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Inverters: How To Pick A Good One.

So you’ve got quotes for a few different solar PV systems. Everyone says their inverter is the best, surprise, surprise. So how do you decide which one to go for?

First of all you need to realise that there are different ways to make inverters, each with their strengths and weaknesses:

Firstly, solar inverters can be either:

a transformer type or
a transformerless type.

They can also be

a pure sine wave type (also called a sinusoidal type)
or the modified sine wave type.

You can usually find out what type of inverter is being offered by looking at the specification sheet for the inverter. The retailer should be able to supply you with this. If they won’t then don’t give them your business!

Types of Solar Inverter

Let’s go through each type:

a) Transformer type solar inverters

Most current inverters available in Australia contain a transformer. As indicated above the transformer increases the voltage of the electricity produced by the solar panels to the same voltage as the mains electricity grid (240 volts).

Transformers are quite heavy and but simple to make. Therefore, inverters with transformers are generally heavier and a little cheaper than equivalent transformerless inverters. They also tend to be marginally less efficient in converting the energy.

Transformers can also create a humming noise and this can be a problem if the inverter is near where people want quiet space (e.g. bedrooms).

b) Transformerless solar inverters

Transformerless solar inverters are very common in Europe and are just entering the Australian Market.

The lack of a transformer generally makes them lighter and slightly more efficient than equivalent inverters with transformers. They can generally react faster to changes in power and are a bit more sophisticated in how they behave.They are also less likely to create a humming sound although the trade-off is they can make high frequency noise; but only annoying if you are a dog!

It is likely that transformerless grid connected inverters will replace transformer type grid connected inverters in Australia in future.

c) Modified sine wave solar inverters

A solar inverter has to convert DC electricity from solar panels into AC electricity.

i.e. It has to turn the electricity from a constant (flat-line) into a mathematical pattern called a sine wave (the red line in the pic below). This makes it compatible with your household appliances.

(And if you are wondering why the 'standard' electricity supply takes a wacky form like this - it is historical. The standard output from a spinning coal fired steam turbine which is driving a rotating generator is the sine wave)

Converting a flat-line signal into a perfect sine-wave is a pretty freaking tricky thing to achieve. So if you really want to cut corners in your inverter design you can approximate the sine wave (red line) with the blocky green line in this picture, call it a "modified sine wave" and hope no one notices.

Modified sine wave inverters were originally used in early non-grid connected (stand alone) solar systems. However, some household electrical appliances did not run well on the modified sine wave. Most household appliances prefer electricity with a pure sine wave for optimum performance.

For this reason, modified sine wave inverters are not generally used in solar power systems any more.

I personally wouldn’t have one in my house!

d) Pure sine wave inverters

These types of inverters are simply a modified sine wave inverter that has funky electronics that creates a “pure” sine wave as opposed to the square sine wave.

The end result is a single sine wave with a frequency matching the frequency of the grid. (50 Hz in Australia).

The conversion process is not 100% efficient so you do not get a 100% pure sine wave. The amount of impurity is measured as Total Harmonic Distortion (THD). Most good inverters will have a THC of less than 5% which is an acceptable level. This will be in the spec sheet.

If you want to give the sales guy a really hard time - ask him what his Inverter's Total Harmonic Distortion is!

All grid connected solar inverters approved for sale in Australia are pure sine wave grid connected inverters.

What to look for in a good inverter.

a) Is it legitimate?

The first thing to look for in an inverter is to see if it complies with the relevant Australian Standard. (AS4777). You can quickly do this by checking out the Clean Energy Council's list on their website.

(See this list from the Clean Energy Council.)

All certified grid connected inverters should be on this list. If it is not on this list don't buy it (unless you live for danger!).

It’s also worth looking at the company benhind the inverter – how long have they been around and what is their expertise?

b) How much should I pay?

This is a difficult question to answer as grid connected inverters are mostly offered as part of a solar PV system package. The inverter can represent around 20% of the cost of a system.

Retail prices range from around $2000 for a good 1kW inverter up to $9000 for a 10 kW inverter. As with solar PV panels, grid connected inverters are constantly being improved and their prices will tend to reduce in the future.

In general, the more features an inverter has, the more you will pay. Thus, it is best to work out what you need in an inverter and compare prices within the overall system costs.

If the particular inverter you want is not offered by a retailer, ask if you can have the inverter you want in their system. Some retailers will offer this service and you may be able to reduce your overall costs by choosing a more basic inverter.

c) What size should I buy?

There is often some confusion around this question.

In general, as a minimum, the inverter needs to be able to handle the maximum power that your solar power system can generate For example, a 1.5 kilowatt (kW) solar PV system needs a grid connected inverter of 1.5 kW DC or above.

A word of caution here to be aware that inverters are rated in DC and AC output terms. Make sure your inverter is rated to suit the output of your system in DC!

Some retailers will offer you a larger inverter so you can add extra solar panels at a later date. Before making a decision to buy this inverter you need to consider the following:

Do I have enough space to put new solar PV panels? (kind of pointless otherwise)
will my current panels be available in the future?

