10 Questions You Should to Know about grid connected inverter

Please note: As you can see from the date of this post – it was written in – it was accurate then, but now it is out of date – most inverters in Australia are now ‘transformer-less’.

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In my “Dummies Guide” to Solar Power I provide a very brief (some may say shallow!) overview of what a grid connected solar inverter does and why.

For those of you who want to know more details about how inverters really work, including the nerdgasm inducing concept of “Maximum Power Point Tracking” (which is all about maximising the power from your panels), then this blog post is for you!

Inverters have 2 main roles in life:

• Converting constant, Direct Current (DC) electricity from the panels into oscillating, Alternating Current (AC) electricity for your power points.
• Transforming the low(ish) voltage from your panels to the high(ish) voltage needed by your appliances.

But they also have to:

• Keep you safe – i.e., the grid connected inverter should shut down if there is a power blackout or a fault with your Solar Power system.
• Export electricity to the grid (and get paid for it) when your solar panels are producing more power than you are using.
• and (ideally) do some funky electronic stuff to maximise the power from your panels if, for example,one panel is giving more output than its neighbour

Let’s go through how that box of electronics bolted to the wall manages to do all these things:

a)         Converting DC electricity to AC electricity

Solar panels produce direct current (DC) electricity. This is the type of electricity used in car batteries. However, the electricity we use in our homes for lighting and power is 240 volt Alternating Current (AC) electricity.

The transformation of DC electricity to AC electricity is achieved by use of very efficient electronic switches to alternate the flow of the DC electricity produced from solar panels.

That is, switch one opens and switch 2 is closes and the current flows one way across the circuit. Then switch 1 closes and switch 2 opens and the current runs the opposite way across a circuit. Thus the DC electricity is converted to AC electricity

b)         Transforming the voltage of the electricity produced by solar panels

The transformation of the voltage from the solar panels (23 to 38 volts) to match the voltage of the electricity grid (240 volts) is achieved by using a transformer.

Put simply, transformers have 2 sets of coiled wire on each side of a circuit. The current of electricity flowing through the first coil causes a current to flow through the second coil.

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The voltage across each coil is related to the number of coils in each set of coils. The higher the number of coils the higher the voltage. Thus, in a grid connected inverter, there are more coils in the second set of coils than in the first set of coils and this increases the voltage of  electricity produced. The number of coils in the second set is adjusted so the output voltage matches the voltage of the electricity grid (240 volts). See diagram below.

c)     Maximising the electricity you get from your solar panels

In order to get the most electricity out of your solar panels the grid connected inverter uses a maximum power point tracker (MMPT).

Each cell in a solar panel has a maximum power point (MPP). This is the point at which the maximum power (i.e. electricity) can be extracted from a cell. The MPP is determined by the voltage and current of the cell, the cell temperature and the amount of sunshine hitting the cell. The MPP for a given cell can vary by as much as 25%.

Thus, to get the maximum power out of a cell we can use a MPP tracker (MMPT) to vary the current and voltage of the cell (using software logic or circuitry controls) so the cell  is operating at its MPP.

Most current grid connected inverters have one MPPT that will seek to get the MPP of a system as a whole. Since individual cells will each have a different MPP (due to differences in manufacturing, differences in shading, etc.) then some cells will not be operating at their MPP. Therefore, there is a loss of efficiency.

Some inverters do have more than one MMPT. These are called Multi String Inverters. This can be useful if you have a system where you might have strings of panels in different situations (e.g. a string facing north and a string  facing west). As each panel will have a different MPP depending on where they are facing, each MMPT can find the MPP for each string of panels. This will increase the overall efficiency of the system.

Multi string inverters may also be useful if you want to connect different panels types provided your inverter can handle the increase power from the solar panels (e.g. if you want to upgrade but your original panels have been superseded).

d)    Safety of your grid connected inverter

All grid connected inverters are required to have certain safety features to protect you, the grid technicians, your electrical appliances and your inverter. These are:

• A DC isolation switch isolates the inverter from the solar panels. This prevents damage to the inverter or household appliances if there is a fault with the solar panels.

• An AC isolation switch isolates the inverter from the grid to protect the inverter in case of a fault in the grid.

• A safety switch that will turn off the inverter and isolate it from the grid in case of a blackout. If this did not happen your Solar System could send electricity into the grid. This, in turn would be a danger to any mains grid technicians that come to check the grid.

e) Exporting Electricity to the grid (and measuring the power output from your panels)

If you want to get paid for the electricity coming out of your panels then you need to do two things: measure the electricity, and export it to the grid (if you don’t use it all in your home).  If you are on a Gross Feed In Tariff then you need a “Gross” Meter that measures all the electricity coming out of those panels whether you export it or not. If you are on a Net Feed In Tariff then you need a meter that measures the difference between what you generate and what you use, because you are only gonna get paid for the exported electricity.

