These battery systems can be retrofitted to a home with an existing solar power system and grid-tie solar inverter. They are expensive but they are also the best in the market.

See also: How to Choose the Right Solar Inverter for Your Solar Power System (Part 1)

How to Choose the Right Solar Inverter for Your Solar Power System (Part 1)

Photo by Alex Csiki from Pixabay

The solar inverter converts direct current (DC) electricity generated by your solar panels into usable alternating current (AC) electricity used by household appliances. If you have decided to go solar and install solar power system for your home, you have to do some research and have an initial knowledge about the solar inverter. 

The solar inverter consists of complex power electronics and software and it is crucial to the performance of your solar power system. When you choose your solar panels for your home it is important also to consider buying the right solar inverter for your solar power system. In this post, I’ll give you some basic information about solar inverters, their types, how they work, the best brands, and how to size an inverter to be right for your solar power system.

There are three groups of solar inverters: grid-tie, off-grid, and hybrid. There are three main types for grid-tied solar power systems: string inverter, microinverter, and string inverter + power optimizer. 

String inverters are central inverters and they are the most commonly used inverters for home solar power systems. When few solar panels of the same output voltage and power are wired together in series we have what is known as a "string”. All of the energy the panels produce is sent to a single inverter that is typically located on the side of your house, garage or utility room. String inverters are easy to install and maintain, and they also have the lowest price. 

This type of inverter is suitable when your panels are not facing multiple angles and have no shading issues. The disadvantage here is that if one panel is in shade and suffers reduced output, every panel in the string drops to that reduced output. 

Microinverters are small devices also known as module-level power electronics (MLPEs) that are attached directly to each solar panel, usually on the back. Thus they need to be designed to be resistant to humidity and heat. Microinverters convert DC power to AC right at the panel, providing a better performance of the solar array thus better performance of the whole solar power system. Just make sure the microinverter capacity matches that of the solar panel.

Microinverters allow you to monitor individual panel performance, giving you a more clear view of efficiency. Also, if something goes wrong with a particular solar panel, you can fix it easily. Because of all of this, and the need for multiple inverters, microinverters are the higher cost option. Besides that, another disadvantage is that they are located on the roof, so their maintenance is complicated. 

Power optimizers are small devices as well that is located on the roof alongside or integrated with each solar panel, allowing for more accurate performance monitoring. But unlike microinverters, they don’t convert power from DC to AC directly. Instead, they simply “optimize” the DC electricity before it is sent to a string inverter for conversion into AC. 

Power optimizers, similar to microinverters, make the solar energy system more efficient, but you can buy them at a more affordable price. And like microinverters, maintenance and repair cost power optimizers are high with power optimizers given their rooftop location. Microinverters and power optimizers typically come with a 20 to 25-year warranty while standard inverters typically have 12 to 15-year warranties.

Microinverters and power optimizers (together referred to as module-level power) are gaining popularity in residential solar markets. It’s always nice to have options, and string inverters and MLPEs each have pros and cons. 

The three best string solar inverters for 2020 are: Fronius (Primo and Galvo), SolarEdge (SE and HD wave), SMA (Sunny boy series). Enphase is a leading manufacturer of microinverters.

The inverter sizing refers to choosing the right size of solar inverter for your solar power system. As a general rule of thumb, you’ll need an inverter that matches the watts of your solar panel installation, For example, if you have a 3kW solar panel system, then you would install a 3kW inverter. In the case of microinverters, the size of the inverters should correspond to the energy output of each solar panel they’re connected to versus the entire system. 

If you plan to go off-grid, to choose the right inverter for your solar power system you must calculate the load your inverter can handle. You should know that inverters are rated in continuous/running watts and surge/peak watts. It’s important to consider both the continuous load and surge load when it comes to inverters (inverters have two capacity values printed on the manufacturer’s label: continuous watts and surge watts).

Surge watts are the amount of power the inverter can support for a very short time, usually no longer than a second when the appliance starts up. Appliances with motors require about 1.5 to 2 times the running wattage before the motor will start. Appliances and tools with induction motors such as refrigerators, freezers, air conditioners, microwave ovens, and pumps, need a much higher power level at startup. They may require a start up surge of 3 to 7 times the continuous rating.

Find out the appliance’s description and read the specific wattage requirements. If the wattage of the appliance is not listed on the specification, you may calculate the standard watts each of your appliances requires using the formula:

Amps x Volts = Required Watts

For example, if you have a microwave 7.5 amps x 120 (using a 120-volt ac) = 900 watts. It means that the inverter has to handle both the 900 running watts and the 2,700 watts surge requirements of your microwave. 

If the running wattage of your fridge is indicated as 600 watts, the surge wattage will be about 1,800 watts, but this surge is needed for only a fraction of a second. To run this refrigerator, you will need an inverter that can handle 600 running wattage and a surge of about 1,800 watts for a split second. At discount stores or home centers, you can find a typical inverter that provides 1,500 watts of continuous power and 3,000 watts of surge power. https://homeguides.sfgate.com/run-refrigerator-inverter-49672.html

For your inverter to be right for your system, its surge watts rating must be approximately equal to (or greater than) the potential surge watts of each appliance. You can find this out by looking at the label on the back of all of the appliances.

If you want to use multiple house appliances at the same time use the appliances running and surge wattage to determine which devices you can run simultaneously. Just add up all the continuous watt ratings of all the appliances that may be running simultaneously. The sum of continuous watts you get will determine if you're inverter can handle it. Always keep the running load under the maximum limit of your inverter.

