Considering more solar and a battery

Martyn1981

NedS said:

Would you not be better putting all your solar, new and existing, into the PW3?

This is a good question and indeed is an option.

If I put all the solar into the PW3, I will have 7.2kWp solar connected to a 5kW inverter (that's the DNO limit of the offer), so I'll be clipping 2.2kW in summer (I currently see clipping on my 3.6kWp array on a 3kW inverter for ~6 months of the year - we are south facing, unshaded in a cooler area of the south of the UK). By the time we start clipping, the battery is already going to be full, so I'm giving up 2.2kWp of generation during the middle of the day on sunny days - for what benefit?

That was the basis of my first suggestion. Can't you have the battery exporting upto 5kW*, so that when the generation exceeds 5kW, it will take up to 6hrs to fill the DC side battery and cause clipping. [As I mentioned, I have 6.725kWp of PV and a 3.68kW inverter, but experience no clipping, so long as I manage the battery SOC.]

*Obviously, depending on export rates, you don't have to be exporting all this, just discharging it from the battery, with you using as much of it as you want.

NedS

Martyn1981 said:

NedS said:

Would you not be better putting all your solar, new and existing, into the PW3?

This is a good question and indeed is an option.

If I put all the solar into the PW3, I will have 7.2kWp solar connected to a 5kW inverter (that's the DNO limit of the offer), so I'll be clipping 2.2kW in summer (I currently see clipping on my 3.6kWp array on a 3kW inverter for ~6 months of the year - we are south facing, unshaded in a cooler area of the south of the UK). By the time we start clipping, the battery is already going to be full, so I'm giving up 2.2kWp of generation during the middle of the day on sunny days - for what benefit?

That was the basis of my first suggestion. Can't you have the battery exporting upto 5kW*, so that when the generation exceeds 5kW, it will take up to 6hrs to fill the DC side battery and cause clipping. [As I mentioned, I have 6.725kWp of PV and a 3.68kW inverter, but experience no clipping, so long as I manage the battery SOC.]

*Obviously, depending on export rates, you don't have to be exporting all this, just discharging it from the battery, with you using as much of it as you want.

If the battery can be configured to do that, then yes that could be manageable.

I believe the order of priority is fulfil the house load requirements first, then charge the battery, then export the excess, so if the battery starts the day near empty, it will fully charge the battery first (by mid morning), before it starts exporting. The battery can charge at a rate of 5kW, so with a 7.2kWp array, it's going to fully charge in 2-3 hours

Until I get the thing and have a play, I really don't know how much fine user control is available. It's all controlled by the Tesla App and seems designed for most users to just set and forget.

It basically offers two modes - Self-Powered mode which uses stored solar energy to power your home and tries to minimise grid imports, and  Time-Based Control which uses stored energy to maximize savings based on ToU tariffs. The AI looks at your typical usage and works out how best to manage the battery, depending which operational mode you've chosen.

I know I can cause the battery to force charge (from the grid) by setting the backup reserve to 100%

I know I can prevent the battery from discharging by setting the backup reserve at or above the current SOC.

I think you can allow the battery to discharge to the grid at 'peak times', but I think it's based on if the AI considers it beneficial to do so.

Qyburn

NedS said:

Qyburn said:

Limiting PW3 output will also limit power that can come from the battery. So if set to 3.0kW then any time house load is over 3.0kW the extra will be drawn from the grid, even if the battery's full. 

The PW3 inverter is 11.04kW and is capable of supplying sustained loads of 11.04kW. When the inverter setting is limited in software to a lower value (either 5kW or 3.68kW in my case), presumably they are saying that this limit only affects/applies to the MPPT generation capacity, and does not affect what the battery can deliver under load. So, for example, if limited to 5kW and the house is pulling 10kW, a max of 5kW can come from solar and the remaining 5kW from stored battery capacity, but the total 10kW load can be met without having to draw from the grid.

We don't have the benefit if knowing exactly what the DNO specified, or why. But normally when they state a maximum system size it refers to maximum output irrespective of whether that comes from battery or from solar.

