Home solar power questions

[rover-56]

Have been looking at grid connected rooftop solar for a few weeks now and it looks financially viable.
There are a couple of things I don't quite understand, so I hope someone can enlighten me.

1. Seems that a solar inverter will not supply power to the house when the mains power is interrupted, unless I install a 'hybrid' inverter and batteries. (expensive).
I understand 'anti islanding' but I would have thought a double pole mains isolating switch would make things safe. Backup generators seem to be usable in that situation, why not a solar system?
Can someone explain please?

2. I would like to be able to heat hot water in the afternoons from the solar system by pressing the 'boost' button on my smart meter.
I have had about a 50/50 split of answers from various sources which some say the solar will supply the power and others say the power will still come from the mains supply. Be nice to get a clear answer.
There are add on devices (greencatch) which can apparently divert solar power to the water heater, so why are people telling me solar won't heat water?

Most likely will go ahead with it, but quite hard to get clear info sometimes, and a bit of a learning curve here for me.

Thanks, Terry

[JDNSW]

Have been looking at grid connected rooftop solar for a few weeks now and it looks financially viable.
There are a couple of things I don't quite understand, so I hope someone can enlighten me.

1. Seems that a solar inverter will not supply power to the house when the mains power is interrupted, unless I install a 'hybrid' inverter and batteries. (expensive).
I understand 'anti islanding' but I would have thought a double pole mains isolating switch would make things safe. Backup generators seem to be usable in that situation, why not a solar system?
Can someone explain please?

2. I would like to be able to heat hot water in the afternoons from the solar system by pressing the 'boost' button on my smart meter.
I have had about a 50/50 split of answers from various sources which some say the solar will supply the power and others say the power will still come from the mains supply. Be nice to get a clear answer.
There are add on devices (greencatch) which can apparently divert solar power to the water heater, so why are people telling me solar won't heat water?

Most likely will go ahead with it, but quite hard to get clear info sometimes, and a bit of a learning curve here for me.

Thanks, Terry

1. My understanding is that, apart from the anti islanding issue, inverters depend on the mains waveform to ensure they operate in phase and at the correct voltage relevant to the mains. While it would be possible to design a system that, when mains power shuts down, automatically disconnects, handles changes in local load correctly, shuts down when power demand exceeds available sunlight, and when mains power returns, adjusts phase and voltage and reconnects. Without batteries this becomes a very complex design, and there is the question whether it is worthwhile if the lights go out when the sun sets. Automatic changeover to a generator is much simpler - just switch from one to the other - there will be at least a few seconds gap unless the generator is running all the time, but there is no requirement for the voltage and phase of the generator to match mains.

2. I think that when you press boost, the water will be heated by whatever power is available, depending on how it is wired. And how it is wired probably depends on which provider installed the system and smart meter.

[BradC]

I understand 'anti islanding' but I would have thought a double pole mains isolating switch would make things safe. Backup generators seem to be usable in that situation, why not a solar system?
Can someone explain please?

The issue is while a double pole mains isolator will make things safe for the linesman, you have 3 potential situations.
- Normal. Grid is connected (and up) and solar is making power
- Islanded. Grid is connected (and down) and solar is not allowed to make power
- Stand alone. Grid is disconnected and solar is allowed to make power.

How do you manage 2 & 3 without having the potential to accidentally squirt power back into the grid. With a genny its a simple changeover as you never want the Gen and grid connected together.
The simple solution from a grid perspective is mandate the inverters can't run isolated. That means even if someone does something dumb, there's a significantly reduced risk of splatting a linesman.

As for the HWS. Its just another appliance. It'll use whatever is available from wherever it can get it. If you are consuming more than you are making then the grid makes up the shortfall.

I've heard of some pretty clever widgets that will work with a smart meter and inverter to determine what your surplus is and divert that to another load (like the HWS), so you only consume what you use, but all the ones I've seen in passing have been fairly homebrew and quite complex.

[rover-56]

1. My understanding is that, apart from the anti islanding issue, inverters depend on the mains waveform to ensure they operate in phase and at the correct voltage relevant to the mains. While it would be possible to design a system that, when mains power shuts down, automatically disconnects, handles changes in local load correctly, shuts down when power demand exceeds available sunlight, and when mains power returns, adjusts phase and voltage and reconnects. Without batteries this becomes a very complex design, and there is the question whether it is worthwhile if the lights go out when the sun sets. Automatic changeover to a generator is much simpler - just switch from one to the other - there will be at least a few seconds gap unless the generator is running all the time, but there is no requirement for the voltage and phase of the generator to match mains.

2. I think that when you press boost, the water will be heated by whatever power is available, depending on how it is wired. And how it is wired probably depends on which provider installed the system and smart meter.

