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What are home solar inverters?

Avatar for Dan Hahn
Updated 02/28/2020
SMA Solar inverters

If your entire home solar system was like a human body, the inverter would be the brain. Inverters are the central component which binds the system together, and they are becoming more intelligent as the years tick by. The duties of home solar inverters are constantly evolving as the grid and solar projects get smarter.

They are also the most likely piece of equipment to fail in the first 10 to 15 years of operation, and we have been accounting for their need to be replaced in our state by state payback calculations for over a decade. So, when considering inverter options for your home solar system, high reliability is important and so are capabilities. We’ll review both below.

Why does your home solar panel system need an inverter?

The inverter is the hardest working component of your solar system. Its main job is to convert direct current (DC) flowing from your solar panels into alternating current (AC) that your home uses.

current changes between ac and dc

DC moves in one direction from point A to B. The current that flows into and out of batteries and solar panels alike is DC. Most low voltage equipment suitable for camping or easily plugged into your car operates in DC (lights, fans, pumps, etc.).

AC electricity moves in two directions at 120 volts and 220 volts, and you need an inverter to translate that DC straight line current from your solar panels into the AC you can use for all your home appliances.

Electrical History Sidebar Our homes and appliances could have been run on DC, but there was a disagreement about power transmission engineering back in the 19th century between Thomas Edison and George Westinghouse that we would have liked to have involved a barfight and shootout.

Instead, there was the reality that transformers at the time only worked well on AC voltages to transmit power longer distances without as much loss, so DC fell by the wayside. Now, some of the longest transmission lines actually run on DC.

What else does a home solar inverter do?

A home solar installation animated to show the solar inverter working

Aside from converting DC to AC, your solar inverter has three other functions.

First, they are responsible for continually tracking your solar array’s voltage to determine the maximum power at which your solar panels operate, ensuring the system produces the most and cleanest power at all times.

Second, inverters need to interface with the grid. They ensure if there’s a temporary power outage, that no power from your solar panels winds up making it out to the transmission lines outside your home so lineworkers who may be troubleshooting or repairing wires don’t get zapped. Your inverters also feed loads into the grid when your home doesn’t need the power and your batteries are full.

Third, inverters are also required to shut down when they detect a hazardous electrical arc, caused by system ageing and material degradation within your home’s wiring and solar panels. Some inverters perform better than others at safety shutoff. According to a PVEL’s 2019 ground and arc fault test, Delta and Fronius inverters had higher accuracy rates of arc detections.

Inverter sizes

Inverters come in all shapes and sizes. Small inverters can convert DC output from car batteries to run AC appliances:

a small inverter suitable for small devices

On the other end, there are huge inverters that can convert the entire output from an enormous solar farm into AC current and then feed it into the grid:

huge inverter for solar farms

For most home applications, inverters are available in sizes ranging from 500 to 10,000 watts. That’s the same as .5kW and 10kW respectively. The inverter pictured below is 5kW in size:

Example size of a typical home solar inverter

Your installer will determine which size is best for you. They’ll take the wattage of the highest power appliance in your home and add that figure to the watts of any appliances you might be running at the same time to make sure your system can handle your usual loads.

For example, if you regularly use an 1800 watt hair dryer while your husband makes a smoothie in a 1000 watt blender, your solar inverter needs to be sized at least to around 3kW for your system to work smoothly.

Your solar array does not need to match the size of your inverter exactly, especially when your system is tied to the grid. Some even say that your inverter works more efficiently when paired with a solar system rated slightly higher than it in kilowatts. After all, it is in very rare circumstances when your solar system generates its peak capacity to tap out your inverter’s capabilities.

Again, your installer will take a look at your habits, appliances and usage and pair your solar panel system size well with your inverter size.

Solar inverter efficiency

Any time you convert electricity or move it from one location to another, there are losses. In the case of transforming power from DC from your panels to AC for your home, the electrical loss takes the form of heat.

Modern inverters are about 95% efficient, which is a pretty good step up in efficiency from the early inverters of the late 20th century.

What about smart inverters? What are they?

smart inverters solar

Inverters have advanced a lot over the past 10 years to include features like data monitoring and advanced utility controls. These abilities allow your solar panel system to talk to the grid and relay voltage and frequency information. Such functions promote grid resilience and help prevent blackouts.

Smart inverters are programmed to automatically respond to the grid when it shuts down and also provides support to the grid when needed. Regular inverters without these capabilities simply turn off after sensing any grid problems like voltage fluctuations, even if those fluctuations are small and meaningless.

In areas where a lot of solar is installed, small disturbances like these which cause thousands of inverters to flip off can create grid load havoc. These power fluctuations can require your utility to turn on other electrical generation resources, which can be very expensive for them over time.

Some utilities are now requiring smart solar inverters to be installed with every new solar panel system. For example, California established a rule in 2017 which requires all new solar projects to have smart inverter features.

Other states like Hawaii, Nevada, Arizona, Vermont, and Massachusetts are close to establishing similar standards.

What about batteries and inverters?

If you’re considering adding battery backup power to your home solar system, you may be wondering if you need to install a battery charging capable inverter.

If you recently went solar in the past few years you can take heart knowing you can add batteries to any solar inverter using a technique called AC coupling.

An AC coupled battery system like the Tesla Powerwall 2 can be tied to any inverter. You don’t need a special battery charging capable inverter, so if your installer says you need one, you might mention AC coupling and save yourself a few hundred dollars.

What about solar microinverters?

microinverters installed on roof

If you’ve got a particularly challenging roof with more shading and would like to make sure each solar panel in your installation is performing to its peak capacity, microinverters might be a good solution for you. Microinverters are installed behind every panel on your system.

If there’s a fleet of geese who can’t help but obliterate a panel or two, the rest of your installation will not suffer the consequences, since the power is sourced and converted to AC from each panel individually instead of as a whole unit. If one panel gets knocked out the others will work just as well.

Microinverters also run at a lower voltage overall than string inverter setups, so if you have a very old home with flammable roofing materials, microinverters might just give you some peace of mind knowing things aren’t getting nearly as hot in the wires up there.

With a microinverter setup, you can monitor the performance of each individual panel. Most people who install microinverters are pretty geeked about the ability to do this every day, for several months. Afterward, the novelty tends to wear off.

It’s important to note that microinverters add about 20% to the cost of your system.

If you’re interested in learning more about them, we have written about how they work from a basic overview and a more detailed engineering perspective.

Our recommendations for working with your installer on your inverter choice

We recommend going for premium inverters over premium solar panels. If your inverter does fail, you’ll be able to get good service for it from the manufacturer. The more you pay for your inverter, typically the less you’ll need to worry about things going awry.

Standard inverter warranties are 5 years but there are plenty of brands which offer 10 year warranties. Go for the 10 year warranty if you can. If you have to pay a little more, insist on it.

In 15 years when your inverter goes bust, the replacement price may be significantly less. It’s unlikely to be more. So, worst case, be prepared to replace your inverter 10-15 years from now.

A good solar installer will be showing you this information in their sales proposal. If they don’t ever mention the possibility of your inverter needing replacement over the life of your solar system, that’s a red flag.  

The good news is that by the time your inverter needs to be replaced, your system has probably paid for itself already. Even better, for the next 10 to 15 years of your solar panel life, future energy savings should more than offset this expense.

Last modified: February 28, 2020

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bob

contact trump

Ben Zientara
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What should I ask him?

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we just had 31 solar panels installed, and we have to upgrade the electric panel. How do we do that?

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