Warning: Make sure that any quote you get includes a new meter and connection of the meter to the grid. It amazes me that anyone can quote a system as “fully installed” without doing this, but a lot of  companies are doing just that. Kind of useless to get a solar power system for your home that isn’t connected to the grid. The worst case scenario is that they connect it to your existing meter which doesn’t know the difference between what you have generated and what you have used, then you will actually be charged for the solar power you generate (not credited) !  Ouch.

Hello All,

I installed a 14.8kw Grid Tied Enphase solar system in . It's been great but generation buy back has really taken a hit in Texas to the point it's no longer worth it.

I'm pretty close to pulling the trigger on installing a Hybrid Inverter and some batteries so I have somewhere to put the excess power I produce. I also have switched to a Free Nights plans which has been great as I have moved my EV Charging and Pool pump to run during the free times and I've been able to move almost 90% of my usage (in winter) to the free period. I know in summer in the Houston Area the AC will change this a lot. So I'm hoping the battery will help offset this and maybe even be able to get me a negligible power bill.

And of course I want to be able to run my critical loads if the grid goes down which has happened a number of times in recent years.

Cost is a pretty big factor for me as I'm trying to justify this to my boss (wife) by cost savings.

My hope is to have Solar and Battery run all my power requirements during the day, and free nights to cover me in the evenings - while I charge my car run the pool pump and charge the solar battery.

So to my questions.
After some research I am learning towards the RUiXU RX-12K Hybrid inverter. I was looking at the EG4-18kPV but it's over $1K more and I'm not sure it will provide any benefit. Same as the Grid/FlexBoss combination. Is there anything with the RUiXU RX-12K I should be considering that I'm not? I was looking a the DEYE inverters as they are a great price point but then I read about the whole Sol-Ark debacle and the bricking so obviously that's not an option. Are there any other options I am not considering that I should.

I am looking at combining 8 x EcoWorthy 12V 280ah battery's. 4S2P. Giving me 26.88kWh. Sorry if this is a silly question but do I need a battery balancer for each of the 4 Series connected battery. I'm assuming an additional BMS connected to 4 batteries with built in BMS"s isn't something you should do.

I already have 2 panels one main one sub. The plan was to move all my critical loads to my sub panel and wire that to the load of the inverter. Wire the Enphase combiner to the GEN connectors, And the main panel to the Grid connectors. I assume the Grid panel will still be powered by my excess solar, is this correct? and the system will continue to sell back unused solar to the grid?

Maybe another silly question but in the event of Grid Down, with the critical load panel running off the battery, will excess solar produced power the Grid panel while we have enough solar production?

I also have a 12kw "portable" generator that I converted to Tri-Fuel that I have wired to an interlock in my main panel. My plan was to wire this to a dual throw manual transfer switch so I can switch from the Generator to Solar in the event of Grid down. This give me the ability to switch to the generator during extended outages if I run out of battery when the Sun isn't out (night or cloudy). Does anyone see any issues with this?

I think that's all I have for now. Any thoughts or advise you may have would be greatly appreciated.

TIA

Cam

Yeah thanks. Just noticed you need specific high volt batteries for it. So I guess I'm back to the RUIXU RX12k or the EG4 18kpv. Still researching the support Solix S6. But it doesn't look like it has an input for the AC solar. How do I wire it in?

The Solis has 4 mppts up to 600v.

It can also ac couple on the load side, or ac retrofit in the grid side

Please give my ignorance but when would I need HV vs LV?

Just whatever you are more comfortable with and whichever is the best value.

Couple more questions if you have the answers.

Do I need a battery balancer if I connect 4 12v batteries in series?

I've heard it's not a good idea to do that but I don't know exactly why. I'll try to find the link I read

I'm pretty sure the answer is yes but just wanted to check with the AC coupled solar connected to the inverter, the solar will still cover the circuits on the grid panel while the grid is up?

Yes that is correct

Thanks for all you knowledge

Cam

Here is a thread on the topic of 12v batteries in series

As a novice in battery making I see that so many people are connecting 4 LIFPO batteries in series to make 48v, and for my understanding this seams to be a very bad practice as there is no way to keep the batteries balanced and the failure of one battery or BMS can create huges problems and thermal runaway as the voltage is way to high for the 12v batteries, it would seam like you would at least need a external type of BMS to monitor the 4 string voltage of individual battery that would trip a breaker to shut off the charging. If one of the mosfet for charging fails as the 3 other ones are...