I don't know what settings are available on the PW3 but if it could be limited to 10kW, then ditching the old inverter and putting everything through the PW3 would be more efficient that splitting your  allowed capacity between two inverters. Still with the 5kW export limit.

Martyn1981

NedS said:

Martyn1981 said:

NedS said:

Would you not be better putting all your solar, new and existing, into the PW3?

This is a good question and indeed is an option.

If I put all the solar into the PW3, I will have 7.2kWp solar connected to a 5kW inverter (that's the DNO limit of the offer), so I'll be clipping 2.2kW in summer (I currently see clipping on my 3.6kWp array on a 3kW inverter for ~6 months of the year - we are south facing, unshaded in a cooler area of the south of the UK). By the time we start clipping, the battery is already going to be full, so I'm giving up 2.2kWp of generation during the middle of the day on sunny days - for what benefit?

That was the basis of my first suggestion. Can't you have the battery exporting upto 5kW*, so that when the generation exceeds 5kW, it will take up to 6hrs to fill the DC side battery and cause clipping. [As I mentioned, I have 6.725kWp of PV and a 3.68kW inverter, but experience no clipping, so long as I manage the battery SOC.]

*Obviously, depending on export rates, you don't have to be exporting all this, just discharging it from the battery, with you using as much of it as you want.

If the battery can be configured to do that, then yes that could be manageable.

I believe the order of priority is fulfil the house load requirements first, then charge the battery, then export the excess, so if the battery starts the day near empty, it will fully charge the battery first (by mid morning), before it starts exporting. The battery can charge at a rate of 5kW, so with a 7.2kWp array, it's going to fully charge in 2-3 hours

Until I get the thing and have a play, I really don't know how much fine user control is available. It's all controlled by the Tesla App and seems designed for most users to just set and forget.

It basically offers two modes - Self-Powered mode which uses stored solar energy to power your home and tries to minimise grid imports, and  Time-Based Control which uses stored energy to maximize savings based on ToU tariffs. The AI looks at your typical usage and works out how best to manage the battery, depending which operational mode you've chosen.

I know I can cause the battery to force charge (from the grid) by setting the backup reserve to 100%

I know I can prevent the battery from discharging by setting the backup reserve at or above the current SOC.

I think you can allow the battery to discharge to the grid at 'peak times', but I think it's based on if the AI considers it beneficial to do so.

My Bold - Thanks, that's what I was missing, sorry should never assume.

So yep, all comes down to what you can and can't do with it. Hopefully someone with a PW3 can comment, or maybe there is info on the interweb, as that might really make a difference to your plans.

NedS

Qyburn said:

NedS said:

Qyburn said:

Limiting PW3 output will also limit power that can come from the battery. So if set to 3.0kW then any time house load is over 3.0kW the extra will be drawn from the grid, even if the battery's full. 

The PW3 inverter is 11.04kW and is capable of supplying sustained loads of 11.04kW. When the inverter setting is limited in software to a lower value (either 5kW or 3.68kW in my case), presumably they are saying that this limit only affects/applies to the MPPT generation capacity, and does not affect what the battery can deliver under load. So, for example, if limited to 5kW and the house is pulling 10kW, a max of 5kW can come from solar and the remaining 5kW from stored battery capacity, but the total 10kW load can be met without having to draw from the grid.

We don't have the benefit if knowing exactly what the DNO specified, or why. But normally when they state a maximum system size it refers to maximum output irrespective of whether that comes from battery or from solar.

I don't know what settings are available on the PW3 but if it could be limited to 10kW, then ditching the old inverter and putting everything through the PW3 would be more efficient that splitting your  allowed capacity between two inverters. Still with the 5kW export limit.

In this case the offer is for up to two inverters, each at a maximum of 5kW. They have also allowed a total export limit of 5kW, regardless of whether there are one or two inverters.

