Inverters: Solar Power Components

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Inverters

This blog post is about Inverters.

This is the fourth in a series as we take a look at some of the main components in a solar electric system: the solar panels, batteries, solar charge controllers (SCC) and inverters. Technology is moving forward at a very fast rate. Most of the generalities are true today, but there are likely to be exceptions.

If you missed the earlier blog posts, I recommend that you read them to get a good foundation:

All other solar power articles may be found HERE.

Inverters

The primary job of an inverter is to convert the direct current (DC) power (red line in the picture above) from the battery bank to alternating current (AC) power needed for most appliances.

In order to do that, we must take the constant DC voltage and change it to a sine wave curve that goes above and below zero volts.

When inverters first came out, the most common way to do this was to make the voltage go straight up and down creating a blocky signal. This is called a modified sine-wave (blue line in the picture above).

More advanced modified sine waves make multiple steps trying to get as close as possible to a pure sine wave. You can see the output of a modified sine-wave on an oscilloscope (bottom left of picture above). The bottom right of the picture is of a pure sine-wave.

Other than how the signal looks, let us see the difference between the two outputs.

Modified Sine Wave Inverter

A modified sine wave inverter can be used for simple systems that don’t have any delicate electronics. CRT TVs and motors with brushes are usually fine with modified sine-wave but your digital clock will likely act funky and battery rechargers quite often just plain won’t work.

Some equipment may seem to be working fine but it may run hotter than with a pure sine wave and reduce the life of it. It is very difficult to say exactly what will and won’t work with modified sine wave inverters.

For simplest systems

Typically inexpensive

Fine for older TVs, incandescent lights, motors with brushes

Generally not good with: electronics, audio, induction motors, rechargeable batteries, or digital clocks

Pure Sine Wave Inverters

Pure sine wave is always needed for grid-tied systems. It is generally needed for the newer LED TVs, CFL and LED light bulbs and inductive loads like brushless motors.

Mandatory for grid-tied

Preferred for off-grid homes or larger systems

Generally more expensive than modified sine wave inverters

Necessary for electronics, fluorescent lights and dimmers, inductive loads

Generally, modified sine wave inverters are less expensive than a pure sine wave inverter so they are still commonly used in simple systems.

As technology advances the cost of pure sine wave inverters is coming down making them much more affordable and really the favorite option these days.

Inverters are used in three different types of solar power systems:

Grid-tied
Off grid
Grid tied with battery backup

A grid-tied inverter only connects the solar panel directly to the electric companies grid through fuses and breakers. Through net metering, if you make more power than you use, your electric meter spins backwards. More often than not, your electric meter will spin slower as your house uses all the power you make. You buy less power from the grid. At night or during cloudy days, you buy your power from the grid same as usual. If the grid goes out, so will your house power even if the sun is shining. The only noticeable difference is that your electric bill will be lower than before you got solar. When selecting a grid-tied inverter, base it on the size of your solar array. A 5KW array will use about 5KW inverter. You need to match the electrical service you get from the grid.

Microinverters have become very popular for grid-tied systems. Instead of one or two large string inverters, microinverters install in the back of the solar panels, generally one per panel. They convert the DC power to AC at the panel. The advantage of this is that if you have partial shading on some of the panels, it doesn’t affect the output of the whole string of panels like it would with a string inverter. You can also have a deeper view into the system monitoring down to the panel level instead of on the whole inverter.

Microinverters are very helpful if you have future expansion in mind. You add more panels and more inverters as needed. You don’t even need to match the panels as you would with a string inverter. The inverter is outside but shaded by the solar panel. It is more exposed to the weather than a string inverter that’s installed inside a building. Microinverter systems can also be a little bit more expensive.

Off-grid inverters are for stand-alone systems where the grid is not available. Once the charge controller has charged up your battery bank, the off-grid inverter converts the 12, 24 or 48V battery bank to AC voltage. The AC output depends on your requirements. Depending on how you wire the output of the inverter and which inverter you get, you could have a dual output. You need to determine what your loads need and select and configure the inverter.

An off grid inverter cannot sell extra power back to the grid. An inverter charger can connect to the grid if available, to act as a battery charger. If you have an inverter charger, you can connect to grid power and use the grid to charge your battery bank when the solar doesn’t provide enough power. That AC connection is one directional, it will only take power from the grid but not send it back. Likewise you can often connect a generator to the AC input of an inverter charger to top off the batteries when needed. This is a common configuration for off-grid homes that need more power than the sun can provide.

When selecting an inverter you must determine what your maximum wattage draw will be if all your appliances that may be on at the same time are on. If you have an 800W well pump, a 100W refrigerator, five (5) 10W light bulbs and a 50W laptop, you will need to add the wattages together to get at least a 1000W inverter. You also have to make sure the inverter is able to handle the surge as motors turn on. For example, if your refrigerator and well pump both turned on at once, the surge could be three times the rated wattage. You must make sure the inverter can handle the surge.

Inverters are rated both by continuous wattage and the surge capability. Additionally you need to select the battery bank size and buy the inverter to match. Inverter voltage is not field selectable, they are either 12V, 24V or 48V.

A grid-tied battery backup inverter is the best of both worlds. Under normal circumstances it converts the power from the DC battery bank and provides AC power to the house selling any extra back to the grid. At night, your house gets its power from the grid same as usual. But when the grid goes out that is when this is the best.

When installing a grid-tie battery backup system, you select which appliances in your house you want to back up with battery power and take these off your main breaker panel and connect them instead to a critical loads panel. When the grid is out, only the items wired to the critical loads panel get powered. So you can have your selected appliances and lights remain on while the rest of your house is off . This allows you to have a smaller battery bank and PV array than if you were to have an off-grid system because you’re only powering a small subset of your house when the grid is out. You reduce your electric bill but you can also keep important things running when the grid is out without having to run a generator.

When selecting a grid-tied battery backup inverter you need to base it both on the battery bank voltage as well as the size of your array. You also have to base it on the wattage of your critical loads including the surge. There is a bit of planning when selecting the right inverter.

Battery based inverters have a lot of options to choose from. Not all inverters have the same features. You need to decide the required features and select the inverter based on which has them.

Some of the features are the ability to charge a battery bank from an AC source like the grid or generator. Some can start the generator when the batteries are low and turn it off when charged. Some can use the generator to assist with high loads.

The inverter is often installed in an out-of-the-way location near the batteries. A remote controller or display in the living area is useful to keep an eye on the system. Some inverters even have the ability to monitor the system via the web. This is very useful for part-time locations that you are not always there to keep an eye on it.

Many inverters are stackable to increase either the voltage or the current or both. This allows you to use multiple inverters in a master-slave configuration. Inverters turn on as needed conserving battery power.