Help with Growatt setting and Octopus Agile tariff (UK)

[OldCodger]

Newbie here. I just bought a house here in the UK which has a Growatt SPH 3000 system already installed. It is 3kW of panels and 3 kWh of batteries.


I’ve managed to get access to the system via ShinePhone and the web interface at server.growatt.com

I am on the Octopus Agile tariff, which has different prices of electricity for every half hour of the day.

I am trying to set up the system to charge the batteries when electricity is cheap and to discharge them when it is expensive.

Thanks to several posts here I’ve worked out how to set the system to charge from the grid overnight. However what I can’t do is see how to “disconnect” the batteries so they don’t start discharging until 4pm when (for now at least) the rates are much higher.

Is there a combination of settings that will achieve this?

Apologies if this has been answered before, I did search the forum before joining to see if there was a previous solution.

OldCodger - aka Ian

 

[OldCodger]

Commenting on my own post, which is bad form I know,but having spent an afternoon researching this - is this something simple? Do I just need to set “Grid First” between 4pm and, say midnight? So I use “all” of the battery capacity.

Not sure how this pans out in sunnier times!

I really am still learning this stuff and I don’t know all the jargon, or how to use it properly!

 

[rpdom]

I can't comment on the Growat - I've got Solis, but I'm on Agile for import and I charge my batteries overnight, use them during the day and if it hasn't been sunny I top them up a bit mid afternoon during the cheapest slot there. Like today there was a cheaper (12.9p) slot between 3pm and 3:30pm. Also for export I'm on Outgoing at a fixed 15p/kWh at all times, so I don't have to export at peak times (although I tend to do that just to help the grid).

 

[OldCodger]

I can't comment on the Growat - I've got Solis, but I'm on Agile for import and I charge my batteries overnight, use them during the day and if it hasn't been sunny I top them up a bit mid afternoon during the cheapest slot there. Like today there was a cheaper (12.9p) slot between 3pm and 3:30pm. Also for export I'm on Outgoing at a fixed 15p/kWh at all times, so I don't have to export at peak times (although I tend to do that just to help the grid).

Thank you! For now, I’m doing roughly what you say, however my batteries are usually exhausted by mid morning, so the top up at 14:30 is basically a full charge. It at least means I use “12.9p” electrons not “30.7p” electrons!

 

[SeaGal]

however my batteries are usually exhausted by mid morning

sounds like you need a lot more kWh storage

 

[OldCodger]

sounds like you need a lot more kWh storage

It’s on the list. Is there a rule of thumb? Ie panels/inverter rating/battery capacity. I’m not looking to go totally solar (not enough roof!) but to get through the afternoon/evening on “stored” electricity would be good.

 

[SeaGal]

Is there a rule of thumb?

Yes... more is better; lots is best

I built 14.3kWh pack as that matched typical daily usage. So in rainy Winter day I can cover 17h between Economy 7 rates.

But still hoping to double that somewhen

 

[OldCodger]

Yes... more is better; lots is best

Understood. But there must be some limit on RoI? I’m not sure what the cost of a 3kWh battery pack is - but I’ve seen prices of £1,400 mentioned plus installation. I seem to use about 15kWh a day so 4 more packs will cover it - but with only 3kW of panels, getting 5 hours a day of full sunshine in the British winter is a bigger hurdle

 

[rpdom]

I've not worked out the RoI. I have a 3kW PV array with 4.8kWh of batteries. I'm looking to add at least another 4.8kWh battery to that. It will take a couple of years to cover the extra cost.

One thing to note is that battery prices are falling, and about 6 minutes before I typed this, additional batteries for Solar use became exempt from VAT, so have just dropped 20% in cost.

 

[OldCodger]

One thing to note is that battery prices are falling, and about 6 minutes before I typed this, additional batteries for Solar use became exempt from VAT, so have just dropped 20% in cost.

Now that is worth looking into! Thank you.

 

[OldCodger]

Commenting on my own post, which is bad form I know,but having spent an afternoon researching this - is this something simple? Do I just need to set “Grid First” between 4pm and, say midnight? So I use “all” of the battery capacity.

Not sure how this pans out in sunnier times!

I really am still learning this stuff and I don’t know all the jargon, or how to use it properly!

Going back to my original question

Does anyone with a Growatt inverter know how to stop the batteries discharging until a fixed time in the day ?


Thanks to @SeaGal and @rpdom for their help I learned a bit more about solar on the way!

 

[SeaGal]

But there must be some limit on RoI? I’m not sure what the cost of a 3kWh battery pack is - but I’ve seen prices of £1,400 mentioned plus installation. I seem to use about 15kWh a day so 4 more packs will cover it - but with only 3kW of panels, getting 5 hours a day of full sunshine in the British winter is a bigger hurdl

I built my DIY 14.3kWh battery 18 months ago for about £2500. Today, that would be less - estimate of around £1800 to £2000. 5kWh packs in the UK, pre-built, are about £1000. Obviously ROI will depend hugely on your consumption patterns and purchase price of electrons and how much you can time-shift in Winter when we'll never have enough PV input. I have 6.6kWp of panels at the moment, but about to double that in the next month of two. Even so, I won't be self sufficient come Winter time - will still need to top up on Economy 7. For me, though, it's as much a hobby come buswoman's holiday, so the ROI of sub 5 years is a bonus

One thing to note is that battery prices are falling, and about 6 minutes before I typed this, additional batteries for Solar use became exempt from VAT, so have just dropped 20% in cost.