Solar PV panel technology is changing so rapidly that your current solar PV panels may not be available when you want to upgrade. Your inverter may not be able to accommodate the mismatch of solar PV panels. Then you may need to buy a new inverter. A way to overcome this potential problem is to perhaps consider a multi string or a MMPT expandable inverter (see below).

In general, the best advice is to put up as many solar PV panels as you can afford (and fit on your roof), and purchase an inverter to fit the maximum power of the system.

What about the physical size of the inverter?

Grid connected inverters come in a variety of shapes, sizes and weight. The smallest inverters are around the size of a large briefcase. The larger inverters are around the size of a small travel case.

They are generally located as near as practicable to your electricity meter and should always be located in the shade for best performance. The best advice is to have a look at the inverter (or check out it's specification sheet) and see whether it will fit near your electricity meter and what it will look like.

d) Is it weather proof?

Some grid connected inverters are weather proof so they can be located in areas that may have some exposure to the weather. Other grid connected inverters are not weather proof and may require the addition of a weather proof cage (at extra cost to you) if the inverter is to be exposed to weather. As a general rule, your inverter is a box of sensitive electronics and the better it is protected from heat and weather, the better it will perform and the longer it will last.

Check this out on the specification sheets or ask your retailer where they intend to install your inverter.

e) Does it have a Warranty?

Typically, grid connected inverters have a life span ranging form 10 to 20 years. You should expect most inverters to last 10 years minimum.

Grid connected inverters have warranties ranging from 5 to 10 years with an increasing number of manufacturers offering pay for service warranty extension. Most retailers are now offering 8 to 10 years warranty. Obviously, the longer the warranty the more protection you have.

So check out the Warranty on your inverter and balance this against other features of your system to help you make an informed decision...

f) Is the inverter expandable?

This is an important consideration if you are intending to expand your solar system in future.

The best solution at present would be to consider a multiple MMPT inverter. These inverters have multiple MMPT trackers allowing you to add additional solar PV arrays at a later date or install solar PV arrays at different orientations. This will enable you to readily add a new string of panels (even if the panels are not exactly the same as the original panels) to your current system.

In future, individual solar PV panels may have a MMPT inverter attached to the rear of the solar PV panel, typically called micro inverters. This would potentially enable different individual solar panels to incorporated in the same solar power system, would overcome shading issues and make individual solar PV panel monitoring possible. There is an increasing trend towards these products in the USA and they are starting to appear in Australia.

g) What about the display?

Most grid connected inverters have display lights indicating whether the unit is on, off or in standby. They can also have displays (often scrolling displays) that indicate some of the following information:

the amount of power (kilowatt hours) you have produced on a daily basis,
the amount of power (kilowatt hours) you have produced since the unit was installed
the amount of power (kilowatts) the unit is currently producing
the number of hours the unit has been producing power

Some of this information (e.g the total amount of power generated) is also available on your meter.

Some meters also offer a data-logging feature so you can download information to a computer, or transmit it over Bluetooth or your Wi Fi network. This means you can either see your power information on your PC, smartphone or on a special in home display.

I personally don’t want to look in the meter box (or wait for the bill) to see if my system is performing properly. I want to have a remote monitor in the house that I can check every day. (Hey I’m a geek!). So I think remote monitoring / data logging is a must.

If your supplier wants to charge you more than about $250 for remote monitoring, then you may want to consider doing it yourself. The best solution I’ve seen is the Current Cost Meter from www.smartnow.com.au . For about $200 you get a wireless display that shows both your solar power and your household power consumption, and it is all downloadable to your PC if you want to get all analytical.

h) How efficient is a grid connected inverter?

Efficiency is a measure of how well the grid connected inverter converts the power it receives from the solar panels into power available to your home or to the grid. Most grid connected transformer based inverters have an efficiency of around 93% or better, transformerless are typically around 95% or better.

Efficiency would only be a factor if two similar inverters had significantly different efficiency.

Efficiency is usually listed on the specification sheet.

i) What about surge capacity?

Surge capacity only becomes an issue if you have a lot of appliances (e.g. refrigerators) or equipment that require large amounts of power for a short time in order to start. Most grid connected inverters have a surge capacity the same or slightly higher than their continuous power output.

If you need extra surge power check the specification sheets to find an inverter to suit your purpose.

How to spot a bad inverter.

Most grid connected inverters supplied by reputable retailers in Australia are OK.

If you are unsure about an inverter check out if is listed on the Clean Energy Council list of inverters Certified to the Australian Standard (AS 4777). If it is not listed there then run a mile! Its also worth checking some of the forums and blogs that discuss such topics.

Also walk away, if there is no Warranty offered on the inverter or no specification sheet is supplied. And make sure the organisation issuing the warranty has an Australian office. You don't want to be calling Shanghai (or Munich) to get a new inverter!

If you have concerns about the retailer offering you a system, you can also contact the Clean Energy Council (see http://www.cleanenergycouncil.org.au/cec/contactus.html ) to see if the retailer is registered with them. The Clean Energy Council is the industry body for the Solar Power Industry and is supposed to only list reputable Solar Power retailers.

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