Newbie question about how power is supplied from grid if using solar

[Danny42]

Hi apologies for the silly question:

Assume I don't want to do a grid-tie (I believe is the phrase for the ability to feed power back into the grid, current rate averaging 4p, vs 52p they charge me...)

If I have an inverter that can supply a maximum of 5kw, and say I have full batteries, or the sun is out and I'm generating 5kw, but then my heat pump kicks in with a compressor startup inductive load (10kW), plus I'm charging my EV(7kW) and my cooker(4kW) is on plus 3 computers(0.7kW) and a bunch of lights(0.3kW), so say I need 22Kw for a while, I need another 17kW.

I know some invertors (although I'm struggling to wrap my rubbish brains about whether this requires them to be a hybrid or a grid tie-in or what designation), have a mains input that can help charge the batteries, or perhaps it can top up the supply.

Here's my question: If the electronics in my invertor are rated to supply 5kW, and then any topping up I need from the mains cant come 'through' the inverting circuits. Does it get 'mixed-in' somehow, and does that mean that the invertor is always syncing the voltage, frequency and phase with the mains to allow for that?

I've been looking at specs for invertors and combined 'solar charge controller & invertor' beasts, and they'll give you all the ratings for the invertor but wont say what the limit of how much they can top up the supply wiht mains is. Maybe no one does that? But for a house with heat pumps and 32A EV chargers and electric cookers, you'd need an impossibly large PV array and giant invertor to cope with the occasional loads of all of that at once.

Please note, I did ask for some clarification on the subject to the people who just charged my dad 24k for a grid tied system on his roof, and they couldn't answer the question. Figured you guys would know!

Thanks, and again apologies for stupid questions.

[Spacey73]

Battery is king in this situation, at the moment I have 12kwh lithium hooked up to a dedicated 5.5Kw hybrid inverter with 9x 540w panels. Without the battery I would constantly have the EV disconnect and also miss out on the solar whilst out in the car. This is a dedicated EV charger setup and is not connected to house.

 

The house at the moment has a 320AH 48v (roughly 16Kwh) battery capable of supplying 200Amps (roughly 8Kw) to the house hybrid inverter which is also hooked up to 2 x 10 series of 390W panels.

 

It copes really well at the moment but have ordered another 16Kwh battery and another hybrid inverter as I’ll be heating the house with heat pumps hooked up to air conditioning. Plus 2 more freezers for the coming apocalypse…..

 

The inverters will only give the house what it needs and anything else goes to the battery. If that’s full then nothing goes to the grid because of grid limiting (I have no interest in exporting).

 

Without  batteries it will be very hard to power any big loads as you would have to time it perfectly to the weather.

 

If the weather is terrible and battery can’t cope with house demand then it will take from grid. Solar first, battery 2nd then grid last is the way to setup. 

Edited September 6, 2022 by Spacey73

[Spacey73]

Just to add, the hybrid inverter will tell you how much pass through from the grid it can take…I’ve wired mine up a bit different though as my meter just works out how much electricity I’ve used in the last few mins then if I have the solar or battery it literally chucks out enough juice to balance out…I literally watch my meter wheel go forward for a few mins then reverse back to the starting kWh reading.

[Dillsue]

2 hours ago, Danny42 said:

Hi apologies for the silly question:

Assume I don't want to do a grid-tie (I believe is the phrase for the ability to feed power back into the grid, current rate averaging 4p, vs 52p they charge me...)

If I have an inverter that can supply a maximum of 5kw, and say I have full batteries, or the sun is out and I'm generating 5kw, but then my heat pump kicks in with a compressor startup inductive load (10kW), plus I'm charging my EV(7kW) and my cooker(4kW) is on plus 3 computers(0.7kW) and a bunch of lights(0.3kW), so say I need 22Kw for a while, I need another 17kW.

Feeding power back into the grid is called exporting which you may or maynot get paid for- google smart export guarantee(SEG). Grid tie is whether your inverter is connected to the grid or not(off grid). If the inverter is feeding the house and the house is connected to the grid, then youve got a grid tied inverter.

 

Your heat pump is only going to pull 10kw for a few seconds during startup and a fraction of that once its running. Unless you cook on full power, your cooker could well run lower than the 4kw you mention. If you design your PV/battery system to supply all the peak house load you could end up with a overly large and very expensive system that rarely gets used. Youd likely be better letting the grid supply the short term peaks like the heat pump startup. I think alot of EV chargers let you schedule charging time so you could charge the EV away from meal times so youve not got cooking and charging load at the same time.

