Solar Inverters – What Are They and What Do They Do?

For all that solar electricity can seem like the stuff of science fiction, and for all that the technology can get very complicated in many ares of detail, when you consider rooftop solar photovoltaic systems on their broadest level they are actually remarkably simple. Typically, a small scale grid-connected rooftop solar photovoltaic system has only two principal components: the panels, and the inverter. Off-grid systems are only marginally more complicated, containing batteries as well, so that the homeowner can turn their lights on after the sun goes down. Beyond this there are only minor components to consider, such as wiring and rails for the panels.

It’s fairly easy to figure out what the solar panels and the batteries are doing in a small-scale rooftop photovoltaic system – the panels are there to turn the sunlight into electricity, while the batteries are there to store any extra electricity that your panels generate so that you can use it at night or during periods of peak demand during the day.

Fairly simple stuff. So what’s this business about inverters then? What exactly is a solar inverter? What does it do? Why does it matter?

The inverter sits at the heart of your rooftop solar system. It takes the electricity that your solar panels produce, and changes it to a form that your lights and appliances can use. Without it, your panels are useless for the purpose of running your home.

Solar panels produce a low voltage DC output. Your fridge, your television, and your computer were all built for high voltage AC electricity, also known as “mains power”. Solar panels can’t produce mains power on their own. They need a helping hand. This is where an inverter comes in. Some inverters will also supply a constant voltage low DC output, to devices that can use that. This is generally of limited use to the average home, but can be useful for running camping equipment or to run the low voltage circuits found on houseboats. Some inverters can even output AC power at voltages above regular mains power, which is useful for people who have commercial or industrial equipment set up to use 3 phase power.

The biggest and most obvious attribute of any solar inverter is its power rating. This is simply a measure of how much power it can take from your panels and provide to your home. There’s absolutely no point putting 20 300 watt panels on your roof

Solar inverters also play a role in ensuring that your photovoltaic panels produce as much power as they can, through a process known as “Maximum Power Point Tracking”, or MPPT. Solar panels will provide differing amounts of power under differing resistive loads, and the amount of resistance required to deliver the maximum amount of power can depending upon the temperature and level of solar irradiation that the panel receives. Through MPPT, the inverter will determine what load to place on the panel array to provide the most power given the prevailing conditions.

Some inverters will have more than one MPPT tracker, allowing a different load to be applied to different groups of panels. If the layout of your roof requires you to place your panels in separate arrays, a dual MPPT inverter can be very useful in ensuring that your panels perform at their best. This is especially true when the panel arrays are angled differently or face different directions. Connecting a single MPPT inverter to panels placed on areas of the rooftop facing in different directions will dramatically affect your system’s performance. A dual MPPT inverter can also provide some incremental benefit to a group of panels all facing the same direction at the same angle, in cases where some of the panels are going to experience temperature differences, or some might be partially shaded at certain times of the day.

Solar inverters also often play a role in reporting your system’s performance. This can be useful in tracking the return on your investment from going solar. Some inverters even hook into your home’s wireless network, so that you can log in and have a look at how your system is doing from your computer or your smartphone.

Some systems have a small inverter attached to each panel – this is known as a “micro-inverter”. This can be useful for many applications. Each panel will have it’s own MPPT, meaning that the ideal load will be applied to each panel to ensure maximum power output. It also means that arrays of solar panels can be added to or downsized over time, as on-site power demands change. It further means that less of the system will go offline in the event of equipment failure.

Choosing the correct inverter for your circumstances is crucial to the performance of your system, and to ensuring that you get the best return that you can on your investment.

Mary J. Gibson

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