With an inverter, with a continuous rating of approximately 1500 watts and with a surge rating of approximately 3000 watts, you can run a wide range of household appliances simultaneously by managing the running load. You can run LED lights, TV, stereo, laptop, fans, fridge, and a microwave (depending on the wattage) at the same time. Just put a current display meter to check the running load before you switch on a new appliance.

It is also recommended to provide a safety room of 10% - 20% watts (more than your largest load) when sizing up an inverter.

Remember: always seek professional help for the installation of your solar power system because of the specific power needs and circumstances of every home.

Next post: How to Choose the Right Solar Inverter for Your Solar Power System (Part 2)

Sources: unboundsolar.com, altenergy.org, homeguides.sfgate.com, batteryspace.com 

Hybrid Solar Power System - the Best of Grid-Tied and Off-Grid Systems

The hybrid solar power system is a combination of grid-tied and off-grid solar power systems. This system is described also as off-grid solar with utility backup power, or grid-tied solar with battery storage. The hybrid solar power system comes with a special hybrid inverter (sometimes referred to as a multi-mode inverter) that can transmit direct current (DC) power to and from your batteries and provides alternating current (AC) power between the grid and your home when necessary. The hybrid inverter can simultaneously manage inputs from both solar panels and battery storage, charging batteries with either solar electricity or electricity from the grid.

The hybrid inverter takes advantage of changes in the utility electricity rates throughout the day. You can temporarily store whatever excess electricity your solar panels produce in batteries, and put it on the utility grid when you are paid the most for every kWh. And you can rely upon your battery stored solar electricity while electricity is expensive and switch to the grid when it is cheap. 

The ability to store and use your solar energy when desired is known as self-use or self-consumption. 

In case you have already installed a grid-tied solar panel system you also have a traditional inverter (solar inverter) which converts DC into AC. If you would like to have a hybrid solar power system you should buy battery storage and a separate battery-specific inverter. 

An option is to buy a hybrid inverter for your grid-tied solar panel system if you are planning soon to add battery storage. Thus you won’t need to purchase a separate battery-based inverter. The hybrid inverter can function as both an inverter for electricity from your solar panels and a solar battery. It combines a solar inverter and battery-specific inverter, so it can charge and discharge battery storage. On the other hand, there are some modern energy storage systems, like the Tesla Powerwall 2, which comes with an inverter already built-in, eliminating the need for a hybrid inverter. 

The hybrid solar panel system is configured in such a way that your home uses solar power first, then stored power in the battery, and then grid power. With a solar hybrid system, you are less reliant on the grid and solar battery storage is used as a back up when solar power is not available. When the stored energy is depleted, the grid is there as a backup, allowing consumers to have the best of grid-tied and off-grid solar systems. And the battery storage is discharged less frequently, which extends its life. It needs to be replaced less often and saves you the cost of purchasing a backup generator. 

The hybrid solar power system is at least half the price of an off-grid system but they are more expensive than an on-grid system – typically double the price of the grid-connected solar power system. The hybrid solar power system is also more complicated for installation and takes more space. 

In summary, the hybrid solar power system has the following advantages: 

• gives you a continuous uninterrupted power supply 
• stores solar electricity or low-cost electricity 
• allows using solar power during the peak times of the electricity grid 
• solar power is available during a blackout or grid outage 
• reduces the power consumption from the grid 
• you are not dependent on the weather conditions 
• gives you more independence than on-grid solar power system

Solar Power and Net Metering

Grid-tied solar power system (with net metering) diagram

Net metering, or also known as net energy metering (NEM), is another big reason why the solar panels are a good investment for your home or business.

Net metering is possible when your home maintains a connection to the grid even after you install solar panels, and of course, when the net metering is available in your area. The grid-connected (grid-tied) solar power system with net metering consists of solar panels, a grid-tied solar inverter that converts DC (direct current) to AC (alternating current), and net meter. DC generated from your solar panels is converted into AC, which is the type of current that is used by the electrical appliances in your home.

Let's explain simply what is net metering and how it works.

When your solar panels produce excess power, it goes to the grid. The utility company compensates you for the excess grid supply with credits added to your electric bill. For the time, when your solar panels produce less power than your household is needed, you can draw electricity from the grid. A net meter (bi-directional meter) is installed to register both the excess solar energy that you export to the grid and the energy that you consume from the grid. This ensures that consumers are only charged for their “net” energy use (energy consumed minus energy sent to the grid). 

Monthly net metering allows consumers to use solar power generated during the day at night. Similarly, during the spring and summer, when the sun is shining, and your solar panels are producing more electricity than you need, that extra energy goes to the grid. Then in the winter, when the days are shorter, you can draw on those energy credits to help offset some of your energy needs.

The grid acts as an energy storage system for your excess power and saves it for later use. The grid connection ensures that you still have power regardless of daily or seasonal variations in solar panels production levels.

So, with grid-tied solar power system and net metering billing mechanism, you save money besides that you help the environment and reduce your carbon footprints.

Net metering originated in the United States, where solar panels and small wind turbines were connected to the electrical grid, and consumers wanted to be able to use the electricity generated at a different time or date from when it was generated. In 1979 an apartment complex and a solar test house in Massachusetts were the first two projects to use net metering. Minnesota is commonly cited as passing the first net metering law, in 1983, and allowed anyone generating less than 40 kWh to either roll over any credit to the next month, or be paid for the excess. 

Keep in mind, however, that net metering policies can vary significantly by country and by state or province. It is not available everywhere in the U.S., which means that there is a need for other nighttime power supply options such as solar battery storage.

Net metering was slow to be adopted in Europe, especially in the United Kingdom, because of confusion over how to address the value-added tax (VAT). Only one utility company in Great Britain offers net metering. In Canada, some provinces have net metering programs.