[Spacey73]

Yeah agreed, no point trying to power everything by solar if you can have grid connected. I still us grid for about 5% of my electricity but that’s really to deal with large peaks and those rare times I run out of battery (several terrible solar days in a row). 
 

Personally I don’t see the point for my situation for exporting. I’d rather tie up any excess I have in heating the Sunamp or charging the car. 

[Danny42]

Hi guys,

 

Thanks for trying to help. I'm afraid I didn't really phrase my questions quite right, let me try again from the point that Dillsue got to:

>> "If the inverter is feeding the house and the house is connected to the grid, then youve got a grid tied inverter."
My invertor would certainly feed the house, my plan for it would be to be connected to my consumer board and able to feed (at least partially) everything I use.

>> "Your heat pump is only going to pull 10kw for a few seconds during startup and a fraction of that once its running. Unless you cook on full power..."
I know the draw might only be very high for a short while, and I certainly don't want to oversize the system based on the temporary max draw. I do get that, I just want to know what happens when the draw does, innevitably, however briefly, go over my system's limit.

I know you might not like the numbers, but if you wouldn't mind indulging me and assuming these are realistic:
I have a temporary demand for 10kW, but I have only a 5kW invertor. Pretend I don't have solar at all, but instead a vast battery that is currently fully charged, lets say I charge it when Octopus are billing me 7p/kwh at night, and now I want to use the battery and invertor while my steady state during the day is around 4kW. But right at this moment it has blipped up to 10kW because I have two rings on the oven going, have just opened the oven door and heat pump is battling the snow outside and is just winding up the compressor. I get that that is rare, and hence wont design the system for it, so have just a 5kW invertor.

My question is this: What happens? Does the invertor shutdown completely? Does it supply the 5kW it is rated to and let the grid supply the rest? And if the latter (I'm hoping the latter), does the inbound grid connection have to somehow go 'through' the invertor, or is it just hooked up in parallel to the grid?

Sorry for the pedantry and I really do appreciate the help!

Thanks,
Danny

 

[Dillsue]

If Octopus are supplying you, then youre connected to the grid. If your inverter is connected to the consumer unit, then its grid tied.

 

Anytime your inverter or batteries cant provide the power you need, the extra will be drawn from the grid

[Danny42]

Thanks Dillsue. Do you know if the 'extra' passes through the invertor, or if it is just supplied by dint of the invertor being hooked up in parallel to mains supply to the busbars on my consumer unit?

[Danny42]

Sorry not to give you time to answer, but again, I need clarity on the nomenclature. You write that if my invertor is hooked up to my consumer board and able to suppliment the grid (and rely on the grid to cope with surge demands higher than its rated output), that is called 'grid tied'.

According to a few (very likely to be wrong) articles, there is some distinction that grid tied systems cannot have batteries. And that the only systems with batteries are 'hybrid'.

I was rather hoping to:
- Have solar charging my batteries if there is an excess of solar generation.
- Charge batteries from the grid at night if the sun didn't fill them up.
- Use the grid to fill in the gaps when my inverter capacity cant cope, or batteries too flat solar not supplying enough.

While I absolutely accept your definition that the invertor is defined as 'grid tied' if it is connected in parallel to the grid supply, does this also mean that it can't use batteries? Perhaps the articles I have read are out of date and that inverter-charger equipment can these days handle everything someone might conceivably want to do.

[Marvin]

Complicated.

 

I think clear questions need to be asked...

 

AFAIK (as far as I know):

 

Grid tied inverters monitor the grid. If the grid has a power cut the inverter will stop working / producing power/ doing anything. 

 

You will need an inverter that will decide to charge your batteries (with programmed parameters) otherwise what will decide where the power goes? 

 

So another question is If a hybrid inverter is connected to the grid, PV and batteries, will the inverter continue to work if the grid goes down?

 

Anyone in the know?

[joth]

50 minutes ago, Danny42 said:

According to a few (very likely to be wrong) articles, there is some distinction that grid tied systems cannot have batteries. And that the only systems with batteries are 'hybrid'.

Really these are orthogonal issues. e.g. what about a grid-tied hybrid system?

In principle all four exist of "grid tied", "grid tied hybrid", "off-grid", and "off-grid hybrid".

 

- Whether something can or can't be grid tied or have batteries is both a technical and a regulatory question, and regulations vary by country, so be very cautious trying to find a universal globally accepted nomenclature by reading online articles that may be written for different countries and regulatory constraints.

 

Generally grid tied means it is not "off grid only" system - i.e. has the necessary circuitry to synchronize the waveform to the grid, and has necessary safety features to work on the grid of a given country. It may also support grid failover (in case of a blackout) but that's something else again.

