Considering more solar and a battery

Qyburn

If you go with separate systems the old system will be using 3.0kW of your 5kW allowed export. Leaving 2.0kW for the new system on the PW3. When at full power, that's 1.56kW potentially clipped, or going into the battery.

Nick_Dr1

NedS said:

Nick_Dr1 said:

Yes, you've hit the nail on the head there. Batteries are good a balancing load but can't sink it forever. The only way is to force discharge to a predicted level for the following day to top it up, which requires a crystal ball (or a solar prediction site - they do exist!). Come the winter though its the opposite problem and you then have to top up the battery from cheap grid electricity to a level that will see you though the day. You'll never get it exact - like playing golf!
Best (least stressful) way is to accept some clipping in the summer and hope the oversized array gets you some benefit in the winter.(in my view, unless you have plenty of time to tune it every day)

Winter is not an issue - we will switch to the Octopus Cosy heat pump tariff, and will always be fully charging the battery from the grid during the cheap rate slots. Solar won't cover our usage, even on a good day, so we will always use what we generate (maybe with a small amount of battery buffering).

The concern is really for the 6 months of summer - from mid march to mid September I currently see clipping (flattened curve) from 12:00-14:10pm on my 3.65kWp array and 3.0kW inverter. Our roof is south facing (-2 deg off south) and is unshaded so we pretty much see our kWp value (or very close) for 2h at midday.

Just to illustrate this, this is one day from my system. You can clearly see the problem with a low export limit as generation drops when the battery is full.

NedS

Qyburn said:

If you go with separate systems the old system will be using 3.0kW of your 5kW allowed export. Leaving 2.0kW for the new system on the PW3. When at full power, that's 1.56kW potentially clipped, or going into the battery.

The other difference is if I keep the old system there will be no clipping as such, as everything generated will be inverted, so it then becomes a use it (or store it) scenario (export 5kW and use as much as possible of the remaining ~1.5kW), whereas if I put both arrays on the Tesla PW3, there is up to 2kW of clipping that can only be stored, not used, as it's above the inverter limit.

That said, I think the Tesla PW3 inverter is likely to be more efficient and it's easier for me to monitor if all the solar is on the PW3, plus the install will be neater.

Modelling showed it's manageable if I can force export from around 8am, the battery.

I'm still really no closer to deciding, there are arguments for both approaches.

QrizB

Are you saying that if you have two inverters (current plus PW3) the DNO will let the PW3 have a 5kW inverter rating and limit export dynamically, but if you only have the PW3 you'll be stuck with a 5kW inverter so you'll never be exporting 5kW due to house loads?

That sounds weird to me, in the single inverter case why wouldn't they let you have the PW3 set higher but with a 5kW dynamic export limit?

Qyburn

QrizB said:

That sounds weird to me, in the single inverter case why wouldn't they let you have the PW3 set higher but with a 5kW dynamic export limit?

It doesn't sound right to me either. Nor does it sound correct that the DNO would limit the inverter to 5kW from solar, but allow it to put out 11kW from battery+solar.  (I'm referring to the G.100 size limit, not export limit)

And there are still unknowns about the PW3 as well, maybe Screwdriva can confirm. For example if the output is limited to 5kW can that be configured to affect output from solar only, leaving battery->house unrestricted, or can it only limit total output?

And comparing with Solaredge which I know Screwdriva is also familiar with, does it offer modes equivalent to Solaredge "Discharge to minimise import", "Discharge to maximise export" and "Charge from clipped solar"

NedS

QrizB said:

Are you saying that if you have two inverters (current plus PW3) the DNO will let the PW3 have a 5kW inverter rating and limit export dynamically, but if you only have the PW3 you'll be stuck with a 5kW inverter so you'll never be exporting 5kW due to house loads?

Yes, I know.

Let me explain how we got here...

We did a regular G99 application, and the DNO were concerned about voltage rises, so they said you can have the proposed 7.2kWp of solar and you can set your inverter rating to 8kW to cover this, but you must limit export to 1kW so we don't cause the grid voltage to rise by more than the allowable amount (253v AC). That offer simply didn't stack up for me as the 1kW export limitation would be too limiting.

The DNO then informed me about a fast track process they have available based on SGI 3 that will permit (up to) two inverters, each of a maximum of 5kW, and a 5kW total export limit from the property:

https://connections.nationalgrid.co.uk/get-connected/solar-and-wind/fast-track-g99/

They said there is no voltage calculation performed for this fast track application process, just an impedance test we must pass. They said this process often gives a better outcome for those who would otherwise receive a very low export offer (my feeder from the substation is already saturated with solar generators as a third of the connected properties have solar installs).

This seems a far more preferable situation than an effectively unlimited inverter (generation) rating but with a 1kW export limit, so this is what we are going with.

