Solar 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?
Well, every bit of solar power produced by your panels will pass through your inverter, and it also contains critical safety shutdown electronics (called Anti Islanding Protection). So it is critical that you choose one that is up to the job.
Solar Inverter Brands
I'm guessing that you don't know one brand of inverter from another. So the simplest, quickest way I can think of to describe how each inverter brand stacks up is to compare them to cars - because I reckon most of you will know the difference between a Mercedes, a Holden and a Hyundai.
So if you are in a hurry and you don't want to "Geek Out" over the technicalities of inverters - this chart I knocked up will teach you enough to be dangerous when talking to shonky sales people or evaluating quotes:
Now let's get technical.
If - after all that - you are still yearning for a deeper explanation of inverters, then read on...
First of all you need to realise that there are 2 main ways to make inverters, each with their strengths and weaknesses:
Firstly, inverters can be either:
- a transformer type or
- a transformerless 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 Inverters
Let’s go through each type:
a) Transformer type solar inverters
About a third of all the 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 inverters are a newer technology that hit the Australian market in about 2006 and are becoming the defacto standard.
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 this is only annoying if you are a dog!
It is likely that transformerless grid-tie inverters will replace transformer type grid-tie inverters in Australia in the future.
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 behind 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.
Wholesale prices in Australia are currently hovering around 30c per Watt. So a 3kW inverter will start at around $900, and a good (SMA) 10kW inverter costs around $3,000. As with solar PV panels, grid connected inverters are constantly being improved and their prices will tend to reduce in the future.
But whatever you do: Never buy the cheapest inverter on the market! The real el cheapo inverters have no chance of lasting 15 years plus. Trust me on this. It is hard to design and make a good inverter that will last. Never buy a bottom of the range inverter from a no name brand. It is false economy. It just won't last (rather like the companies selling them!).
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. That usually means they if you want a 5kW solar system, you get 5kW of panels and a 5kW inverter to suit. Simple eh? Actually it can get a little more complicated than that. Let me explain.
Because of system losses in the panels, your solar inverter can actually be rated at up to 25% less (in AC kW) than your panel array - in some situations. Confused? So are a number of installers! The Clean Energy Council guidelines for inverter sizing recently changed, and I provide a detailed explanation of when 'undersized' inverters are allowed here.
A final word of caution: Be aware that inverters are rated in "DC input" and "AC output" terms. Make sure your inverter is also rated to suit the output of your panels in DC! (The previous link explains how to check this)
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 on my roof 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 MPPT (Maximum Power Point Tracking) inverter. These inverters have multiple MPPT 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 small MPPT inverter attached to the rear of the solar PV panel, typically called micro inverters. This would potentially enable different models of solar panel to incorporated in the same solar power system, would overcome shading issues and make individual solar PV panel monitoring possible. There is an rapidly increasing trend towards these products in the USA and they are starting to appear in Australia.
g) What about the display?
Most 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". 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?
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.
How to spot a bad inverter
Most inverters supplied by reputable retailers in Australia are OK. But there are some horrible ones that have got through the Aussie compliance testing.
Before you consider buying a very cheap inverter - check out my ever growing list of inverter reviews. These are reviews by people that have owned the inverter for at least 12 months. If there are lots of 1 and 2 star reviews - that usually means the inverters are failing in the first 12 months. Not a good sign!
Next have a look at my solar inverter comparison chart. It compares independent efficiency measurements of lots of inverters. A very low efficiency is a bad sign.
It's also worth checking some of the forums (like whirlpool.net.au) that discuss such topics.
Also, walk away if there is no warranty offered on the inverter or no specification sheet is supplied. 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!