 

A hybrid inverter can simultaneously support battery and PV and DC inputs. But if you have a non-hybrid grid tied inverter you can still add batteries using its own inverter -- a so-called AC coupled system.

 

Your requirements can be met with either of "grid tied PV and a separate AC coupled battery" or "grid tied hybrid inverter supporting both PV + DC coupled battery". There's some regulatory and power efficiency gains in going for the latter, but (currently) some cost savings and flexibility in sourcing the former.

 

 

Edited September 22, 2022 by joth

[Dillsue]

1 minute ago, Marvin said:

So another question is If a hybrid inverter is connected to the grid, PV and batteries, will the inverter continue to work if the grid goes down?

 

Anyone in the know?

Absolutely not. It MUST shutdown or it could electrocute someone trying to repair the cause of your power cut

[Dillsue]

1 hour ago, Danny42 said:

Thanks Dillsue. Do you know if the 'extra' passes through the invertor, or if it is just supplied by dint of the invertor being hooked up in parallel to mains supply to the busbars on my consumer unit?

No. Both the grid  and your inverter feed your consumer unit and the power from either or both feeds the house circuits fed from the consumer unit. Your inverter is connected to an MCB(maybe fuse) in your consumer unit in the same way as all the other circuits in the house. The difference being during daytime power flows from the inverter TO the consumer unit rather than power flowing FROM the consumer unit for all other circuits. At night when the inverter isnt generating, a small amount of power flows TO the inverter. None of the circuits are connected to the busbars

[joth]

23 minutes ago, Dillsue said:

Absolutely not. It MUST shutdown or it could electrocute someone trying to repair the cause of your power cut

Caveat on this is if it has an automatic grid disconnection switch or a separate non-grid tied  battery-backed AC output then it can remain powered. But, this is orthogonal feature it may or may not have depending on both technical and regulatory constraints

 

(For example my SolarEdge SE-8000H hybrid invert supports this technically, but it is not legal to use in the UK as it doesn't yet have the necessary certifications)

 

Edited September 22, 2022 by joth

[Dillsue]

1 hour ago, Danny42 said:

I was rather hoping to:
- Have solar charging my batteries if there is an excess of solar generation.
- Charge batteries from the grid at night if the sun didn't fill them up.
- Use the grid to fill in the gaps when my inverter capacity cant cope, or batteries too flat solar not supplying enough.
 

You can do all of those things. Plenty of good schematic diagrams and write ups on the web BUT... If youve read that grid tied systems cant have batteries you need to be a bit more choosy where you get your info as that is wrong. Make sure you are looking at systems for the UK market as our rules differ to other countries.

 

Have a look at "Solaredge storenergy" for a DC coupled hybrid system.  Have a look at "Tesla Powerwall" for an AC coupled battery pack. Lots of other manufacturers, but those two have good schematics.

[Dillsue]

4 minutes ago, joth said:

Caveat on this is if it has an automatic grid disconnection switch or a separate non-grid tied  battery-backed AC output then it can remain powered. But, this is orthogonal feature it may or may not have depending on both technical and regulatory constraints

 

(For example my SolarEdge SE-8000H hybrid invert supports this technically, but it is not legal to use in the UK as it doesn't yet have the necessary 

Is that a UK spec G99 compliant inverter?? AFAIK that feature wont be in a UK spec inverters firmware???

[Danny42]

15 minutes ago, Dillsue said:

No. Both the grid  and your inverter feed your consumer unit and the power from either or both feeds the house circuits fed from the consumer unit. Your inverter is connected to an MCB(maybe fuse) in your consumer unit in the same way as all the other circuits in the house. The difference being during daytime power flows from the inverter TO the consumer unit rather than power flowing FROM the consumer unit for all other circuits. At night when the inverter isnt generating, a small amount of power flows TO the inverter. None of the circuits are connected to the busbars

You write that "At night when the inverter isnt generating, a small amount of power flows TO the inverter" Do you mean that at night the solar panels aren't 'generating'? I don't think of an inverter 'generating' power, just transforming it. I guess we used to use 'function generators' at uni to create arbitrary waveforms, it just sounds a bit odd in this context to refer to an invertor as 'generating'; I certainly do think of the solar panels or wind turbine as 'generating' power.