Ah... staying up to get your order in then ... like those people who camp out at Apple stores when the latest iPhone is released Seriously though, the way I read the rules is that the VAT exemption is for 'supply and installation' of energy saving materials until 2027 now including retrofit batteries. So, I didn't think that you could just buy a battery rack from a retailer and have the VAT zero rated if said retailer didn't 'install' it... not that installing a battery pack involves that much physical work. Do correct me if I'm wrong there!

Going back to my original question

Yep, sorry about the diversion... Like @rpdom, I have a Solis so don't know exactly how the Growatt works. Do I assume you just want to use PV and grid when the rates are average and kick in the battery to supplement your house usage when the grid rate is high? (As opposed to wanting to actively dump/export your battery charge to the grid to get export payments?).

Presumably you can enable and disable the battery via manual pressing buttons on the Growatt inverter itself or via the web interface you mention, but I guess you want something more automated.

If so (and the following is just my guess work from what I understand about Octopus Agile), is that you can receive the half-hourly rates the day before and then adjust your usage accordingly. Do you have software development skills? Most inverters support some form of real-time control, typically via an RS485 port with modbus RTU comms on it. I would therefore imagine that you'd need some external control software (maybe home assistant will do this out of the box... no idea as I write my own software) that can query the Octopus Agile rates via a web service type call one day and then send data to your Growatt the following day to enable/disable the charge/discharge parameters to align optimal use of the battery to the half-hourly rates. I'd be surprised if no one has created that sort of control system before, though maybe not necessarily for a Growatt. Hopefully someone who has a Growatt will chip in.

 

[OldCodger]

Do I assume you just want to use PV and grid when the rates are average and kick in the battery to supplement your house usage when the grid rate is high? (As opposed to wanting to actively dump/export your battery charge to the grid to get export payments?).

Yes. Thats the idea.


Although charging over night at under 10p and selling them back their own electricity at 15p has its attractions

 

[OldCodger]

Presumably you can enable and disable the battery via manual pressing buttons on the Growatt inverter itself or via the web interface you mention, but I guess you want something more automated.

Theoretically yes, but it’s in the attic so not easy to get at..

 

[OldCodger]

Do you have software development skills? Most inverters support some form of real-time control, typically via an RS485 port with modbus RTU comms on it. I would therefore imagine that you'd need some external control software (maybe home assistant will do this out of the box... no idea as I write my own software) that can query the Octopus Agile rates via a web service type call one day and then send data to your Growatt the following day to enable/disable the charge/discharge parameters to align optimal use of the battery to the half-hourly rates.

I’ve seen something called Solar Assist which seems to do this. But I need to invest in a Rspberry Pi and my skills with that are limited….

Introduction to SolarAssistant

Read a high level overview of SolarAssistant.

solar-assistant.io

(Sorry for all the individual replies this version of the forum software doesn’t seem to have multi quote like other forums I’m on do.. )

 

[SeaGal]

Sorry for all the individual replies

You can...

this version of the forum software

...just highlight the text you want to quote and

doesn’t seem to have multi quote like other forums I’m on do.. )

.. hover back over it and press reply... like this...

 

[OldCodger]

...just highlight the text you want to quote and

hover back over it and press reply... like this...

Ah. So you can - even on an iPhone. Thank you!

 

[rpdom]

Although charging over night at under 10p and selling them back their own electricity at 15p has its attractions

Even better when they have a "plunge" event and they pay you to charge your batteries

I’ve seen something called Solar Assist which seems to do this. But I need to invest in a Rspberry Pi and my skills with that are limited….

I believe there are also modules for Home Assistant that can use the Octopus API to get the rates and set your inverter. Again, I don't know if it works on Growat. Like @SeaGal I write my own software, most of which runs on Raspberry Pis. But I haven't fully automated the charging yet. I generate a daily report that shows me the six cheapest overnight slots, then set up a charge schedule for the best of those.

 

[OldCodger]

Just to close this out so anyone looking in future finds the outcome.

The system vendor eventually came back to me and confirmed that i need to use the "Battery First" setting. This will use Solar first, then Grid power to keep the batteries topped up to the "Stop Charging SOC" percentage (100% is the default)

They did advise against keeping the batteries at 100% all day as it could impact battery life.

So the strategy I now use is to charge the batteries at night using Grid power when it is cheap. The controller goes into Battery First mode between 3am and 7 am. This means we have full batteries as the day starts. At 7am it switches back to the default "Load First" so we start using the batteries to reduce the power we import. Any solar power during the day helps keep battery use to a minimum and even tops the batteries back up if there's an excess. (I start the charge at 3am as that is when electricity is often the cheapest)

Mid afternoon I "top up" the batteries using "Battery First" / Grid power so I reduce the amount of higher priced electricity I use during the peak price period (which at this time of year is 4pm-7pm) Under the Octopus Agile tariff afternoon electricity has been typically 15p / kWh but at peak rate it is regularly over 30p / kWh

(This is the Feb 2024 situation if you are reading this in the future)

In the hope it is of use to others, here's the screen shot of my settings page which acheives this.



Thanks to those who responded. I learned a lot!