Thanks for that. I read though it and past the first few replies most seem to be saying it's okay. The batteries I'm looking at are listed as 4s4p by the manufacturer. But I'm still concerned.

I believe the main concern is a one of the 12v batteries dying and having a thermal runaway issue but if each of the 12v batteries has a BMS with thermal monitoring, shouldn't that kill the battery before that happens?

Please know I'm not disputing you, I really just started learning all this 2 weeks ago, but just trying to understand.


I understand if you can get 48v certainly do it but there is a 50% price increase per kWh if I don't want to build my own batteries. (Which I may end up doing it I need to). I was hoping 4s with a balancer 2s would work.

I looked more into the Solis S6 but I can't find anything about a split phase version outside of AliExpress. They all seem to be single phase. I read you can wire single phase inverters for split phase but is that normal?

Thanks again for all of your help The S6 US is split phase with built in backup capability. They are making a MID (microgrid Interconnect Device) that will allow more options, but I don't think it's available yet. Search for Solar Guppy for his install and experiences. High voltage batteries are a different animal, but more and more options are coming out all the time, and they are looking like the future. I've been looking into the S6 quite a bit to use as a grid tie inverter for now, and maybe adding 10kW or so of backup battery in the future.

The Growatt requires an autotransformer or an MID to provide backup power, depending on which version you get of the same part #, which could be argued is an incredibly stupid idea. Why they wouldn't make a different part # for an essentially different inverter is mind boggling. In fact, they made 3 different versions of the same part # of that inverter within 2-3 years of it coming out, and they make at least 2 of them at the same time.

Here is a link to the S6 available in the US. It's available from a bunch of online retailers, I just knew it was easy to get to from signature solar's website. https://signaturesolar.com/solis-s6...-high-voltage-energy-storage-hybrid-inverter/

The S6 US is split phase with built in backup capability. They are making a MID (microgrid Interconnect Device) that will allow more options, but I don't think it's available yet. Search for Solar Guppy for his install and experiences. High voltage batteries are a different animal, but more and more options are coming out all the time, and they are looking like the future. I've been looking into the S6 quite a bit to use as a grid tie inverter for now, and maybe adding 10kW or so of backup battery in the future.

The Growatt requires an autotransformer or an MID to provide backup power, depending on which version you get of the same part #, which could be argued is an incredibly stupid idea. Why they wouldn't make a different part # for an essentially different inverter is mind boggling. In fact, they made 3 different versions of the same part # of that inverter within 2-3 years of it coming out, and they make at least 2 of them at the same time.

Here is a link to the S6 available in the US. It's available from a bunch of online retailers, I just knew it was easy to get to from signature solar's website. https://signaturesolar.com/solis-s6...-high-voltage-energy-storage-hybrid-inverter/

Thank you!

I read though the manual of the S6 and it brought a few questions.

It says that when AC coupled I can wire the ACPV to either the grid side or backup side. But if I wire to the backup side it says the ACPV can't exceed the nameplate size of the inverter. My existing solar array is 14.8kW. and the S6 11.4kW.

So does this mean I need to wire the ACPV onto the grid side. And if I do this won't it get disconnected with the grid is down?

It also says if the solar consumption on the grid side is more than the name plate side when the grid is present an overload could occur. I have an EV charger which will be in my grid side that well pull more than 11.4kW by itself. This sounds weird an I reading it right? Why does the inverter care if it's on the grid side?

Thank you!

I read though the manual of the S6 and it brought a few questions.

It says that when AC coupled I can wire the ACPV to either the grid side or backup side. But if I wire to the backup side it says the ACPV can't exceed the nameplate size of the inverter. My existing solar array is 14.8kW. and the S6 11.4kW.

Correct, that is true of pretty much all ac coupling systems. Look up the Victron factor 1 rule for explanation.

Does your 14.8kw system ever produced more than 11.4kw at any point in time?

Are you and to supply the system up into two different outputs? Some going to backup sides and some going to grid side?

So does this mean I need to wire the ACPV onto the grid side. And if I do this won't it get disconnected with the grid is down?

Yes if is connected to the grid side you will not have solar when the grid is down but your inverter will still operate off battery . Do you have a lot of grid down time?

It also says if the solar consumption on the grid side is more than the name plate side when the grid is present an overload could occur. I have an EV charger which will be in my grid side that well pull more than 11.4kW by itself. This sounds weird an I reading it right? Why does the inverter care if it's on the grid side?

This is a good question. I read that too and was confused