Inverters

uk1

Good morning all.

Another question from a trainee pv'er!

I'm receiving quotes at the moment and I have a question about inverters. 

1.  If a quote includes an inverter that is a 5.0 KW (GivEnergy 5.0) does this imply that a G99 will have to be applied for and given?  Does the 5k give me a worthwhile amount of power over a year on a 6kw array to make a G99 application worthwhile - if it is indeed needed?

2.  If a quote includes an inverter limited at 3.68kw (Solis)  I'm presuming that this implies the easier G98.  But based on instinct, experience or actual calculation what would I lose on a lovely day and perhaps an average day on a 6kwh array.  Or does the excess over 3.68kw that isn’t being currently used divert to the batteries ie no loss?  

Thanks all for the continued patience and replies and helping me with my PV education!

[Deleted User]

Anything that is less than 16amps/phase (3.68kW/phase) just requires the DNO to be informed post installation. An installation greater than 16amps/phase requires prior DNO approval. The DNO could impose a site export limit.

I have a 6.35kWp array with a 5kW SolarEdge inverter. The inverter is sized to take into account array size; orientation and output during the Winter months. My installer was able to show me the SolarEdge graphs on his system which he uses to match array with inverter size.

The inverter is what it is. If you have a 5kW inverter then it will flatline when it reaches 5kW. It doesn’t produce 6kW and send 1kW to the battery. You can see the flatlining in this graph. (The maximum output is less than 5kW due to the temperature of the panels).

I am struggling to understand why you are considering a 3.68kW inverter with a 6kWp array unless the installer intends to uprate it. For example, SolarEdge inverters can be updated to 115%. The only other reason for going for a 3.68kW inverter is that the installer doesn’t want to wait, or pay for, DNO approval. 

Seriously, you need to press your installer to show you the calculations that underpin the choice of inverter. My installer was very happy to do so. As he put it ‘inverter size is always a compromise’. His recommendation was to lean slightly towards Winter which is when people want electricity.

My 6.35kWp array on 2 roofs with a 90 degree difference in orientation produces 5900kWh a year.

uk1

Dolor said:

Anything that is less than 16amps/phase (3.68kW/phase) just requires the DNO to be informed post installation. An installation greater than 16amps/phase requires prior DNO approval. The DNO could impose a site export limit.

I have a 6.35kWp array with a 5kW SolarEdge inverter. The inverter is sized to take into account array size; orientation and output during the Winter months. My installer was able to show me the SolarEdge graphs on his system which he uses to match array with inverter size.

The inverter is what it is. If you have a 5kW inverter then it will flatline when it reaches 5kW. It doesn’t produce 6kW and send 1kW to the battery. You can see the flatlining in this graph. (The maximum output is less than 5kW due to the temperature of the panels).

I am struggling to understand why you are considering a 3.68kW inverter with a 6kWp array unless the installer intends to uprate it. For example, SolarEdge inverters can be updated to 115%. The only other reason for going for a 3.68kW inverter is that the installer doesn’t want to wait, or pay for, DNO approval. 

Seriously, you need to press your installer to show you the calculations that underpin the choice of inverter. My installer was very happy to do so. As he put it ‘inverter size is always a compromise’. His recommendation was to lean slightly towards Winter which is when people want electricity.

My 6.35kWp array on 2 roofs with a 90 degree difference in orientation produces 5900kWh a year.

Many thanks for that.

When looking at your graph it seems to me that you are only gaining the extra kwh over 3.68 for a couple of hours at most on a sunny day.  For the rest of the time your 6k array is below 4 kwh?  Have I understood that correctly?  I think my array will be looking like 5.7kwh if I take the quote that I’m currently favouring.

ABrass

A bit of undersizing is fine, you don't lose much. Some people recommend it to get better performance at the low generating end. 10-20% is within the normal range.

With a split orientation system then a smaller inverter makes sense as both aspects will rarely both be fully exposed to the sun.

With 5.7kW of panels and a 3.68KW inverter you're losing 35% of the potential power at the top end. That's larger than normal, I would be looking for a 5kW inverter.

Most inverters can be configured to only ever export 3.68kW to the grid, but allow you to use the full power of the inverter to power your house or charge a battery.

If you're in the South and covered by UK Power Networks then G99 applications are free, it's just a bit of paperwork for your installer. Otherwise going for a 5kW inverter and capping the grid export might save some time and cost.

uk1

Thanks for that …. I’m learning all the time.  I hadn’t realised that a 5k could be restricted in that way and still allow the rest to be used or stored. 

The Solis is the one being proposed and I noticed they have a 5k version.

https://www.segen.co.uk/product/solis-5kw-s6-dual-mppt-single-phase-with-dc/

Martyn1981

Hiya, Givenergy produce a hybrid 5kW inverter, so if you are getting batteries too, then you could cap at 3.68kW, but direct extra into the batts.

But it's not as easy as that, since you may fill the batteries up during the morning as the generation rises, and then by mid day when gen is peaking at say 5kW, the batts can't take any more, and the 3.68kW export would kick in and restrict generation.

Hope that makes sense, just showing that there may be a number of variables.

Totally agree about the 10-20% guide for the UK, as we get a lot of poor weather, so an undersized inverter (smaller) can be more efficient at low levels, and make up for what it loses capping peak gen at times. Obviously for countries with lots of good sun (eg Spain), things are different.

uk1

Many thanks.

If you buy a system with a 5k inverter that is set to 3.68 does this require the G99 or G98 process?

Am I right in thinking that a G99 system cannot be commissioned until approval has been received but a G98 is retrospective and commissioning proceeds. 

Alnat1

Martyn1981 said:

But it's not as easy as that, since you may fill the batteries up during the morning as the generation rises, and then by mid day when gen is peaking at say 5kW, the batts can't take any more, and the 3.68kW export would kick in and restrict generation.

I've got 5.25kW on the roof with a 3.6kW hybrid inverter. If I know it's going to be sunny most of the day, I set the battery fill rate to 0% early in the morning and adjust it later when the output is heading towards 3.6kW. While ever the batteries are filling the output won't flatline at 3.6. It also works well with Outgoing Agile for export, as I'm filling the batteries 12-3 when the prices tend to be lowest.

Unfortunately, my Lux app doesn't have the option to set this by time and I have to manually remember to change the %. Also, there's the risk that the sky clouds up at lunchtime and the batteries don't fill but that hasn't happened yet 

uk1

Just a hopeful bump of my question ….

If you buy a system with a 5k inverter that is set to 3.68 does this require the G99 or G98 process?

Am I right in thinking that a G99 system cannot be commissioned until approval has been received but a G98 is retrospective and commissioning proceeds. 

QrizB

uk1 said:

Just a hopeful bump of my question ….
If you buy a system with a 5k inverter that is set to 3.68 does this require the G99 or G98 process?

FWIW I think it needs G100, as you're using export-limiting technology.
https://www.energynetworks.org/assets/images/Resource library/ENA_EREC_G100_Issue_1_Amendment_2_(2018).pdf

uk1

Thanks co-NorthHampshiree …..:)

Asking in case you know to save me reading it all.  If it is G100 does that imply prior approval before installing and commissioning?