To summarise, I have two options:

Option 1:

Solar Array 1 (3.645kWp) remains on existing non-hybrid 3kW inverter

Solar Array 2 (3.56kWp) in installed on MPPT1 of the Tesla PW3

Overall export is limited to 5kW (DNO restriction)

Option 2:

Solar Array 1 and Solar Array 2 (7.2kWp) attached to the PW3, connected MPPT1 and MPPT2 respectively, limited to 5kW total (DNO restriction) with potential for ~2kW clipping due to the restriction.

Ditch the existing dumb non-hybrid inverter

Overall export is limited to 5kW (DNO restriction)

The inverter in the PW3 is an 11.04kW inverter that can be software limited in 1kW steps from 3.68kW, then 5kW through to 11kW. I believe this only affects/limits the solar generation (MPPTs) DC/AC conversion, not the DC/AC conversion from battery as the battery can still deliver the full 11.04kW of sustained load (as there are 3 MPPTs, and 11.04/3 = 3.68, I suspect it's effectively 3 x 3.68 inverters across the 3 MPPTs)

Qyburn

NedS said:

I believe the order of priority is fulfil the house load requirements first, then charge the battery, then export the excess, so if the battery starts the day near empty, it will fully charge the battery first (by mid morning), before it starts exporting. 

If the option exists with the PW3 you're better to charge from "clipped" power.  

As an example we have 8.5kW solar, and 6kW export. Default setting has our battery full by 09:00 or 10:00, meaning that at peak time there's nowhere for the surplus to go.  

Here's an example using the charge from clipped setting.  Blue is solar power, yellow battery charge/discharge. See middle of the day as the power goes over 6kW the overflow goes into the battery.

Nick_Dr1

Qyburn said:

NedS said:

I believe the order of priority is fulfil the house load requirements first, then charge the battery, then export the excess, so if the battery starts the day near empty, it will fully charge the battery first (by mid morning), before it starts exporting. 

If the option exists with the PW3 you're better to charge from "clipped" power.  

As an example we have 8.5kW solar, and 6kW export. Default setting has our battery full by 09:00 or 10:00, meaning that at peak time there's nowhere for the surplus to go.  

Here's an example using the charge from clipped setting.  Blue is solar power, yellow battery charge/discharge. See middle of the day as the power goes over 6kW the overflow goes into the battery.

Wish mine had that. I have to do the same thing from HomeAssistant, which isn't the most friendly system in the world.

NedS

Qyburn said:

NedS said:

I believe the order of priority is fulfil the house load requirements first, then charge the battery, then export the excess, so if the battery starts the day near empty, it will fully charge the battery first (by mid morning), before it starts exporting. 

If the option exists with the PW3 you're better to charge from "clipped" power.  

As an example we have 8.5kW solar, and 6kW export. Default setting has our battery full by 09:00 or 10:00, meaning that at peak time there's nowhere for the surplus to go.  

Here's an example using the charge from clipped setting.  Blue is solar power, yellow battery charge/discharge. See middle of the day as the power goes over 6kW the overflow goes into the battery.

Yes, agreed. I'm currently just unsure if (a) the Tesla has the ability to fine control / manage it, and (b) if the battery will ever have spare capacity. In peak summer (days we are concerned about clipping) we will be generating 4 times our daily usage (generating ~50kW, using around 12-13kW). I expect the battery is always going to be full at the end of the day, and we are only likely to use ~3kW overnight, so come the following morning there is going to be very little spare capacity and it is going to fill quickly. Unless I can force discharge the battery early morning and/or control the charge rate / prevent from charging, there will be no scope to charge from clipped solar.

I've been looking at my existing non-hybrid inverter in a bit more detail (it's a cheapie basic Fox ESS 3kW model). It clips at around 3.26kW being fed in from the 3.645kWp array, with an efficiency of around 96.5% (The Tesla quotes 97.5% efficiency), so I'm maybe leaving 400W max of clipping on the table there due to the under-sized inverter. The DC startup voltage of both inverters is similar at 60v.

I'm going to see if I can find a Tesla user forum to get some first hand advice from users with similar configurations.

Nick_Dr1

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)

NedS

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.