>> If you've read that grid tied systems cant have batteries you need to be a bit more choosy where you get your info as that is wrong.
https://www.solarreviews.com/blog/grid-tied-off-grid-and-hybrid-solar-systems
First section contrasts 'grid tied' as a definition only applying to systems wihtout batteries.
Also:
https://skyelectric.com/blog/hybrid-vs-grid-tied-solar-energy-systems/#:~:text=2- GRID-TIED SOLAR ENERGY SOLUTIONS&text=These systems are less effective,be stored for future consumption.
Again it explicitly states that: Grid-tied solar energy systems also lack energy backups. So excess energy cannot be stored for future consumption. 
They both seemed well written but were promptly contradicted by the specs on devices advertised as 'grid tied'.

Please understand: I'm not saying they're right and you're wrong, I expect you are absolutely correct. I'm just saying that I did check a couple of sites, before finding the information rather contradictory! As for telling me to 'be a bit more choosy where you get your info', well I read quite a bit, found it contradictory and then came here, here is where I chose to get some more definitive info, as I think should be clear from all the questions

 

[Dillsue]

10 minutes ago, Danny42 said:

They both seemed well written but were promptly contradicted by the specs on devices advertised as 'grid tied'.

Enough said!! I only looked at the first site which is a US site. Youre better sticking to UK focussed sites as rules and terminology differ from other countries.

 

You are correct that its the panels that do the generating and inverters the conversion.

[joth]

19 minutes ago, Dillsue said:

Enough said!! I only looked at the first site which is a US site. Youre better sticking to UK focussed sites as rules and terminology differ from other countries.

 

The second looks to be based in Pakistan. Not going to the most authoritative source to learn about UK compliant equipment.

 

[joth]

49 minutes ago, Dillsue said:

Is that a UK spec G99 compliant inverter?? AFAIK that feature wont be in a UK spec inverters firmware???

Yes it is G99 compliant. It's disabled in firmware, and as well as firmware change I believe needs additional external hardware to enable the off-grid failover. I've really not looked into it as it's really not something I currently care that much about. (But ask me again when we start the winter of discontent rolling blackouts)

 

[Danny42]

1 hour ago, Dillsue said:

Enough said!! I only looked at the first site which is a US site. Youre better sticking to UK focussed sites as rules and terminology differ from other countries.

 

You are correct that its the panels that do the generating and inverters the conversion.

I think you may be misunderstanding, I was explaining why I came to this site for better answers. It was a compliment and explanation, I wasn't suggesting they were good sources of data, I was just trying to say that the first few hits on google for 'grid-tie vs hybrid' sent me to places that contradicted specs and hence I came to this site.

Regarding: "you are correct", yes, I know I'm correct about use of that term within this context. I intentionally referred to using function generators at uni. in the hope that you might better judge the context of my statement. 4 years at university studying physics and then various technical careers have not left me in the position of asking people for the definition of the word 'generating'. I was trying to explain to you that your statement was a little confusing. You wrote that: "At night when the inverter isnt generating, a small amount of power flows TO the inverter" I found this to be a tad unclear, and was explaining to you that perhaps you didn't really mean 'generating' and was hoping you might clarify what you had meant. For instance you could have written: "I meant that when the solar panels aren't generating your invertor will draw a small current from the grid."

Also, above you wrote that the invertor's output in a grid-tied system is not connected to the busbars, but is connected to the consumer unit just all other load circuits are, "via an MCB(or fuse)". I get that a circuit breaker will likely be involved, I wasn't really enquiring about the minutia of protective circuitry present when I asked if: the invertor output is hooked up to the busbars. What I meant was: is it connected in parallel to the grid connection and hence all the load circuits. And I think the answer is yes.

Anywho, I think I've taken this line of enquiry as far as I can just here. Thank you to everyone for your help, its really appreciated.

 

[Spacey73]

On 22/09/2022 at 14:27, Dillsue said:

Absolutely not. It MUST shutdown or it could electrocute someone trying to repair the cause of your power cut

This is true, but mine has a switch over setting for this scenario and will power the house. It takes 10ms to switch over, as soon as it detects a grid signal again it goes back to islanding mode again.

[Radian]

On 06/09/2022 at 19:56, Danny42 said:

Here's my question: If the electronics in my invertor are rated to supply 5kW, and then any topping up I need from the mains cant come 'through' the inverting circuits. Does it get 'mixed-in' somehow, and does that mean that the invertor is always syncing the voltage, frequency and phase with the mains to allow for that?

Have you looked at the Victron Energy MultiPlus range? I don't know where these fit in with the other hybrid inverters being mentioned but it sounds to me like the ideal grid supported battery source for this application. While generally marketed as a supply for boats that can operate from on-shore power and/or battery (it claims it can boost shore power) it's also frequently used in RV's and houses.