I believe the Tesla system limits export at the Gateway (which sits between the meter and consumer unit), and doesn't know or care where it's coming from, be it the old solar inverter, new solar array / PW3 inverter, or forced discharge of the battery. The DNO doesn't care as long as it doesn't exceed 5kW .

To answer the last part of your question above, I think we could export at the full 5kW and meet the house load by forced discharge of the battery (which can deliver sustained loads of up to 11.04kW even if the inverter setting is limited to 5kW (at least this is what Tesla support tells me). The challenge will be to manage the peak midday generation to manage that 2kW of clipping - I can export until my hearts content later in the day/evening as long as I can store that peak generation at midday, so only exporting 4kW whilst using 1kW isn't an issue

That sounds weird to me, in the single inverter case why wouldn't they let you have the PW3 set higher but with a 5kW dynamic export limit?

Yes, I agree, but they make the rules and I'm just glad to be able to go ahead with the install and have some kind of sensible export limit, otherwise it's a non-starter with a 1kW export limit. Of course I would much rather have one inverter at 7kW than 2 x 5kW inverters giving 10kW in total, but they are not my rules. The rules are simply that I cannot connect generating equipment (inverter) capable of generating more than 2 x 5kW max, and I cannot export more than 5kW total.

Trying to summarise the pro's and con's of each:

If I keep the old inverter alongside PW3:

• Max inverted generation of ~6.5kW, with minimal clipping
• Export 5kW, use or store the remaining 1.5kW of peak time generation
• Old inverter is less efficient, losing upto 0.5kW due to lower efficiencies and minimal clipping
  
• More complicated to collect and record data as solar generation from two inverters

If I ditch the old inverter and just use the PW3:

• 7.2kWp on 5kW inverter - substantial clipping, will need to carefully manage battery SOC to ensure space to store the clipped excess, or lose it
• Can only invert 5kW in total to use/export unless forced battery discharge
• Newer inverter, more efficient (97.5% solar to home/grid)
• Neater install as getting rid of old inverter
• Easier to manage data as one source of data, not two

I still have no clear view on which is preferable

NedS

An update:

We finally have DNO approval (limited to 5kW) and an installation date in a few weeks.

We are having a Tesla PW3 with two strings of south facing solar (3.645kWp and 3.56kWp) connected to MPPT1 and MPPT2, respectively. So we will have a 7.2kWp oversized array connected to a 5kW inverter, with 5kW export limit.

Is there any interest in starting a Powerwall owners thread to discuss Powerwall settings / operation and other Powerwall news?

QrizB

I don't have a Powerwall but a thread would complement the one for Lux inverters. I think there are enough PW owners on the forum to make it worthwhile, and the rest of us can see what we're missing out on!

NedS

My PW3 and second solar array was finally installed this week. The installers did a great job, so I'm happy.

I'm still getting to know the system but am getting a feel for what it's capable of.

I will clarify a couple of technical points we discussed earlier in the thread.

The Powerwall 3 is very flexible in the way it can be commissioned and set up which allows it to be used in almost any (single phase) deployment regardless of what DNO restrictions may exist.

The first of this is the inverter rating which can be set during commissioning. The documentation says this can be configured at 3.68kW, 5, 6, 7, 8, 9, 10 or 11.04kW. It's actually configured by max output current (Amps), with the max (default) setting being 48A corresponding to a nominal 11.04kW at 230v. How many kW the PW3 will output under load will vary depending on voltage - with a voltage of around 240v it will deliver ~11.5kW. This setting exists to allow installation where the DNO has placed a restriction on the maximum inverter size, or allow a PW3 to be installed as a direct replacement of an existing inverter and set to the same rating. Some installers are installing PW3 under G98 and limiting it to 3.68kW saving them the hassle of a G99 application. If the inverter output is limited, this will limit the amount of power the PW3 battery can deliver to the home as well as the amount of PV solar that can be inverted. 

There are also settings for import and export limits. Import limits can be set to prevent the system drawing more power than the connection can handle. PW3 is capable of grid charging at 5kW (or 8kW if an expansion pack is fitted), and you can install up to 4 PW3, each with 3 expansion packs. In theory 4 PW3's could charge at 20kW (87A), or 32kW (139A) if each PW3 has expansion packs installed which would exceed the 80A fuse commonly fitted at most residential single phase properties. Setting the import limit prevents the system drawing excess current but slows the ultimate charge rate on a multi-PW3 system. The system can also be configured to allow grid charging or not. If Grid charging is permitted, this can be toggled on/off by the user in the Tesla App.

Export limits can also be independently set to conform with any DNO restrictions on export. The PW3 can be configured for no export, export from solar only, or to allow export from solar and battery, and the export limit can be set in Watts. If export has been allowed during commissioning, then there is an option in the Tesla App for the user to export export on/off.

Screwdriva

Well done! Glad to see another fan of the PW3