Thanks John,
Yes I can see that the need to match phase would be an issue, I suppose the 'hybrid' inverter has the required smarts to do all the switching and matching - hence the price asked.
I suppose the problem with the basic inverter is that someone would need to be home (and remember) to isolate the grid supply to keep the power grid repairers safe. A problem there when no one is home when the power goes out.

Differences in wiring probably would explain the varying answers to the boost power question. I suppose if the HW power is wired to only use mains power that would be an annoying problem.
Cheers,
Terry.

[rover-56]

The issue is while a double pole mains isolator will make things safe for the linesman, you have 3 potential situations.
- Normal. Grid is connected (and up) and solar is making power
- Islanded. Grid is connected (and down) and solar is not allowed to make power
- Stand alone. Grid is disconnected and solar is allowed to make power.

How do you manage 2 & 3 without having the potential to accidentally squirt power back into the grid. With a genny its a simple changeover as you never want the Gen and grid connected together.
The simple solution from a grid perspective is mandate the inverters can't run isolated. That means even if someone does something dumb, there's a significantly reduced risk of splatting a linesman.

As for the HWS. Its just another appliance. It'll use whatever is available from wherever it can get it. If you are consuming more than you are making then the grid makes up the shortfall.

I've heard of some pretty clever widgets that will work with a smart meter and inverter to determine what your surplus is and divert that to another load (like the HWS), so you only consume what you use, but all the ones I've seen in passing have been fairly homebrew and quite complex.

Thanks Brad,
I suppose the whole system is set up to keep linesmen safe from dopey housholders, and I can see a problem with a manual switch over if no one is home.
I am assuming that these 'hybrid' inverters are capable of doing the required switching and phase matching to allow unattended operation. They are quite a bit more pricey compared to the basic grid connected units.
With the hot water boost, as John says, the wiring configuration would affect the power source. Mine might be ok and it might not. I guess we'll see.
Those clever widgets (greencatch is one) look like they work ok, but for $1500 installed I would rather just press the boost button if I can.

I used to work as a fitter at a plastic moulding plant once. They used 5 natural gas fuelled 2000kW ship engines to generate the power needed. A 6th engine was on standby for backup if needed, and the connection phase matching was done manually - a 300mm dia motor coupling was regularly sent through the plant roof. They eventually set up an electronic system.

Terry

[Vern]

https://www.google.com/url'sa=t&sour...akTCUyKEKMHZIE

I have installed these in the past, work great.

[Graeme]

My Fronius non-hybrid inverter has a data management option whereby a user-supplied relay with 12V coil and 240V contacts is switched on and off according to the amount of surplus power being exported. I generally set the limits to switch on just below the maximum allowed export of 5kW and off at 1kW as the HWS element is 3.6kW but mid-winter I set the limits to switch on when 4kW export and off when nil export. A lower rated element (eg 2400W) would allow the HWS to switch on more often but I always have the option to switch on the original HWS timer set to off-peak times (overnight here).

My electrician installed the relay in parallel with the HWS timer (which had been set to off-peak but now almost always OFF) and I supplied the 12V supply for the relay coil.

Because my PV system including the inverter is distant to the house, I provided a master/slave wifi system that is switched by the inverter's 12V o/p which in turn switches the relay coil at the house. If the PV system was on the house where the HWS timer is located then its a simple 12V connection from the inverter to the relay.

[Disco4Dave]

Be sure you are better off using solar power for your hot water. We are on the solar-savers plan with AGL which pays a feed-in of 20c/kWh, and charges approx 14c/kWh (inc GST) for off peak. For us, it was better to keep the hot water connected to off peak, and export instead.

[rover-56]

Rooftop solar makes financial sense at the moment long term, the issue of power with the grid down would have been icing on the cake but the cost of a grid connected battery system is not financially attractive - even with the various government rebates intended to make solar look competitive with coal power.
I can see that lineman safety is a very important issue.
As for the HW boost from solar sources we will have to see what happens after installation, maybe I can get some wiring or programming changed.
I heat water in the winter from a kitchen wood stove, so don't pay for power then. using the boost control would avoid the installation of a dedicated solar water heater on the roof in parallel with the stove heat exchanger. All very interesting.

Thank you for the replies everyone.
Terry

[rover-56]

Be sure you are better off using solar power for your hot water. We are on the solar-savers plan with AGL which pays a feed-in of 20c/kWh, and charges approx 14c/kWh (inc GST) for off peak. For us, it was better to keep the hot water connected to off peak, and export instead.

Yes I'll do the sums when I see the real world costings.
I am with Red, and they only pay $0.12 feed in atm.
Terry